Home
Search
Collections
Journals
About
Contact us
My IOPscience
Enhanced sensitivity of double gate junctionless transistor architecture for biosensing applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 145201 (http://iopscience.iop.org/0957-4484/26/14/145201) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.174.255.116 This content was downloaded on 03/05/2015 at 20:31
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 145201 (8pp)
doi:10.1088/0957-4484/26/14/145201
Enhanced sensitivity of double gate junctionless transistor architecture for biosensing applications Mukta Singh Parihar and Abhinav Kranti Low Power Nanoelectronics Research Group, Electrical Engineering Discipline, Indian Institute of Technology Indore, India E-mail: [email protected] Received 10 October 2014, revised 16 February 2015 Accepted for publication 16 February 2015 Published 16 March 2015 Abstract
In the present work, we demonstrate the potential of double gate junctionless (JL) architecture for enhanced sensitivity for detecting biomolecules in cavity modulated field effect transistors (FETs). The higher values of body factor, achieved in asymmetric gate operation under impact ionization is utilized for enhanced sensing margin which is nearly five times higher than compared to symmetrical mode operation. The intrinsic detection sensitivity is evaluated in terms of threshold voltage change, and the ratio of drain current in the presence and absence of biomolecules in JL nanotransistors. It is shown that asymmetric mode JL transistor achieves a higher degree of detection sensitivity even for a partially filled cavity. The work demonstrates the potential of JL channel architecture for cavity based dielectric modulated FET biosensors. Keywords: junctionless, MOSFET, biosensors, double gate (Some figures may appear in colour only in the online journal) Introduction
FET based sensors have faced challenges relating to selectivity and poor signal-to-noise ratio owing to intrinsic variation and non-controllability of parameters related to detection of signals [20]. To overcome these, FET devices with enhanced sensing margins are needed so as to effectively detect the presence of biomolecules. Previous studies of using FETs for biosensing applications have demonstrated the potential of conventional inversion (INV) mode transistor using dielectric modulated cavity either, in the gate oxide or at the side gate electrode [16–19]. The various designs have used either very high drain-tosource voltages (7.8 V) [19] or a very wide cavity area (3000–5000 nm × 50 nm) [17] for a meaningful detectable shift in drain current (Ids)—gate voltage (Vgs) characteristics which has been used as a sensing metric. Buitrago et al, have demonstrated sensing capability of long channel (∼500 nm) JL architecture from subthreshold characteristics for the ion sensitive functionalized biosensor [21]. They have not implemented impact ionization (II), which can further improve the subthreshold characteristics and can substantially enhance the biomolecule detection capability in both ion-
Over the past few years junctionless (JL) transistors [1–4], have shown significant potential for low power logic [5], analog/RF [6] and memory applications [7, 8], owing to unintentional underlap effect, reduced parasitic capacitances, enhanced short channel controllability and higher degree of bipolar effects [9, 10]. Apart from these applications, a possible use of JL transistor architecture can be for sensing biomolecules. The complementary metal–oxide–semiconductor compatible field effect transistor (FET) sensors have attained greater attention for electronic detection of biomolecules as they facilitate simultaneous fabrication of sensor arrays, complementary error detection and integrated signal processing electronics [11–14]. In one such sensing approach, MOSFET sensor can be functionalized with proper receptor to capture unlabeled macromolecules with increased selectivity and sensitivity [15]. In another approach, biomolecules accumulate at the cavity [16–19], located in the front gate oxide of FET, and can easily be detected by the change in the electrical characteristics of the device. Conventional 0957-4484/15/145201+08$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
sensitive or cavity based sensor architecture. Also, to detect very small concentration of biomolecules, the dimensions of biosensors must be scaled down to increase higher sensitivity with improved signal to noise ratio [22]. Hence, in this work, we investigate the potential of a JL biosensor with 50 nm gate length. According to published literature, the size of proteins lies within deca-nanometer range [10]. The thickness of biotin functionalized layer and of streptavidin are within ∼3–5 nm and ∼5 nm, respectively. Hence, a nanogap of less than 10 nm is highly desirable to recognize subnanomolar target proteins [23]. Also, there is a possibility of these proteins getting trapped within the nanogap embedded in the transistor architecture. Moreover, the experimental demonstration of current change due to the trapping of streptavidin within 10 nm cavity has been shown by Kim et al [24], which can detect the very small concentration of the same ranging from 20 pM to 2 nM concentration. In this work, we demonstrate the utilization of JL channel architecture for a cavity based dielectric modulated sensor to achieve high detection sensing margin at relatively low drain bias (∼1.2 V). As classical INV mode transistors do not exhibit II at the selected drain bias [9, 10], they are not expected to offer any significant improvement in the sensing metric. Hence, we have restricted the subsequent discussion only to JL transistors.
different dielectric constant has been demonstrated experimentally by Im et al, [15, 27]. All characteristics are compared to absence of biomolecules in the cavity which corresponds to a structure with dielectric constant of 1. Figure 1(b) shows the drain current (Ids)—front gate voltage (Vgf) characteristics of JL transistor for symmetrical gate operation (Vgf = Vgb). At a drain bias of 1.2 V, the II triggered JL architecture exhibits steep subthreshold swing (Sswing) at Vgf ∼ −1.2 V. Due to the accumulation of biomolecules in the cavity, the Ids − Vgf characteristic shifts towards higher gate voltages. The accumulation of biomolecules changes the dielectric constant of cavity and increases the front gate capacitance, which weakens the lateral electric field. This reduced electric field results in lower II rate and the turn-on behavior is achieved at higher Vgf values. For APTES, which has the highest dielectric constant (εκ = 3.57), the JL transistor offers maximum threshold voltage (Vth) shift of ∼0.58 V. Vth is selected as value of Vgf corresponding to the steep transition of the drain current. This shift in Vth can be further increased if JL transistor is biased in the asymmetric mode with back gate bias (Vgb) of −0.2 V. As shown in figure 1(c), this shift in Vth increases to ∼2.9 V (figure 1(c)) at the same Vds. Figure 1(d) demonstrates the threshold voltage shift ( ΔVth = Vth (εκ ) − Vth (εκ = 1) ) due to the presence of different biomolecules. The Vth shift increases with dielectric constant of biomolecules for both asymmetric and symmetric gate operation. ΔVth shift is ∼5× higher in the asymmetric gate operation for APTES. The results for the case, when II is off, are also shown in the figure. It is clear that the occurrence of II benefits the device for sensing applications as ΔVth increases the shift by ∼2 times in asymmetric and ∼1.3 times symmetric design. Thus, the use of asymmetric mode operation over symmetric operation improves the detection sensitivity for biosensing applications. Biomolecules carry different charge configuration depending upon their isoelectric point, pH of solution and its molar concentration, and can be calculated by Henderson– Hasselbalch equation [22]. Higher the difference between the isoelectric point (pI) and pH of the solution, higher will be the charge associated with the biomolecules i.e. net positive charge for pH < pI and the polarity of charge becomes negative for pH > pI. The pIs of biomolecules used in this work i.e. biotin, streptavidin and APTES are 3.5 [24], 5–6 [24] and 8.73 [28], respectively. It has been suggested to ensure large difference between pH of the solution and the pI of the molecule selected to increase the detection sensitivity [12]. Unfortunately, the specific quantitatively calculated charge densities for each individual biomolecules, at different molar concentration and different pH, have not been reported in the literature [18]. Therefore, it is not possible to model a biomolecule with certain fix charge density as the charge on the biomolecule is not constant, it rather varies with the pH and molar concentration [29]. Hence, biomolecules in this work are modeled with different dielectric constants and the effect of various charge densities (Qbio) on the sensing parameter is also incorporated in the present work.
Results and discussion In order to demonstrate the potential of JL transistor architecture for label free biosensing applications, these devices (figure 1(a)) were simulated using ATLAS simulation software [25] with Lombardi mobility model [26] and modules for doping, bipolar and II effects. The gate length (Lg) and silicon film thickness (Tsi) were selected as 50 and 10 nm, respectively. These transistors were doped with a concentration (Nd) of 5 × 1018 cm−3 and the operating drain bias (Vds) was fixed at 1.2 V. The front gate oxide had a cavity (figure 1(a)) of 10 nm where biomolecules can be accumulated for detection. The length of this cavity (Lgap) was selected as 20 nm. The back gate oxide had the same thickness as front gate oxide. A unique property of JL device is the occurrence of bipolar effects at relatively lower drain voltages in comparison to conventional INV mode MOSFETs [9, 10]. Our simulation results, reported elsewhere [7, 10] for II induced double gate JL MOSFETs depicting the occurrence of steep subthreshold swing and hysteresis, along with their variation with applied drain bias, are in good qualitative agreement with the experimental results for JL transistors [9]. In this work, we have focused on the evaluation of the biosensing capabilities and its enhancement in cavity based dielectric modulated JL nanotransistors. The occurrence of II has been exploited for biosensing applications in our present work. The effect of back gate (Vgb) biasing on the detection sensitivity of JL transistors is also analyzed. The biomolecules which are represented in the analysis are: biotin, streptavidin and 3-aminoppropyltriethoxysilane (APTES) which have the dielectric constants (εκ) 2.63, 2.1 and 3.57, respectively [18]. Representation of biomolecules with 2
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
Figure 1. (a) The schematic diagram of cavity based dielectric modulated JL transistor for biosensing. Ids − Vgs characteristics of JL transistor with impact ionization (II) for cavity filled with various biomolecules for (b) symmetric gate operation (Vgf = Vgb) and, (c) asymmetric gate operation with Vgb = −0.2 V. (d) Threshold voltage shift (ΔVth) for different biomolecules of different dielectric constants (εκ) for both symmetric and asymmetric modes. (e) Dependence of threshold voltage change on the charge density (Qbio) for different biomolecules for asymmetric mode operation for Qbio range −1010 to −1012 C cm−2.
The dependence of threshold voltage shift (ΔVth) on charge density (Qbio) of the biomolecules is shown in figure 1(e). Generally, the solution is prepared in neutral medium i.e. water based (pH = 7) [22] or in phosphate-buffered saline (pH ∼ 7.4) solution [30], and pI of biotin and streptavidin is less than 7. Hence, the developed charge will be negative in the polarity. Although the pI of APTES is ∼9,
the polarity of net solution when APTES is used as an immobilizer for biotin and streptavidin ensemble, remains negative [18]. Therefore, for the chosen biomolecules in this work, Qbio is considered as negative values. The threshold voltage shift increases by ∼29% to ∼12% for εκ = 2.1 to 3.57 with increase in the negative charge magnitude from 1010 to 1012 C cm−2. In order to model the binding of biomolecules, 3
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
interface charge ranging from −1011 to −1012 C cm−2 has also been used by several researchers [19, 31–33]. It can be seen that for the magnitude as lower as 1010 C cm−2, the inclusion of charge density does not facilitate any significant shift in threshold voltage as the shift is mainly caused due to the change in the permittivity of the cavity due to the accumulation of biomolecules. However, as magnitude of Qbio increases, threshold voltage shift increases. The negative charge of biomolecules assists in depleting the silicon thin film thus pushing the threshold voltage towards more positive values. Even at the constant level of Qbio the strength of depleting field is different for different biomolecules due to different dielectric constant, which results in different values of ΔVth. To achieve the same pH of solution and its molar concentration can be also tuned. To understand the reason behind the significant change in Vth in case of asymmetric mode operation Vgb was varied from −3 to 0 V, and Vth is evaluated for with and without II cases (figure 2(a)). Device performance is analyzed at Vgb of −2, −1 and 0 V which corresponds to point A, B and C, respectively in the curve with II, and A′, B′ and C′ for without II. For the case, when II is off, Vth decreases linearly with the increase in Vgb form −3 to 0 V, whereas, in the case with II, a saturation in Vth for Vgb < −1 V and a sharp decrease for Vgb > 1 V is observed. The same can also be inferred by extracting the body factor γ = dVth/dVgb [34] which is plotted in figure 2(b). JL transistor without II exhibits a nearly constant γ of ∼ −2 which confirms well the almost linear variation of threshold voltage with Vgb. II induced JL transistor offers a very large variation in body factor as shown in figure 2(b). For Vgb < −1 V, the value of |γ| is less than 1 that corresponds to a lower variation in Vth values with an increase in Vgb. However, as Vgb increases beyond −1 V, γ reduces very sharply exhibiting ∼×4 increase in |γ|. In the case of JL transistor without II, the peak electron concentration shifts from upper half (Lg/2, y < Tsi/2) to the lower half (Lg/2, y > Tsi/2) of the silicon film with the successive increase in Vgb from point A′ to C′ (figure 2(c)). At point C′, the peak electron concentration is located in the lower half of the silicon film, which requires a more negative Vgf to deplete the entire film and turn the device off. This results in the decrease in the value of Vth as shown in figure 2(a). Also, the progressive shift in electron concentration (conduction channel) from upper half (Lg/2, y < Tsi/2) to lower half (Lg/2, y > Tsi/2) with increase in Vgb from point A' to C', results in almost linear variation in threshold voltage or nearly constant γ. JL transistor with II, for Vgb less than −1 V exhibits lower electrostatic potential values at the back surface (y = Tsi) of silicon film. Generated holes accumulate at the lower potential pocket which restricts the electron concentration to the upper half of the silicon film. Due to the holes accumulating at the back surface, the electron concentration does not vary much by changing Vgb as shown in figure 2(d) for bias points A and B, and hence, Vth remains nearly constant [35]. However, as Vgb shifts towards relatively positive values, the degree of holes accumulation at back surface reduces, and the electrostatic potential profile becomes almost symmetrical
across silicon thin film at bias point B, which almost converges with B' point. Any further increase in Vgb leads to an increase in back surface potential, which further increases the current density. Due to the weakening of depleting electric field from back interface along with high II rates, Vgf is required to shift to very negative values to deplete the silicon film. This results in the sharp decrease in Vth with increase in Vgf and is the reason for higher magnitude of body factor as shown in figure 2(b). A large body factor increases the inherent sensitivity of the device for detection of biomolecules. Hence, it is advantageous to bias the JL transistor in asymmetric mode with a carefully selected back gate bias value for enhanced sensing metric (ΔVth). The distribution of electrostatic potential across the channel length is shown in figures 2(e) and (f) for asymmetric mode operation with Vgb = −0.2 V. All the structures were analyzed at Vgf = −3 V. The potential profile is evaluated 1 nm below the front gate oxide/silicon film interface. The gate is located from 100 to 150 nm. As shown in the figure 2(e), due to the II, the channel potential increases in the absence of biomolecule. When the biomolecules accumulate in the cavity, the effective gate capacitance changes [27], which alters the electrostatic coupling between channel and the gate, which results in modification of the potential distribution in the channel. Due to the higher dielectric constant of the biomolecule (κ > 1), vertical electric field increases and carriers are depleted in the silicon film which results in the potential lowering across the channel as shown in figure 2(e). In case when the II is turned-off (figure 2(f)), the potential in the film is lower as compared to the absence of biomolecules, which results in the relatively lower potential change (Δϕ) when compared with the II case. Thus, II phenomenon occurring in JL MOSFETs at lower drain voltages improves the sensing margin in cavity based nanotransistors for biosensing applications. It is also important to analyze the sensing metric (ΔVth) for semi-filled cavities. A uniform dielectric constant representing the biomolecule is assumed within the percentage fillin inside the cavity volume. The results for 20% to 100% fillin factor are shown in figures 3(a) and (b) for symmetric and asymmetric mode of operation, respectively. In the case with symmetric operation, ΔVth is restricted to less than 250 mV for less than 50% cavity occupancy. For a significant change in Vth, the cavity must be more than half filled for symmetric mode of operation. However, in an asymmetric mode operation with a 20% fill-in factor, a shift of 640 mV can be achieved with streptavidin and 750 mV for APTES. Therefore, asymmetric mode JL transistor can be used to effectively sense biomolecules even with lower cavity occupancy. It has been demonstrated experimentally that even under the precisely controlled experiments, the complete fill-in is difficult to achieve [17]. Therefore, the use of asymmetric JL topology as FET sensor provides an opportunity to enhance the detection sensitivity owing to its higher body factor even in the partially filled cavity. Along with the fill-in factor, the location of possible accumulation site of the biomolecule within the nanogap (cavity) can differ and, therefore, should be analyzed. In this 4
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
Figure 2. Dependence of (a) threshold voltage (Vth) and (b) body factor (γ) on for back bias for JL device. Variation of electron concentration (ne) across the silicon film at mid gate (x = Lg/2, y) for three different voltages for (a) without II and (d) with II cases. Variation of electrostatic potential at y = 1 nm across the channel for the absence and presence of biomolecules in the cavity in the case (e) with impact ionization and (f) without impact ionization in JL transistor. The structures were analyzed at Vds = 1.2 V, Vgf = −3 V and Vgb = −0.2 V.
work, we have considered a simple case of a 50% filled cavity with biomolecules which possibly can take three different orientations (denoted by A, B and C) within the cavity (figure 3(c)). In real time, the accumulation of biomolecules can be asymmetric and random and quite complex due to low binding probability in a carved nanogap [30]. As shown in figure 3(d), for any configuration (A to C) the threshold
voltage shift for a particular molecule is limited within a range. For example, for biotin (εκ = 2.63) the ΔVth lies between 1.2 and 1.5 V, similarly for streptavidin (εκ = 2.1) ΔVth is around 1.0–1.2 V and for APTES (εκ = 3.57), the range of ΔVth shift is between 1.6 and 1.9 V. Therefore, it can be concluded that although the exact value of sensing metric may vary depending upon the percentage fill-in and its position, 5
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
Figure 3. The dependence of ΔVth on the cavity fill-in factor for different biomolecules evaluated for (a) symmetric and (b) asymmetric gate operation. (c) Different configuration (case A, B and C) depicting possible accumulation sites of biomolecule in 50% filled cavity in the junctionless biosensor (d) threshold voltage shift (ΔVth) corresponding to each configuration for different biomolecules. Variation of (e) (Ids)air/(Ids)bio and (f) ΔVth/area for different biomolecules for both symmetric and asymmetric mode operation. Notations for figure 3(f) are the same as in figure 3(e).
but ΔVth stays within a certain range which reflects on the presence of the biomolecule. The variation in the threshold voltage, observed for different accumulating position is due to variation in the gate field lines passing through biomolecules, which in turns affects channel potential. Although a higher value ΔVth signifies the presence of a particular biomolecule, the possible drawback of this method can be the
distinguishability of different types of biomolecules from each other. Another metric apart from threshold voltage shift, which can be used to detect the biomolecules accumulated in dielectric cavity, is the change in drain current. Figure 3(e) demonstrates the ratio of drain current in the absence to presence of the biomolecules, (Ids)air/(Ids)bio for different εκ 6
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
values. The (Ids)air/(Ids)bio is evaluated at Vgf of −1 V for symmetric operation (Vgf = Vgb) and −3 V for asymmetric mode operation (Vgb = −0.2 V) for. These values of Vgf are selected after a careful investigation of transfer characteristics (figures 1(b)–(c)) to ensure a significant change in Ids between the absence and presence of bio-molecules at Qbio = −1010 C cm−2. Asymmetric mode with completely filled cavity offers a ratio of ∼108 for different biomolecules, while in symmetric mode of operation sensitivity increases linearly from ∼104 to ∼107 for biomolecules with increasing dielectric constant. It has been further demonstrated that, if the charge density of each biomolecule can be tuned near −1012 C cm−2 by tuning the molar concentration and respective pH of solution to be farther from the pI of the biomolecule used, high detection sensitivity (∼108) in terms of (Ids)air/ (Ids)bio can be achieved. The high values of (Ids)air/(Ids)bio will ensure the efficient detection even in the presence of signal degrading sources such as intrinsic device noise due to mobility and/or carrier fluctuations, thermal fluctuations of the environment and the interaction between biomolecules and the silicon film surface [20]. Transistor operated at lower drain bias, should be designed such than maximum shift in the threshold voltage is achieved for the smallest possible cavity area for a given biomolecule. This can be evaluated as a ratio of ΔVth to cavity area i.e. ΔVth/area. This should be achieved with minimum possible drain voltage. Figure 3(f) demonstrates the variation of this metric (ΔVth/area) for different biomolecules in symmetric and asymmetric mode of operation. Asymmetric mode JL transistor with Lg of 50 nm achieves a value of 5–7.2 mV nm−2 for ΔVth/area, which reduces to 0.8–1.5 mV nm−2 for symmetrical mode of operation at a drain bias of 1.2 V for Qbio of −1012 C cm−2. ΔVth/area is extracted to be of 2.4 × 10−2 mV nm−2 at Vds = 50 mV and 5.3 mV nm−2 at Vds = 7.8 V, from the available data in literature [17, 19] for cavity dielectric constant of ∼3. Considering a higher charge on the biomolecules (Qbio = −1012 C cm−2), the performance metric (ΔVth/area) increases when compared with the case of low a biomolecule charge (−1010 C cm−2). These high ΔVth/area values even at smaller cavity dimensions reflect the increased biomolecule detection ability of asymmetrical mode operation of JL transistor for sensing applications. Based on these results, it is expected that nanoscale JL transistor architecture would enhance the detection sensing margin for FET based biosensors in the nanometer regime. The occurrence of II in JL transistor architecture improves the detection sensitivity of the sensor. As the position and uniformity and the percentage fill-in of biomolecules within the cavity is process variant [30], it is difficult to model this variation accurately for nano-cavity structures, and beyond the scope of this work. The principal limitation of cavity based sensors comes from the uncertainty of the accumulation pattern and probability of biomolecules inside the cavity. However, as the work suggests that the use of JL transistor over conventional INV mode devices is expected to improve the performance of cavity based nanoscale sensors. The performance of the proposed sensor design may be
further improved in future with the detailed knowledge of charge density of different biomolecules at different pH for femto/pico molar solutions and the fill-in probability analysis for few decameter long cavities.
Conclusion The effect of transistor architecture and biasing conditions on detection sensitivity of cavity modulated FET sensors has been analyzed. The occurrence of II at relatively lower voltages in JL transistor can be efficiently utilized for biosensing applications. The sensing margin for the biomolecular detection can be enhanced by operating in asymmetric biasing mode which exhibits a higher body factor and has been demonstrated in this work. The potential of asymmetric gate biasing in II induced JL transistor architecture can be further utilized to improve the sensing capability of receptor based sensors.
Acknowledgments This work is supported by the Science and Engineering Research Board, Department of Science and Technology, Government of India, under Grant SR/S3/EECS/0130/2011.
References [1] Lee C-W, Afzalian A, Dehdashti Akhavan N, Yan R, Ferain I and Colinge J-P 2009 Junctionless multigate fieldeffect transistor Appl. Phys. Lett. 94 053511 [2] Colinge J-P et al 2010 Nanowire transistors without junctions Nat. Nanotechnology 5 225–9 [3] Nazarov A N, Ferain I, Dehdashti Akhavan N, Razavi P, Yu R and Colinge J P 2011 Random telegraph-signal noise in junctionless transistors Appl. Phys. Lett. 98 092111 [4] Raskin J-P, Colinge J-P, Ferain I, Kranti A, Lee C-W, Akhavan N D, Yan R, Razavi P and Yu R 2010 Mobility improvement in nanowire junctionless transistors by uniaxial strain Appl. Phys. Lett. 97 042114 [5] Parihar M S and Kranti A 2014 Revisiting the doping requirement for low power junctionless MOSFETs Semicond. Sci. Technol. 29 075006 [6] Ghosh D, Parihar M S, Armstrong G A and Kranti A 2012 High-performance junctionless MOSFETs for ultralowpower analog/RF applications IEEE Electron Device Lett. 33 1477–9 [7] Parihar M S, Ghosh D, Armstrong G A and Kranti A 2012 Bipolar snapback in junctionless transistors for dynamic random access memory Appl. Phys. Lett. 101 263503 [8] Kranti A, Lee C-W, Ferain I, Yan R, Dehdashti Akhavan N, Razavi P, Yu R, Armstrong G A and Colinge J-P 2010 Junctionless 6 T SRAM cell IET Electron. Lett. 46 1491–3 [9] Lee C-W, Nazarov A N, Ferain I, Dehdashti Akhavan N, Yan R, Razavi P, Yu R, Doria R T and Colinge J-P 2010 Low subthreshold slope in junctionless multigate transistors Appl. Phys. Lett. 96 102106 [10] Parihar M S, Ghosh D, Armstrong G A, Yu R, Razavi P and Kranti A 2012 Bipolar effects in unipolar junctionless transistors Appl. Phys. Lett. 101 093507 7
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
[11] Gao X P A, Zheng G and Lieber C M 2010 Subthreshold regime has the optimal sensitivity for nanowire FET biosensors Nano Lett. 10 547–52 [12] Stern E, Klemic J F, Routenberg D A, Wyrembak P N, Turner-Evans D B, Hamilton A D, LaVan D A, Fahmy T M and Reed M A 2007 Label-free immunodetection with CMOS-compatible semiconducting nanowires Nature 445 519–22 [13] Curreli M, Zhang R, Ishikawa F N, Chang H-K, Cote R J, Zhou C and Thompson M E 2008 Real-time label-free detection of biological entities using nanowire-based FETs IEEE Trans. Nanotechnology 7 651–67 [14] Passi V et al 2011 High gain and fast detection of warfare agents using back-gated silicon-nanowired MOSFETs IEEE Electron Device Lett. 32 976–8 [15] Im H, Huang X-J, Gu B and Choi Y-K 2007 A dielectricmodulated field-effect transistor for biosensing Nat. Nanotechnology 2 430–4 [16] Ahn J-H, Choi S-J, Han J-W, Park T-J, Lee S Y and Choi Y-K 2010 Double-gate nanowire field effect transistor for a biosensor Nano Lett. 10 2934–8 [17] Kim C-H, Ahn J-H, Lee K-B, Jung C, Park H G and Choi Y-K 2012 A new sensing metric to reduce data fluctuations in a nanogap-embedded field-effect transistor biosensor IEEE Trans. Electron Devices 59 2825–31 [18] Kim S, Baek D, Kim J-Y, Choi S-J, Seol M-L and Choi Y-K 2012 A transistor-based biosensor for the extraction of physical properties from biomolecules Appl. Phys. Lett. 101 073703 [19] Kannan N and Kumar M J 2013 Dielectric-modulated impactionization MOS transistor as a label-free biosensor IEEE Electron Device Lett. 34 1575–7 [20] Gao A, Zou N, Dai P, Lu N, Li T, Wang Y, Zhao J and Mao H 2013 Signal-to-noise ratio enhancement of silicon nanowires biosensor with rolling circle amplification Nano Lett. 13 4123–30 [21] Buitrago E, Fagas G, Badia M F-B, Georgiev Y M, Berthomé M and Ionescu A M 2013 Junctionless silicon nanowire transistors for the tunable operation of a highly sensitive low power sensor Sensors Actuators B 183 1–10 [22] Nair P R and Alam M A 2007 Design considerations of silicon nanowire biosensors IEEE Trans. Electron Devices 54 3400–8 [23] Jang D-Y, Kim Y-P, Kim H-S, Park S-H K, Choi S-Y and Choi Y-K 2007 Sublithographic vertical gold nanogap for
[24]
[25] [26]
[27] [28]
[29]
[30]
[31]
[32]
[33]
[34] [35]
8
label-free electrical detection of protein-ligand binding J. Vac. Sci. Technol. B 25 443–7 Kim S, Kim J-Y, Ahn J-H, Park T J, Lee S Y and Choi Y-K 2010 A charge pumping technique to identify biomolecular charge polarity using a nanogap embedded biotransistor Appl. Phys. Lett. 97 053702 ATLAS User Manual 2010 (Santa Clara, USA: Silvaco International) Lombardi C, Manzini A, Saporito A and Vanzi M 1988 A physically based mobility model for numerical simulation of nonplanar devices IEEE Trans. Comput.-Aided Des. Integr. Circuits 7 1164–71 Choi J-M, Han J-W, Choi S-J and Choi Y-K 2010 Analytical modeling of a nanogap-embedded FET for application as a biosensor IEEE Trans. Electron Devices 57 3477–84 Wu Z, Xiang H, Kim T, Chun M-S and Lee K 2006 Surface properties of submicrometer silica spheres modified with aminopropyltriethoxysilane and phenyltriethoxysilane J. Colloid Interface Sci. 304 119–24 Barbaro M, Bonfiglio A and Raffo L 2006 A charge-modulated FET for detection of biomolecular processes: conception, modeling, and simulation IEEE Trans. Electron Devices 53 158–66 Lee K-W, Choi S-J, Ahn J-H, Moon D-I I, Park T J, Lee S Y and Choi Y-K 2010 An underlap field-effect transistor for electrical detection of influenza Appl. Phys. Lett. 96 033703 Baek D J, Duarte J P, Moon D-I, Kim C-H, Ahn J-H and Choi Y-K 2012 Accumulation mode field-effect transistors for improved sensitivity in nanowire-based, biosensors Appl. Phys. Lett. 100 213703 Kim C-H, Jung C, Lee K-B, Park H G and Choi Y-K 2011 Label-free DNA detection with a nanogap embedded complementary metal oxide semiconductor Nanotechnology 22 135502 Shoorideh K and Chui C O 2012 Optimization of the sensitivity of FET-based biosensors via biasing and surface charge engineering IEEE Trans. Electron Devices 59 3104–10 Cristoloveanu S 2001 Silicon on insulator technologies and devices: from present to future Solid-State Electron. 45 1403–11 Lim H-K and Fossum J-G 1983 Threshold voltage of thin-film silicon-on-insulator (SOI) MOSFETs IEEE Trans. Electron Devices 30 1244–51
Search
Collections
Journals
About
Contact us
My IOPscience
Enhanced sensitivity of double gate junctionless transistor architecture for biosensing applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 145201 (http://iopscience.iop.org/0957-4484/26/14/145201) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.174.255.116 This content was downloaded on 03/05/2015 at 20:31
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 145201 (8pp)
doi:10.1088/0957-4484/26/14/145201
Enhanced sensitivity of double gate junctionless transistor architecture for biosensing applications Mukta Singh Parihar and Abhinav Kranti Low Power Nanoelectronics Research Group, Electrical Engineering Discipline, Indian Institute of Technology Indore, India E-mail: [email protected] Received 10 October 2014, revised 16 February 2015 Accepted for publication 16 February 2015 Published 16 March 2015 Abstract
In the present work, we demonstrate the potential of double gate junctionless (JL) architecture for enhanced sensitivity for detecting biomolecules in cavity modulated field effect transistors (FETs). The higher values of body factor, achieved in asymmetric gate operation under impact ionization is utilized for enhanced sensing margin which is nearly five times higher than compared to symmetrical mode operation. The intrinsic detection sensitivity is evaluated in terms of threshold voltage change, and the ratio of drain current in the presence and absence of biomolecules in JL nanotransistors. It is shown that asymmetric mode JL transistor achieves a higher degree of detection sensitivity even for a partially filled cavity. The work demonstrates the potential of JL channel architecture for cavity based dielectric modulated FET biosensors. Keywords: junctionless, MOSFET, biosensors, double gate (Some figures may appear in colour only in the online journal) Introduction
FET based sensors have faced challenges relating to selectivity and poor signal-to-noise ratio owing to intrinsic variation and non-controllability of parameters related to detection of signals [20]. To overcome these, FET devices with enhanced sensing margins are needed so as to effectively detect the presence of biomolecules. Previous studies of using FETs for biosensing applications have demonstrated the potential of conventional inversion (INV) mode transistor using dielectric modulated cavity either, in the gate oxide or at the side gate electrode [16–19]. The various designs have used either very high drain-tosource voltages (7.8 V) [19] or a very wide cavity area (3000–5000 nm × 50 nm) [17] for a meaningful detectable shift in drain current (Ids)—gate voltage (Vgs) characteristics which has been used as a sensing metric. Buitrago et al, have demonstrated sensing capability of long channel (∼500 nm) JL architecture from subthreshold characteristics for the ion sensitive functionalized biosensor [21]. They have not implemented impact ionization (II), which can further improve the subthreshold characteristics and can substantially enhance the biomolecule detection capability in both ion-
Over the past few years junctionless (JL) transistors [1–4], have shown significant potential for low power logic [5], analog/RF [6] and memory applications [7, 8], owing to unintentional underlap effect, reduced parasitic capacitances, enhanced short channel controllability and higher degree of bipolar effects [9, 10]. Apart from these applications, a possible use of JL transistor architecture can be for sensing biomolecules. The complementary metal–oxide–semiconductor compatible field effect transistor (FET) sensors have attained greater attention for electronic detection of biomolecules as they facilitate simultaneous fabrication of sensor arrays, complementary error detection and integrated signal processing electronics [11–14]. In one such sensing approach, MOSFET sensor can be functionalized with proper receptor to capture unlabeled macromolecules with increased selectivity and sensitivity [15]. In another approach, biomolecules accumulate at the cavity [16–19], located in the front gate oxide of FET, and can easily be detected by the change in the electrical characteristics of the device. Conventional 0957-4484/15/145201+08$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
sensitive or cavity based sensor architecture. Also, to detect very small concentration of biomolecules, the dimensions of biosensors must be scaled down to increase higher sensitivity with improved signal to noise ratio [22]. Hence, in this work, we investigate the potential of a JL biosensor with 50 nm gate length. According to published literature, the size of proteins lies within deca-nanometer range [10]. The thickness of biotin functionalized layer and of streptavidin are within ∼3–5 nm and ∼5 nm, respectively. Hence, a nanogap of less than 10 nm is highly desirable to recognize subnanomolar target proteins [23]. Also, there is a possibility of these proteins getting trapped within the nanogap embedded in the transistor architecture. Moreover, the experimental demonstration of current change due to the trapping of streptavidin within 10 nm cavity has been shown by Kim et al [24], which can detect the very small concentration of the same ranging from 20 pM to 2 nM concentration. In this work, we demonstrate the utilization of JL channel architecture for a cavity based dielectric modulated sensor to achieve high detection sensing margin at relatively low drain bias (∼1.2 V). As classical INV mode transistors do not exhibit II at the selected drain bias [9, 10], they are not expected to offer any significant improvement in the sensing metric. Hence, we have restricted the subsequent discussion only to JL transistors.
different dielectric constant has been demonstrated experimentally by Im et al, [15, 27]. All characteristics are compared to absence of biomolecules in the cavity which corresponds to a structure with dielectric constant of 1. Figure 1(b) shows the drain current (Ids)—front gate voltage (Vgf) characteristics of JL transistor for symmetrical gate operation (Vgf = Vgb). At a drain bias of 1.2 V, the II triggered JL architecture exhibits steep subthreshold swing (Sswing) at Vgf ∼ −1.2 V. Due to the accumulation of biomolecules in the cavity, the Ids − Vgf characteristic shifts towards higher gate voltages. The accumulation of biomolecules changes the dielectric constant of cavity and increases the front gate capacitance, which weakens the lateral electric field. This reduced electric field results in lower II rate and the turn-on behavior is achieved at higher Vgf values. For APTES, which has the highest dielectric constant (εκ = 3.57), the JL transistor offers maximum threshold voltage (Vth) shift of ∼0.58 V. Vth is selected as value of Vgf corresponding to the steep transition of the drain current. This shift in Vth can be further increased if JL transistor is biased in the asymmetric mode with back gate bias (Vgb) of −0.2 V. As shown in figure 1(c), this shift in Vth increases to ∼2.9 V (figure 1(c)) at the same Vds. Figure 1(d) demonstrates the threshold voltage shift ( ΔVth = Vth (εκ ) − Vth (εκ = 1) ) due to the presence of different biomolecules. The Vth shift increases with dielectric constant of biomolecules for both asymmetric and symmetric gate operation. ΔVth shift is ∼5× higher in the asymmetric gate operation for APTES. The results for the case, when II is off, are also shown in the figure. It is clear that the occurrence of II benefits the device for sensing applications as ΔVth increases the shift by ∼2 times in asymmetric and ∼1.3 times symmetric design. Thus, the use of asymmetric mode operation over symmetric operation improves the detection sensitivity for biosensing applications. Biomolecules carry different charge configuration depending upon their isoelectric point, pH of solution and its molar concentration, and can be calculated by Henderson– Hasselbalch equation [22]. Higher the difference between the isoelectric point (pI) and pH of the solution, higher will be the charge associated with the biomolecules i.e. net positive charge for pH < pI and the polarity of charge becomes negative for pH > pI. The pIs of biomolecules used in this work i.e. biotin, streptavidin and APTES are 3.5 [24], 5–6 [24] and 8.73 [28], respectively. It has been suggested to ensure large difference between pH of the solution and the pI of the molecule selected to increase the detection sensitivity [12]. Unfortunately, the specific quantitatively calculated charge densities for each individual biomolecules, at different molar concentration and different pH, have not been reported in the literature [18]. Therefore, it is not possible to model a biomolecule with certain fix charge density as the charge on the biomolecule is not constant, it rather varies with the pH and molar concentration [29]. Hence, biomolecules in this work are modeled with different dielectric constants and the effect of various charge densities (Qbio) on the sensing parameter is also incorporated in the present work.
Results and discussion In order to demonstrate the potential of JL transistor architecture for label free biosensing applications, these devices (figure 1(a)) were simulated using ATLAS simulation software [25] with Lombardi mobility model [26] and modules for doping, bipolar and II effects. The gate length (Lg) and silicon film thickness (Tsi) were selected as 50 and 10 nm, respectively. These transistors were doped with a concentration (Nd) of 5 × 1018 cm−3 and the operating drain bias (Vds) was fixed at 1.2 V. The front gate oxide had a cavity (figure 1(a)) of 10 nm where biomolecules can be accumulated for detection. The length of this cavity (Lgap) was selected as 20 nm. The back gate oxide had the same thickness as front gate oxide. A unique property of JL device is the occurrence of bipolar effects at relatively lower drain voltages in comparison to conventional INV mode MOSFETs [9, 10]. Our simulation results, reported elsewhere [7, 10] for II induced double gate JL MOSFETs depicting the occurrence of steep subthreshold swing and hysteresis, along with their variation with applied drain bias, are in good qualitative agreement with the experimental results for JL transistors [9]. In this work, we have focused on the evaluation of the biosensing capabilities and its enhancement in cavity based dielectric modulated JL nanotransistors. The occurrence of II has been exploited for biosensing applications in our present work. The effect of back gate (Vgb) biasing on the detection sensitivity of JL transistors is also analyzed. The biomolecules which are represented in the analysis are: biotin, streptavidin and 3-aminoppropyltriethoxysilane (APTES) which have the dielectric constants (εκ) 2.63, 2.1 and 3.57, respectively [18]. Representation of biomolecules with 2
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
Figure 1. (a) The schematic diagram of cavity based dielectric modulated JL transistor for biosensing. Ids − Vgs characteristics of JL transistor with impact ionization (II) for cavity filled with various biomolecules for (b) symmetric gate operation (Vgf = Vgb) and, (c) asymmetric gate operation with Vgb = −0.2 V. (d) Threshold voltage shift (ΔVth) for different biomolecules of different dielectric constants (εκ) for both symmetric and asymmetric modes. (e) Dependence of threshold voltage change on the charge density (Qbio) for different biomolecules for asymmetric mode operation for Qbio range −1010 to −1012 C cm−2.
The dependence of threshold voltage shift (ΔVth) on charge density (Qbio) of the biomolecules is shown in figure 1(e). Generally, the solution is prepared in neutral medium i.e. water based (pH = 7) [22] or in phosphate-buffered saline (pH ∼ 7.4) solution [30], and pI of biotin and streptavidin is less than 7. Hence, the developed charge will be negative in the polarity. Although the pI of APTES is ∼9,
the polarity of net solution when APTES is used as an immobilizer for biotin and streptavidin ensemble, remains negative [18]. Therefore, for the chosen biomolecules in this work, Qbio is considered as negative values. The threshold voltage shift increases by ∼29% to ∼12% for εκ = 2.1 to 3.57 with increase in the negative charge magnitude from 1010 to 1012 C cm−2. In order to model the binding of biomolecules, 3
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
interface charge ranging from −1011 to −1012 C cm−2 has also been used by several researchers [19, 31–33]. It can be seen that for the magnitude as lower as 1010 C cm−2, the inclusion of charge density does not facilitate any significant shift in threshold voltage as the shift is mainly caused due to the change in the permittivity of the cavity due to the accumulation of biomolecules. However, as magnitude of Qbio increases, threshold voltage shift increases. The negative charge of biomolecules assists in depleting the silicon thin film thus pushing the threshold voltage towards more positive values. Even at the constant level of Qbio the strength of depleting field is different for different biomolecules due to different dielectric constant, which results in different values of ΔVth. To achieve the same pH of solution and its molar concentration can be also tuned. To understand the reason behind the significant change in Vth in case of asymmetric mode operation Vgb was varied from −3 to 0 V, and Vth is evaluated for with and without II cases (figure 2(a)). Device performance is analyzed at Vgb of −2, −1 and 0 V which corresponds to point A, B and C, respectively in the curve with II, and A′, B′ and C′ for without II. For the case, when II is off, Vth decreases linearly with the increase in Vgb form −3 to 0 V, whereas, in the case with II, a saturation in Vth for Vgb < −1 V and a sharp decrease for Vgb > 1 V is observed. The same can also be inferred by extracting the body factor γ = dVth/dVgb [34] which is plotted in figure 2(b). JL transistor without II exhibits a nearly constant γ of ∼ −2 which confirms well the almost linear variation of threshold voltage with Vgb. II induced JL transistor offers a very large variation in body factor as shown in figure 2(b). For Vgb < −1 V, the value of |γ| is less than 1 that corresponds to a lower variation in Vth values with an increase in Vgb. However, as Vgb increases beyond −1 V, γ reduces very sharply exhibiting ∼×4 increase in |γ|. In the case of JL transistor without II, the peak electron concentration shifts from upper half (Lg/2, y < Tsi/2) to the lower half (Lg/2, y > Tsi/2) of the silicon film with the successive increase in Vgb from point A′ to C′ (figure 2(c)). At point C′, the peak electron concentration is located in the lower half of the silicon film, which requires a more negative Vgf to deplete the entire film and turn the device off. This results in the decrease in the value of Vth as shown in figure 2(a). Also, the progressive shift in electron concentration (conduction channel) from upper half (Lg/2, y < Tsi/2) to lower half (Lg/2, y > Tsi/2) with increase in Vgb from point A' to C', results in almost linear variation in threshold voltage or nearly constant γ. JL transistor with II, for Vgb less than −1 V exhibits lower electrostatic potential values at the back surface (y = Tsi) of silicon film. Generated holes accumulate at the lower potential pocket which restricts the electron concentration to the upper half of the silicon film. Due to the holes accumulating at the back surface, the electron concentration does not vary much by changing Vgb as shown in figure 2(d) for bias points A and B, and hence, Vth remains nearly constant [35]. However, as Vgb shifts towards relatively positive values, the degree of holes accumulation at back surface reduces, and the electrostatic potential profile becomes almost symmetrical
across silicon thin film at bias point B, which almost converges with B' point. Any further increase in Vgb leads to an increase in back surface potential, which further increases the current density. Due to the weakening of depleting electric field from back interface along with high II rates, Vgf is required to shift to very negative values to deplete the silicon film. This results in the sharp decrease in Vth with increase in Vgf and is the reason for higher magnitude of body factor as shown in figure 2(b). A large body factor increases the inherent sensitivity of the device for detection of biomolecules. Hence, it is advantageous to bias the JL transistor in asymmetric mode with a carefully selected back gate bias value for enhanced sensing metric (ΔVth). The distribution of electrostatic potential across the channel length is shown in figures 2(e) and (f) for asymmetric mode operation with Vgb = −0.2 V. All the structures were analyzed at Vgf = −3 V. The potential profile is evaluated 1 nm below the front gate oxide/silicon film interface. The gate is located from 100 to 150 nm. As shown in the figure 2(e), due to the II, the channel potential increases in the absence of biomolecule. When the biomolecules accumulate in the cavity, the effective gate capacitance changes [27], which alters the electrostatic coupling between channel and the gate, which results in modification of the potential distribution in the channel. Due to the higher dielectric constant of the biomolecule (κ > 1), vertical electric field increases and carriers are depleted in the silicon film which results in the potential lowering across the channel as shown in figure 2(e). In case when the II is turned-off (figure 2(f)), the potential in the film is lower as compared to the absence of biomolecules, which results in the relatively lower potential change (Δϕ) when compared with the II case. Thus, II phenomenon occurring in JL MOSFETs at lower drain voltages improves the sensing margin in cavity based nanotransistors for biosensing applications. It is also important to analyze the sensing metric (ΔVth) for semi-filled cavities. A uniform dielectric constant representing the biomolecule is assumed within the percentage fillin inside the cavity volume. The results for 20% to 100% fillin factor are shown in figures 3(a) and (b) for symmetric and asymmetric mode of operation, respectively. In the case with symmetric operation, ΔVth is restricted to less than 250 mV for less than 50% cavity occupancy. For a significant change in Vth, the cavity must be more than half filled for symmetric mode of operation. However, in an asymmetric mode operation with a 20% fill-in factor, a shift of 640 mV can be achieved with streptavidin and 750 mV for APTES. Therefore, asymmetric mode JL transistor can be used to effectively sense biomolecules even with lower cavity occupancy. It has been demonstrated experimentally that even under the precisely controlled experiments, the complete fill-in is difficult to achieve [17]. Therefore, the use of asymmetric JL topology as FET sensor provides an opportunity to enhance the detection sensitivity owing to its higher body factor even in the partially filled cavity. Along with the fill-in factor, the location of possible accumulation site of the biomolecule within the nanogap (cavity) can differ and, therefore, should be analyzed. In this 4
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
Figure 2. Dependence of (a) threshold voltage (Vth) and (b) body factor (γ) on for back bias for JL device. Variation of electron concentration (ne) across the silicon film at mid gate (x = Lg/2, y) for three different voltages for (a) without II and (d) with II cases. Variation of electrostatic potential at y = 1 nm across the channel for the absence and presence of biomolecules in the cavity in the case (e) with impact ionization and (f) without impact ionization in JL transistor. The structures were analyzed at Vds = 1.2 V, Vgf = −3 V and Vgb = −0.2 V.
work, we have considered a simple case of a 50% filled cavity with biomolecules which possibly can take three different orientations (denoted by A, B and C) within the cavity (figure 3(c)). In real time, the accumulation of biomolecules can be asymmetric and random and quite complex due to low binding probability in a carved nanogap [30]. As shown in figure 3(d), for any configuration (A to C) the threshold
voltage shift for a particular molecule is limited within a range. For example, for biotin (εκ = 2.63) the ΔVth lies between 1.2 and 1.5 V, similarly for streptavidin (εκ = 2.1) ΔVth is around 1.0–1.2 V and for APTES (εκ = 3.57), the range of ΔVth shift is between 1.6 and 1.9 V. Therefore, it can be concluded that although the exact value of sensing metric may vary depending upon the percentage fill-in and its position, 5
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
Figure 3. The dependence of ΔVth on the cavity fill-in factor for different biomolecules evaluated for (a) symmetric and (b) asymmetric gate operation. (c) Different configuration (case A, B and C) depicting possible accumulation sites of biomolecule in 50% filled cavity in the junctionless biosensor (d) threshold voltage shift (ΔVth) corresponding to each configuration for different biomolecules. Variation of (e) (Ids)air/(Ids)bio and (f) ΔVth/area for different biomolecules for both symmetric and asymmetric mode operation. Notations for figure 3(f) are the same as in figure 3(e).
but ΔVth stays within a certain range which reflects on the presence of the biomolecule. The variation in the threshold voltage, observed for different accumulating position is due to variation in the gate field lines passing through biomolecules, which in turns affects channel potential. Although a higher value ΔVth signifies the presence of a particular biomolecule, the possible drawback of this method can be the
distinguishability of different types of biomolecules from each other. Another metric apart from threshold voltage shift, which can be used to detect the biomolecules accumulated in dielectric cavity, is the change in drain current. Figure 3(e) demonstrates the ratio of drain current in the absence to presence of the biomolecules, (Ids)air/(Ids)bio for different εκ 6
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
values. The (Ids)air/(Ids)bio is evaluated at Vgf of −1 V for symmetric operation (Vgf = Vgb) and −3 V for asymmetric mode operation (Vgb = −0.2 V) for. These values of Vgf are selected after a careful investigation of transfer characteristics (figures 1(b)–(c)) to ensure a significant change in Ids between the absence and presence of bio-molecules at Qbio = −1010 C cm−2. Asymmetric mode with completely filled cavity offers a ratio of ∼108 for different biomolecules, while in symmetric mode of operation sensitivity increases linearly from ∼104 to ∼107 for biomolecules with increasing dielectric constant. It has been further demonstrated that, if the charge density of each biomolecule can be tuned near −1012 C cm−2 by tuning the molar concentration and respective pH of solution to be farther from the pI of the biomolecule used, high detection sensitivity (∼108) in terms of (Ids)air/ (Ids)bio can be achieved. The high values of (Ids)air/(Ids)bio will ensure the efficient detection even in the presence of signal degrading sources such as intrinsic device noise due to mobility and/or carrier fluctuations, thermal fluctuations of the environment and the interaction between biomolecules and the silicon film surface [20]. Transistor operated at lower drain bias, should be designed such than maximum shift in the threshold voltage is achieved for the smallest possible cavity area for a given biomolecule. This can be evaluated as a ratio of ΔVth to cavity area i.e. ΔVth/area. This should be achieved with minimum possible drain voltage. Figure 3(f) demonstrates the variation of this metric (ΔVth/area) for different biomolecules in symmetric and asymmetric mode of operation. Asymmetric mode JL transistor with Lg of 50 nm achieves a value of 5–7.2 mV nm−2 for ΔVth/area, which reduces to 0.8–1.5 mV nm−2 for symmetrical mode of operation at a drain bias of 1.2 V for Qbio of −1012 C cm−2. ΔVth/area is extracted to be of 2.4 × 10−2 mV nm−2 at Vds = 50 mV and 5.3 mV nm−2 at Vds = 7.8 V, from the available data in literature [17, 19] for cavity dielectric constant of ∼3. Considering a higher charge on the biomolecules (Qbio = −1012 C cm−2), the performance metric (ΔVth/area) increases when compared with the case of low a biomolecule charge (−1010 C cm−2). These high ΔVth/area values even at smaller cavity dimensions reflect the increased biomolecule detection ability of asymmetrical mode operation of JL transistor for sensing applications. Based on these results, it is expected that nanoscale JL transistor architecture would enhance the detection sensing margin for FET based biosensors in the nanometer regime. The occurrence of II in JL transistor architecture improves the detection sensitivity of the sensor. As the position and uniformity and the percentage fill-in of biomolecules within the cavity is process variant [30], it is difficult to model this variation accurately for nano-cavity structures, and beyond the scope of this work. The principal limitation of cavity based sensors comes from the uncertainty of the accumulation pattern and probability of biomolecules inside the cavity. However, as the work suggests that the use of JL transistor over conventional INV mode devices is expected to improve the performance of cavity based nanoscale sensors. The performance of the proposed sensor design may be
further improved in future with the detailed knowledge of charge density of different biomolecules at different pH for femto/pico molar solutions and the fill-in probability analysis for few decameter long cavities.
Conclusion The effect of transistor architecture and biasing conditions on detection sensitivity of cavity modulated FET sensors has been analyzed. The occurrence of II at relatively lower voltages in JL transistor can be efficiently utilized for biosensing applications. The sensing margin for the biomolecular detection can be enhanced by operating in asymmetric biasing mode which exhibits a higher body factor and has been demonstrated in this work. The potential of asymmetric gate biasing in II induced JL transistor architecture can be further utilized to improve the sensing capability of receptor based sensors.
Acknowledgments This work is supported by the Science and Engineering Research Board, Department of Science and Technology, Government of India, under Grant SR/S3/EECS/0130/2011.
References [1] Lee C-W, Afzalian A, Dehdashti Akhavan N, Yan R, Ferain I and Colinge J-P 2009 Junctionless multigate fieldeffect transistor Appl. Phys. Lett. 94 053511 [2] Colinge J-P et al 2010 Nanowire transistors without junctions Nat. Nanotechnology 5 225–9 [3] Nazarov A N, Ferain I, Dehdashti Akhavan N, Razavi P, Yu R and Colinge J P 2011 Random telegraph-signal noise in junctionless transistors Appl. Phys. Lett. 98 092111 [4] Raskin J-P, Colinge J-P, Ferain I, Kranti A, Lee C-W, Akhavan N D, Yan R, Razavi P and Yu R 2010 Mobility improvement in nanowire junctionless transistors by uniaxial strain Appl. Phys. Lett. 97 042114 [5] Parihar M S and Kranti A 2014 Revisiting the doping requirement for low power junctionless MOSFETs Semicond. Sci. Technol. 29 075006 [6] Ghosh D, Parihar M S, Armstrong G A and Kranti A 2012 High-performance junctionless MOSFETs for ultralowpower analog/RF applications IEEE Electron Device Lett. 33 1477–9 [7] Parihar M S, Ghosh D, Armstrong G A and Kranti A 2012 Bipolar snapback in junctionless transistors for dynamic random access memory Appl. Phys. Lett. 101 263503 [8] Kranti A, Lee C-W, Ferain I, Yan R, Dehdashti Akhavan N, Razavi P, Yu R, Armstrong G A and Colinge J-P 2010 Junctionless 6 T SRAM cell IET Electron. Lett. 46 1491–3 [9] Lee C-W, Nazarov A N, Ferain I, Dehdashti Akhavan N, Yan R, Razavi P, Yu R, Doria R T and Colinge J-P 2010 Low subthreshold slope in junctionless multigate transistors Appl. Phys. Lett. 96 102106 [10] Parihar M S, Ghosh D, Armstrong G A, Yu R, Razavi P and Kranti A 2012 Bipolar effects in unipolar junctionless transistors Appl. Phys. Lett. 101 093507 7
Nanotechnology 26 (2015) 145201
M S Parihar and A Kranti
[11] Gao X P A, Zheng G and Lieber C M 2010 Subthreshold regime has the optimal sensitivity for nanowire FET biosensors Nano Lett. 10 547–52 [12] Stern E, Klemic J F, Routenberg D A, Wyrembak P N, Turner-Evans D B, Hamilton A D, LaVan D A, Fahmy T M and Reed M A 2007 Label-free immunodetection with CMOS-compatible semiconducting nanowires Nature 445 519–22 [13] Curreli M, Zhang R, Ishikawa F N, Chang H-K, Cote R J, Zhou C and Thompson M E 2008 Real-time label-free detection of biological entities using nanowire-based FETs IEEE Trans. Nanotechnology 7 651–67 [14] Passi V et al 2011 High gain and fast detection of warfare agents using back-gated silicon-nanowired MOSFETs IEEE Electron Device Lett. 32 976–8 [15] Im H, Huang X-J, Gu B and Choi Y-K 2007 A dielectricmodulated field-effect transistor for biosensing Nat. Nanotechnology 2 430–4 [16] Ahn J-H, Choi S-J, Han J-W, Park T-J, Lee S Y and Choi Y-K 2010 Double-gate nanowire field effect transistor for a biosensor Nano Lett. 10 2934–8 [17] Kim C-H, Ahn J-H, Lee K-B, Jung C, Park H G and Choi Y-K 2012 A new sensing metric to reduce data fluctuations in a nanogap-embedded field-effect transistor biosensor IEEE Trans. Electron Devices 59 2825–31 [18] Kim S, Baek D, Kim J-Y, Choi S-J, Seol M-L and Choi Y-K 2012 A transistor-based biosensor for the extraction of physical properties from biomolecules Appl. Phys. Lett. 101 073703 [19] Kannan N and Kumar M J 2013 Dielectric-modulated impactionization MOS transistor as a label-free biosensor IEEE Electron Device Lett. 34 1575–7 [20] Gao A, Zou N, Dai P, Lu N, Li T, Wang Y, Zhao J and Mao H 2013 Signal-to-noise ratio enhancement of silicon nanowires biosensor with rolling circle amplification Nano Lett. 13 4123–30 [21] Buitrago E, Fagas G, Badia M F-B, Georgiev Y M, Berthomé M and Ionescu A M 2013 Junctionless silicon nanowire transistors for the tunable operation of a highly sensitive low power sensor Sensors Actuators B 183 1–10 [22] Nair P R and Alam M A 2007 Design considerations of silicon nanowire biosensors IEEE Trans. Electron Devices 54 3400–8 [23] Jang D-Y, Kim Y-P, Kim H-S, Park S-H K, Choi S-Y and Choi Y-K 2007 Sublithographic vertical gold nanogap for
[24]
[25] [26]
[27] [28]
[29]
[30]
[31]
[32]
[33]
[34] [35]
8
label-free electrical detection of protein-ligand binding J. Vac. Sci. Technol. B 25 443–7 Kim S, Kim J-Y, Ahn J-H, Park T J, Lee S Y and Choi Y-K 2010 A charge pumping technique to identify biomolecular charge polarity using a nanogap embedded biotransistor Appl. Phys. Lett. 97 053702 ATLAS User Manual 2010 (Santa Clara, USA: Silvaco International) Lombardi C, Manzini A, Saporito A and Vanzi M 1988 A physically based mobility model for numerical simulation of nonplanar devices IEEE Trans. Comput.-Aided Des. Integr. Circuits 7 1164–71 Choi J-M, Han J-W, Choi S-J and Choi Y-K 2010 Analytical modeling of a nanogap-embedded FET for application as a biosensor IEEE Trans. Electron Devices 57 3477–84 Wu Z, Xiang H, Kim T, Chun M-S and Lee K 2006 Surface properties of submicrometer silica spheres modified with aminopropyltriethoxysilane and phenyltriethoxysilane J. Colloid Interface Sci. 304 119–24 Barbaro M, Bonfiglio A and Raffo L 2006 A charge-modulated FET for detection of biomolecular processes: conception, modeling, and simulation IEEE Trans. Electron Devices 53 158–66 Lee K-W, Choi S-J, Ahn J-H, Moon D-I I, Park T J, Lee S Y and Choi Y-K 2010 An underlap field-effect transistor for electrical detection of influenza Appl. Phys. Lett. 96 033703 Baek D J, Duarte J P, Moon D-I, Kim C-H, Ahn J-H and Choi Y-K 2012 Accumulation mode field-effect transistors for improved sensitivity in nanowire-based, biosensors Appl. Phys. Lett. 100 213703 Kim C-H, Jung C, Lee K-B, Park H G and Choi Y-K 2011 Label-free DNA detection with a nanogap embedded complementary metal oxide semiconductor Nanotechnology 22 135502 Shoorideh K and Chui C O 2012 Optimization of the sensitivity of FET-based biosensors via biasing and surface charge engineering IEEE Trans. Electron Devices 59 3104–10 Cristoloveanu S 2001 Silicon on insulator technologies and devices: from present to future Solid-State Electron. 45 1403–11 Lim H-K and Fossum J-G 1983 Threshold voltage of thin-film silicon-on-insulator (SOI) MOSFETs IEEE Trans. Electron Devices 30 1244–51
