IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 61, NO. 5, MAY 2014
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Investigation of Sensor Performance in Accumulation- and Inversion-Mode Silicon Nanowire pH Sensors Jieun Lee, Bongsik Choi, Seonwook Hwang, Jung Han Lee, Student Member, IEEE, Byung-Gook Park, Member, IEEE, Tae Jung Park, Dong Myong Kim, Member, IEEE, Dae Hwan Kim, Senior Member, IEEE, and Sung-Jin Choi Abstract— We investigate the performance of accumulation (ACC)-mode and inversion (INV)-mode silicon nanowire (SiNW) pH sensors that are electrically controlled by a liquid gate. The two sensing parameters of the changes of threshold voltage and current are explored in both types of SiNW pH sensors at different pH levels. As device dimensions and channel doping concentration increase, the performance of the ACC-mode biosensor degrades more rapidly than the performance of the INV-mode biosensor. Therefore, INV-mode SiNW pH sensors with a liquid gate could be robust to process variation and provide improved current sensitivity. Index Terms— Accumulation (ACC)-mode transistor, biosensors, inversion (INV)-mode transistor, ion-sensitive field-effect transistor (FET), silicon nanowire (SiNW) FET.
I. I NTRODUCTION
N
ANOSCALE field-effect transistors (FETs) composed of semiconductor nanowires (NWs) or nanoribbons have been introduced as promising, highly sensitive, real-time, and label-free biosensors for the detection of various biomolecules [1]–[5]. In particular, among the various structures that have been examined, top-down fabricated silicon NW (SiNW) FET-based biosensors are currently the objects of intense study because these biosensors exhibit high reproducibility [6]–[8]. In addition, they offer well-understood surface chemistry and can potentially allow conductivity to be finely controlled via the introduction of n- or p-type dopants. Recently, a new type of transistor, the accumulation (ACC)-mode transistor, has
Manuscript received November 13, 2013; revised January 28, 2014; accepted March 3, 2014. Date of publication April 7, 2014; date of current version April 18, 2014. This work was supported in part by the National Research Foundation of Korea through the Ministry of Education, Science and Technology, Korean Government, under Grant 2013057870, in part by the Educational Research Team for Creative Engineers on Material-Device-Circuit Co-Design under Grant BK21+, and in part by the Korean Health Technology Research and Development Project, Ministry of Health and Welfare, Korea, under Grant HI13C0862. The review of this brief was arranged by Editor F. Ayazi. (Corresponding author: S.-J. Choi.) J. Lee, B. Choi, S. Hwang, D. M. Kim, D. H. Kim, and S.-J. Choi are with the School of Electrical Engineering, Kookmin University, Seoul 136-702, Korea (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). J. H. Lee and B.-G. Park are with the School of Electrical Engineering and Computer Science, Seoul National University, Seoul 151-742, Korea (e-mail:
[email protected];
[email protected]). T. J. Park is with the Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2014.2312413
been the focus of many studies because ACC-mode transistors exhibit certain significant advantages, such as high speed and reliability, compared with their conventional inversion (INV)-mode counterparts [9]. In an ACC-mode transistor, majority carriers are predominantly responsible for current transport because the channel, source, and drain regions are implanted with the same dopant type for n-type (n+–n–n+) and p-type (p+–p–p+) transistors; in contrast, an INV-mode transistor is activated through the generation of inverted minority carriers. Both ACC- and the INV-mode transistors can be employed for sensor applications. Certain studies have predominantly utilized ACC-mode transistors for various sensor operations [10], [11], whereas other investigations have employed conventional INV-mode transistors as well [12], [13]. In particular, most of the SiNW biosensors fabricated using a top-down approach can normally operate as INV-mode transistors, whereas most of the SiNW biosensors fabricated using a bottom-up approach typically operate as ACC-mode transistors. Despite the tremendous potential and promising experimental results associated with both types of transistors, few studies have compared the sensor performance of these two transistor types [14], [15]. In addition, previous research comparing these transistor types has only utilized a back-gate (i.e., solid-gate) structure rather than a top-gate (i.e., liquid-gate) structure; however, ACC- and INV-mode transistors can exhibit different biosensor responses with a liquid gate. Therefore, the dependence of device sensitivity on SiNW dimensions and doping concentration in liquidgate structures must be elucidated to allow for the systematic optimization of sensor design. In this brief, we experimentally compare the pH sensor performances of ACC- and INV-mode SiNW transistors with a liquid-gate structure. Numerical simulation is also utilized to evaluate sensor performance for these two transistor types. II. 3-D N UMERICAL S IMULATION We conducted a 3-D numerical simulation [16] to investigate the electrical characteristics of the ACC- and INV-mode transistors at various channel doping concentrations (Nch ) and SiNW dimensions. Devices with long channel lengths (L ch = 1 μm) were used in the simulation to exclude side effects. Moreover, abrupt source/drain (S/D) junctions were used in both types of transistors. Fig. 1(a) and (b)
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Fig. 2. (a) VT and (b) current sensitivity for ACC- and INV-mode pH sensors at different Nch and W values.
Fig. 1. Device structures used for 3-D numerical simulation. (a) Birds-eye view of the SiNW channel. (b) Cross-sectional images of the SiNW channel in the lateral direction and vertical direction. The S/D doping concentration is 1020 cm−3 . The height of the SiNW channel (Hnw ) is 80 nm. Source-to-drain voltage is 1 V. 3-D numerical simulation results indicating (c) initial VT and (d) SS levels for ACC- and INV-mode sensors at various Nch and W values.
shows the SiNW pH sensor structure used in this simulation. We assumed that the region of solution covering the top oxide layer of an SiNW pH sensor could be modeled as a dielectric layer with a dielectric constant εr of 80 and a double-layer thickness Hdl of 4.43 nm because the double-layer capacitance (Cdl = εr ε0 /Hdl ) of the solution is 16 μF/cm2 (where ε0 represents the permittivity of free space) [17]. Fig. 1(c) and (d) shows the simulation results for threshold voltage (VT ) and subthreshold swing (SS) versus Nch at different SiNW widths (W ) for the ACC- and the INV-mode sensors. As shown in Fig. 1(c), as Nch and/or W increase, VT decreases in the ACC-mode transistor but increases in the INV-mode transistor. Notably, Nch -dependent VT variation is greater for ACC-mode transistors than for INV-mode transistors; this difference in Nch -dependent VT variation becomes increasingly evident as W increases. In addition, as shown in Fig. 1(d), Nch -dependent SS variation is greater for ACC-mode transistors than for INV-mode transistors, particularly at large W values. This is because the uncontrollable body current at the subthreshold region can flow to the channel in the ACC-mode transistors, resulting in a large increase of SS [9]. Therefore, the fluctuations in electrical characteristics due to variations in W and Nch can be unavoidable in ACC-mode SiNW transistors. To evaluate the sensor performance of the two examined types of transistors, we simulated the two sensing parameters of VT shift (VT ) and current sensitivity (S = ID /ID ); these results are shown in Fig. 2. VT was extracted using the constant current method (at ID = 10−7 A), and current sensitivity was extracted at the subthreshold region (at ID = 10−8 A). We assumed that the device functions as a pH sensor, and
therefore deliberately added the extra charges on the device surface [Fig. 1(b)] using the site-binding model (Q target = −9.52 × 10−7 C/cm2 ) [17], [18]. As shown in Fig. 2(a), VT remains nearly constant as pH changes for both types of sensors. Moreover, VT is almost independent of Nch and W . However, current sensitivity behaves in significantly different ways for the two types of sensors [Fig. 2(b)]. In particular, current sensitivity decreases with increasing Nch and/or W in both ACC- and INV-mode sensors; however, current sensitivity is more greatly affected by Nch and W values for ACC-mode sensors than for INV-mode sensors. This is because the ACC-mode transistor has larger SS than the INV-mode transistor, as previously shown in Fig. 1(d). In the subthreshold region, current sensitivity is defined as follows: S=
eq(VLG −VT 1 )/mkT − eq(VLG −VT 2 )/mkT I D = ID eq(VLG −VT 2 )/mkT q(VT 2 −VT 1 )/mkT =e − 1 = eqVT /mkT − 1.
(1)
In the above equation, m is the ideality (body) factor, k is the Boltzmann constant, T is the absolute temperature, and q is electric charge. Because VT is independent of sensor type [Fig. 2(a)] (i.e., whether a sensor is an ACC- or INV-mode sensor), Nch , and W , current sensitivity is closely related to m. As Nch and channel width increase, m also increases, resulting in the degradation of the electrical performance of the examined device [19], [20]. As shown in Fig. 1(d), this degradation of SS is worse for ACC-mode sensors than for INV-mode sensors. Therefore, the attainment of high current sensitivity requires smaller device dimensions and Nch values for ACC-mode sensors than for INV-mode sensors. III. E XPERIMENTAL R ESULTS AND D ISCUSSION To experimentally investigate the aforementioned aspects of sensor performance, we fabricated two types of SiNW sensors with liquid-gate structures for pH detection. Details regarding the device fabrication process and the measurement setup are provided in [21]. A scanning electron microscopy (SEM) image of a fabricated SiNW sensor is shown in Fig. 3(a), and the inset image illustrates a polydimethylsiloxane (PDMS) fluidic channel prepared for fluidic transport. Potassium phosphate buffers were used as pH solutions (pH 5–9). The SiNW sensors were functionalized with 3-aminopropyltriethoxysilane to apply an amine (−NH2 )
LEE et al.: INVESTIGATION OF SENSOR PERFORMANCE IN ACC- AND INV-MODE SiNW pH SENSORS
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Fig. 4. Statistical comparisons of the (a) VT and (b) current sensitivity values for the two examined sensor types at different channel doping concentrations and SiNW widths. The measured data from six devices are included in each condition.
Fig. 3. Experimental results for the fabricated SiNW pH sensors. (a) SEM image of a fabricated SiNW sensor (W = 100 nm, L ch = 3 μm, and Hnw = 80 nm) and the PDMS fluidic channel system on the sensor chip. (b) Transfer characteristics of the ACC- and INV-mode pH sensors with a sweeping liquid-gate voltage. (c) VT and (d) ID at different pH levels; these values were extracted from (b).
surface to the top oxide layer. As shown in Fig. 3(b), the transfer characteristics of the two types of SiNW pH sensors were measured at five different pH values. The average VT as pH changed was determined to be 50.3 mV for the ACC-mode sensor and 51 mV for the INV-mode sensor [Fig. 3(c)]. These measured results indicate that the fabricated SiNW sensors exhibit responses close to the ideal defined by the Nernst limit, which is known to be 59.6 mV/pH at room temperature (300 K), and therefore reveal that the fabricated sensors function properly as pH sensors [18]. Importantly, the two sensors demonstrate nearly identical VT values but differ with respect to current sensitivity. These differences are attributed to the different SS values of the two types of sensors; in particular, as shown in Fig. 3(b), the SS of the ACC-mode sensor (215 mV/decade) is almost twice the SS of the INV-mode sensor (125 mV/decade). Consequently, as shown in Fig. 3(d), the average current sensitivity of the INV-mode sensor (S = 1.57) is 2.18 times higher than that of the ACC-mode sensor (S = 0.72). The extracted VT and current sensitivity values for the two examined types of pH sensor at different Nch and W are summarized in Fig. 4. As expected, the extracted VT as pH changes is nearly the same for the INV- and ACC-mode sensors. In addition, as W decreases, current sensitivity is improved for both sensor types. However, it is notable that as W and Nch increase, current sensitivity degrades more sharply for the ACC-mode sensor than for the INV-mode sensor; this finding is expected given the simulation results discussed above. From the experimental results, we can conclude that VT for pH changes is not strongly dependent on Nch and W for both types of sensors, but that the current sensitivity of the ACC-mode sensor is more sensitive to these three parameters, compare with the current sensitivity of the INV-mode sensor, because the electrical characteristics of an ACC-mode sensor are greatly and directly dependent on these
parameters. Therefore, an INV-mode SiNW sensor can provide higher current sensitivity than a corresponding ACC-mode SiNW sensor if Nch and device dimensions are relatively large. Furthermore, it is expected that the INV-mode SiNW sensor can accommodate a large range of potential process variation. IV. C ONCLUSION We used the two sensing parameters of VT and current sensitivity to investigate the sensor performance of ACC- and INV-mode pH sensors with liquid-gate structures. Through numerical simulations and experiments, we determined that as device dimensions and Nch increase, the INV-mode sensor generally exhibits higher current sensitivity than the ACC-mode sensor; in contrast, VT does not depend on device parameters. Moreover, the performance of the ACC-mode pH sensor is sensitive to Nch and device dimensions, and degrades more rapidly than the performance of the INV-mode pH sensor as device dimensions and Nch increase. Therefore, with liquidgate structures, the use of an INV-mode pH sensor instead of an ACC-mode pH sensor could improve sensor performance and increase robustness to process variation. R EFERENCES [1] Y. Cui, Q. Wei, H. Park, and C. M. Lieber, “Nanowire nanosensors for highly sensitive and selective detection of biological and chemical Species,” Sci., vol. 293, pp. 1289–1292, Aug. 2001. [2] S. K. Yoo, S. Yang, and J.-H. Lee, “Hydrogen ion sensing using Schottky contacted silicon nanowire FETs,” IEEE Trans. Nanotechnol., vol. 7, no. 6, pp. 745–747, Nov. 2008. [3] C.-H. Lin, C.-H. Hung, C.-Y. Hsiao, H.-C. Lin, F.-H. Ko, and Y.-S. Yang, “Poly-silicon nanowire field-effect transistor for ultrasensitive and label-free detection of pathogenic avian influenza DNA,” Biosens. Bioelectron., vol. 24, no. 10, pp. 3019–3024, Jun. 2009. [4] E. Stern et al., “Label-free immunodetection with CMOS-compatible semiconducting nanowires,” Nature, vol. 445, pp. 519–522, Feb. 2007. [5] G. Zheng, F. Patolsky, Y. Cui, W. U. Wang, and C. M. Lieber, “Multiplexed electrical detection of cancer markers with nanowire sensor arrays,” Nat. Biotechnol., vol. 23, no. 10, pp. 1294–1301, Oct. 2005. [6] P. Ginet, S. Akiyama, N. Takama, H. Fujita, and B. Kim, “CMOScompatible fabrication of top-gated field-effect transistor silicon nanowire-based biosensors,” J. Micromech. Microeng., vol. 21, no. 6, p. 065008, May 2011. [7] A. Gao et al., “Silicon-nanowire-based CMOS-compatible field-effect transistor nanosensors for ultrasensitive electrical detection of nucleic acids,” Nano Lett., vol. 11, pp. 3974–3978, Aug. 2011. [8] J. Lee et al., “SiNW-CMOS hybrid common-source amplifier as a voltage-readout hydrogen ion sensor,” IEEE Electron Device Lett., vol. 34, no. 1, pp. 135–137, Jan. 2013.
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[9] J.-P. Colinge et al., “Properties of accumulation-mode multi-gate field-effect transistors,” Jpn. J. Appl. Phys., vol. 48, no. 3R, p. 034502, Mar. 2009. [10] A. Gao et al., “Signal-to-noise ratio enhancement of silicon nanowires biosensor with rolling circle amplification,” Nano Lett., vol. 13, no. 9, pp. 4123–4130, Aug. 2013. [11] O. Knopfmacher et al., “Nernst limit in dual-gated Si-nanowire FET sensors,” Nano Lett., vol. 10, no. 6, pp. 2268–2274. May 2010. [12] P. G. Fernandes et al., “Effect of back-gate biasing on floating electrolytes in silicon-on-insulator-based nanoribbon sensors,” IEEE Electron Device Lett., vol. 33, no. 3, pp. 447–449, Mar. 2012. [13] N. Clément, K. Nishiguchi, J. F. Dufreche, D. Guerin, A. Fujiwara, and D. Vuillaume, “A silicon nanowire ion-sensitive field-effect transistor with elementary charge sensitivity,” Appl. Phys. Lett., vol. 98, no. 1, p. 014104, Jan. 2011. [14] N. Elefström, A. E. Karlström, and J. Linnros, “Silicon nanoribbons for electrical detection of biomolecules,” Nano Lett., vol. 8, no. 3, pp. 945–949, Feb. 2008. [15] D. J. Baek, J. P. Duarte, D.-I. Moon, C.-H. Kim, J.-H. Ahn, and Y.-K. Choi, “Accumulation mode field-effect transistors for improved sensitivity in nanowire-based biosensors,” Appl. Phys. Lett., vol. 100, no. 21, p. 213703, May 2012. [16] Sentaurus User’s Manual, Synopsys, Inc. Mountain View, CA, USA, 2012. [17] R. A. Champman et al., “Comparison of methods to bias fully depleted SOI-based MOSFET nanoribbon pH sensors,” IEEE Trans. Electron Device, vol. 58, no. 6, pp. 1752–1760, Jun. 2011. [18] L. Bousse, N. F. De Rooij, and P. Bergveld, “Operation of chemically sensitive field-effect sensors as a function of the insulator-electrolyte interface,” IEEE Trans. Electron Device, vol. 30, no. 10, pp. 1263–1270, Oct. 1983. [19] J.-P. Colinge, Silicon-on-Insulator Technology: Materials to VLSI, 3rd ed. New York, NY, USA: Springer-Veralg, 2004. [20] S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed. New York, NY, USA: Wiley-Interscience, 2006. [21] J. Lee et al., “Complementary silicon nanowire hydrogen ion sensor with high sensitivity and voltage output,” IEEE Electron Device Lett., vol. 33, no. 12, pp. 1768–1770, Dec. 2012.
Jieun Lee received the B.S. and M.S. degrees from Kookmin University, Seoul, Korea, in 2008 and 2010, respectively, where she is currently pursuing the Ph.D. degree in electrical engineering. Her current research interests include design, fabrication, characterization, and modeling of silicon nanowire/CMOS hybrid biosensors.
Jung Han Lee (S’07) was born in Seoul, Korea, in 1981. He received the B.S. degree in physics from Korea University, Seoul, in 2007. He is currently pursuing the Ph.D. degree in nanoscience and technology with Seoul National University, Seoul. His current research interests include nanoscale silicon devices, single-electron transistors, quantum devices, and silicon nanowire biosensors.
Byung-Gook Park (M’90) received the B.S. and M.S. degrees in electronics engineering from Seoul National University (SNU), Seoul, Korea, and the Ph.D. degree in electrical engineering from Stanford University, Stanford, CA, USA, in 1990. He joined SNU as a Professor with the School of Electrical Engineering. His current research interests include nano-CMOS devices, flash memories, and neuromorphic devices.
Tae Jung Park received the B.S. and M.S. degrees from Chungnam National University, Daejeon, Korea, and the Ph.D. degree from the Korea Advanced Institute of Science and Technology, Daejeon, in 1998, 2000, and 2008, respectively. He is currently an Assistant Professor with the Department of Chemistry, Chung-Ang University, Seoul, Korea. His current research interests include nanobiofusion studies, including biosensors and nanomaterial fabrication.
Dong Myong Kim (S’86–M’88) received the B.S. (magna cum laude) and M.S. degrees in electronics engineering from Seoul National University, Seoul, Korea, and the Ph.D. degree in electrical engineering from the University of Minnesota, Twin Cities, MN, USA, in 1986, 1988, and 1993, respectively. He has been with the School of Electrical Engineering, Kookmin University, Seoul, since 1993. His research interests include modeling and characterization of semiconductor devices.
Bongsik Choi received the B.S. degree in electrical engineering from Kookmin University, Seoul, Korea, in 2013, where he is currently pursuing the M.S. degree in electrical engineering.
Dae Hwan Kim (SM’08) is currently an Associate Professor with the School of Electrical Engineering, Kookmin University, Seoul, Korea. He has authored and co-authored more than 200 research publications and patents. His current research interests include nano-CMOS, TFTs, biosensors, exploratory logic, and memory devices.
Seonwook Hwang received the B.S. degree in electrical engineering from Kookmin University, Seoul, Korea, in 2012, where he is currently pursuing the M.S. degree in electrical engineering.
Sung-Jin Choi received the M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2012. He is currently an Assistant Professor with the School of Electrical Engineering, Kookmin University, Seoul, Korea.
