Nanoscale FET-Based Transduction toward Sensitive Extended-Gate

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Article Cite This: ACS Sens. 2019, 4, 1724−1729

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Nanoscale FET-Based Transduction toward Sensitive Extended-Gate Biosensors Jae Kwon,† Byung-Hyun Lee,‡ Seong-Yeon Kim,‡ Jun-Young Park,‡ Hagyoul Bae,‡ Yang-Kyu Choi,*,‡ and Jae-Hyuk Ahn*,† †

Department of Electronic Engineering, Kwangwoon University, Seoul 01897, Korea School of Electrical Engineering, KAIST, Daejeon 34141, Korea

‡

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S Supporting Information *

ABSTRACT: Owing to their simple and low-cost architecture, extended-gate biosensors based on the combination of a disposable sensing part and a reusable transducer have been widely utilized for the label-free electrical detection of chemical and biological species. Previous studies have demonstrated that sensitive and selective detection of ions and biomolecules can be achieved by controlled modification of the sensing part with an ion-selective membrane and receptors of interest. However, no systematic studies have been performed on the impact of the transducer on sensing performance. In this paper, we introduce the concept of a nanoscale field-effect transistor (FET) as a reusable and sensitive transducer for extended-gate biosensors. The capacitive effect from the external sensing part can degrade the sensing performance, but the nanoscale FET can reduce this effect. The nanoscale FET with a gate-all-around (GAA) structure exhibits a higher pH sensitivity than a commercially available FET, which is widely used in conventional extended-gate biosensors. A sensitivity reduction is observed for the commercial FET, whereas the pH sensitivity is insensitive to the area of the sensing region in the nanoscale FET, thus allowing the scaling of the detection area. Our analysis based on a capacitive model suggests that the high pH sensitivity in the compact sensing area originates from the small input capacitance of the nanoscale FET transducer. Moreover, a decrease in the nanowire width of the GAA FET leads to an improvement in the pH sensitivity. The extended-gate approach with the nanoscale FET-based transduction can pave the way for a highly sensitive analysis of chemical and biological species with a small sample volume. KEYWORDS: Field-effect transistor, nanoscale transducer, extended gate, chemical and biological sensor, pH sensor, small volume analysis on-sensitive field-effect transistors (ISFETs) have been utilized for label-free electrical detection of chemical and biological species.1 Owing to their structural similarity to conventional metal-oxide-semiconductor field-effect transistors (MOSFETs), except for the liquid-state gate in ISFETs and the solid-state gate in MOSFETs, ISFETs can be fabricated with well-established semiconductor processes used for massive and reliable production of integrated circuits, such as microprocessors and memories. The structure of the ISFET is modified, compared with that of the MOSFET, by replacing the solid gate with the combination of an ionic solution and a reference electrode. Charged species such as ions and biomolecules selectively bound to the sensor surface (i.e., gate dielectric or channel material) change the surface potential, which modulates the current flowing through the channel material. The sensitivities of ISFETs have been improved using nanomaterials with high surface-to-volume ratios for the channel, such as silicon nanowires (SiNWs),2−4 carbon nanotubes,5,6 graphene,7,8 and MoS2,9 which allow the detection of pH, proteins, and DNA with a limit of detection as low as the picomolar level.

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© 2019 American Chemical Society

As one of the modified structures of ISFETs, extended-gate FETs have several advantages over conventional ISFETs owing to their simple and low-cost architectures.10−16 The sensing part (i.e., extended gate) is separated from the channel region of the FET transducer. As the channel region is isolated from the ionic solution, ion-induced instabilities of sensors such as hysteresis,17 drift,18 and dissolution of channel materials19 can be overcome, and long-term operations of the sensors can be achieved.20 FET transducers are reusable, while the sensing parts are disposable, which simplifies the device fabrication and reduces the cost.12,14,15 A previous study indicated that the extended-gate biosensor, even when a commercially available FET is used as the transducer, has similar performance to a nanoscale ISFET-based biosensor where biomolecules are directly bound in the channel region.21 However, the sensitivity of the extended-gate biosensor is lower than the Received: April 18, 2019 Accepted: June 4, 2019 Published: June 4, 2019 1724

DOI: 10.1021/acssensors.9b00731 ACS Sens. 2019, 4, 1724−1729

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ACS Sensors ideal value, as the sensing area of the extended gate is reduced.22,23 This scaling limitation in the detection area impedes the analysis of a small-volume sample and the fabrication of a high-density sensor array. In this study, we present an approach using nanoscale FET transducers to achieve a high sensitivity in a compact sensing area while preserving the advantages of extended-gate biosensors. We demonstrate gate-all-around (GAA) FETs for pH sensing as an example to confirm that the nanoscale FET can be a highperformance reusable transducer for extended-gate biosensors. An extended gate with an Al2O3 layer on top serving as a replaceable and stable pH sensing part is connected to the GAA FET. The pH sensing characteristics of the GAA FET are compared with those of a commercial FET. In contrast to the commercial FET, the pH sensitivity of the GAA FET is not significantly reduced when the sensing area is reduced. This indicates that the nanoscale FET can detect biomolecules immobilized in the small sensing area, which is suitable for the analysis of a small sample volume. Our analysis indicates that the superior sensitivity of the GAA FET originates from the small input capacitance, which provides a strong capacitive coupling to the sensing region. The sensitivity is further improved with narrower nanowires of the GAA FET owing to the stronger capacitive coupling between the transducer and the sensing region. The nanoscale FET-based transducer can be used for versatile sensing applications. The transduction mechanism determining the sensitivity provides design guidelines for sensitive extended-gate biosensors.

Figure 1. (a) Schematic of the nanoscale FET-based extended-gate biosensor. A disposable extended gate is connected to the nanoscale FET, which is used as a high-performance reusable transducer. Surface charges induced by the pH solution on the ion-sensitive membrane (i.e., Al2O3) of the extended gate change the surface potential and thus the drain current (ID). To bias the FET-based transducer, a liquid-gate voltage (VLG) and drain voltage (VD) are applied to the ionic solution on the extended gate and drain terminal, respectively, while the source terminal is electrically grounded. G: gate, D: drain, S: source. (b) Schematic of the nanoscale FET with the GAA configuration (GAA FET). Three-story SiNWs are vertically stacked; the gate surrounds the nanowires. (c) Schematic of the one-route alldry etching process. “HM” represents the oxide hard mask layer used to form SiNWs during the etching process. The vertically stacked SiNWs are formed by the iterative process of C4F8-based passivation and SF6-based isotropic dry etching. (d) Scanning electron microscopy (SEM) image of the three-story SiNWs after the oneroute dry etching process before the gate formation. The SiNWs are completely separated from each other without the stiction problem. (e) Cross-sectional transmission electron microscopy (TEM) image of the GAA FET. The nanowire widths are in the range of 50−130 nm, depending on the mask layout. (f) Transfer and (g) output characteristics of the fabricated nanoscale FET.

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RESULTS AND DISCUSSION Figure 1a illustrates our approach, in which a high-performance nanoscale FET serves as a reusable transducer, while an extended gate is used as a replaceable sensing part. The external capacitance of the extended gate can reduce the overall sensitivity, but the nanoscale FET can solve this problem through its small input capacitance, as discussed below. Once the fabrication of nanoscale FETs is completed via an advanced semiconductor process, the nanoscale FET can be reused as a sensitive transducer by converting the change in the surface potential of the sensing region into a drain current (ID). The sensing part can be easily replaced depending on the type of experiment. A cost-effective approach for fabricating the sensing parts is to use low-cost and disposable substrates such as paper and plastic.24 Among various types of nanoscale FETs, we selected the GAA FET as a transducer owing to the strong electrostatic control of the channel potential through the surrounding gate structure,25−27 which can effectively convert a voltage signal generated from the extended gate into a channel current (Figure 1b). The fabrication process for the GAA FETs was similar to that reported in our previous study;28 the process steps are described in detail in Supporting Information. Briefly, the fabrication process comprised nanowire fabrication, gate formation, and source/drain implantation. Three-story SiNWs were fabricated on a bulk silicon substrate instead of a siliconon-insulator (SOI) substrate using a one-route all-dry etching process called the Bosch process, as shown in Figure 1c. An oxide hard mask for patterning the nanowire region was formed by deposition of a high-density plasma (HDP) oxide, photolithography using a KrF excimer laser, and dry etching. The one-route etching for fabricating the SiNWs comprised two steps. First, the sidewalls of the hard mask or SiNWs were passivated by a C4F8-based polymer generated from CF4 gas in

order to prevent the subsequent isotropic dry etching. The bottom portion of the SiNWs was then carved and separated from the bulk substrate via SF6-based isotropic dry etching. The vertically stacked three-story SiNWs were obtained through three cycles of iterative etching. Figure 1d shows the stable formation of the vertically integrated SiNWs without stiction. As shown in Figure 1e, the GAA configuration is successfully achieved by thermal oxidation to grow the gate dielectric (5 nm), followed by chemical vapor deposition (CVD) of an in situ-doped polycrystalline silicon (poly-Si) as a gate electrode completely wrapping the three-story SiNWs. After the gate patterning, the source and drain were formed by ion implantation. Unless otherwise mentioned, the nanowire width and gate length of the GAA FETs were 50 nm and 1 μm, respectively. The fabricated GAA transistor exhibits typical n1725

DOI: 10.1021/acssensors.9b00731 ACS Sens. 2019, 4, 1724−1729

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ACS Sensors channel characteristics with a subthreshold swing of ∼70 mV/ dec and an on−off ratio of >105 (Figure 1f and g). The performance of the fabricated GAA FET as a transducer was compared with that of a commercially available FET (CD4007UB, Texas Instruments), which has been widely used for other extended-gate pH sensors owing to its low cost and simplicity.22,29,30 Extended-gate electrodes with 3-nm-thick Cr and 100-nmthick Au layers were fabricated on a Si wafer with a 90-nmthick SiO2 layer on top using photolithography, electron-beam evaporation, and a lift-off process. The square sensing areas were 0.09, 0.36, 1, 4, 9, and 25 mm2. For stable pH sensing, a 15-nm-thick Al2O3 layer was deposited on the electrodes by electron-beam deposition.31 A reservoir to contain the pH solution was prepared on the sensing region using a silicone elastomer (Kwik-Cast, World Precision Instruments, Inc.). With an increase in the pH value of the buffer solution, the drain current (ID) versus liquid-gate voltage (ID−VLG) curve is shifted to the right for both the GAA FET and the commercial FET (Figure 2a and c), as expected. The curve in the log− linear scale is shifted in parallel, without a change in the subthreshold slope (Figure S2). Hydrogen ions reversibly bind to surface sites of the Al2O3 layer, and the surface charge is modulated according to the pH (Figure 2e). The reliable operation of the Al2O3 layer for pH sensing is validated using analytical figures of merit such as the selectivity, response time,

long-term stability, and influence of the background electrolyte on the pH sensitivity, which are analyzed in detail in Supporting Information (Figures S3−S6). The GAA FETbased sensor exhibits a higher pH sensitivity with a smaller deviation compared with the commercial FET-based sensor (Figures 2b and d). Notably, the pH sensitivity of our extended-gate sensor is comparable to those of other ISFETbased sensors where the channel material is directly in contact with the pH solution.32,33 This indicates that the detection sensitivity is not reduced by the extended structure, preserving the advantage of the simple replacement of the extended gate.21 The electrical measurement setup for pH sensing is detailed in Supporting Information. The pH sensing performances of the GAA FET were compared with those of the commercial FET for different sensing areas in the range of 25−0.09 mm2 (Figure 3a). The pH sensitivity of the GAA FET is higher than that of the

Figure 3. (a) Schematic of the extended-gate biosensor and its capacitive model. CDL, CEG, and CTR represent the capacitances attributed to the electric double layer, Al2O3 dielectric layer of the extended gate, and mobile carriers in the SiNWs, respectively. The hydrogen ions bound to the Al2O3 sensing layer cause a potential change Δψ0 at the outer surface of the Al2O3 sensing layer, leading to a change in the floating-gate voltage (ΔVFG) induced at the gate terminal of the FET transducer. (b) Dependences of the pH sensitivity on the sensing area for the different types of FET transducers. The pH sensitivity of the commercial FET is reduced for the smaller sensing area, whereas the pH sensitivity is insensitive to the sensing area in the case of the GAA FET, allowing the analysis of a small sample volume. The experimental data for the pH sensitivity as a function of the sensing area for the commercial FET are empirically fitted with CTR = 9 pF obtained from the electrical measurement. The other curves are obtained by varying CTR. (c) Schematic of the measurement setup with an external capacitor connected to the gate terminal of the FET transducer. The capacitance of the external capacitor (Cext) is modeled as the capacitance of the extended gate in the ionic solution. A lower Cext corresponds to a lower extended-gate capacitance (CEG) or smaller sensing area. (d) Effect of the external capacitance on the transducer operation. The transconductance ratio is defined as the ratio of the maximum transconductance (gm) between the reference Cext of 1 μF and the given Cext.

Figure 2. pH sensing characteristics of the extended-gate FETs. (a) ID−VLG characteristics and (b) threshold voltage (VT) of the GAA FET as a function of the pH value. (c) ID−VLG characteristics and (d) VT of the commercial FET (as a control device) as a function of the pH value. In both FET transducers with the same sensing-region area of 9 mm2, the ID−VLG curves shift to the right with an increase in the pH; the linear dependence of VT on the pH is observed. The GAA FET exhibits a higher pH sensitivity than the commercial FET. (e) Schematic of the origin of the pH response in the Al2O3 layer. Hydrogen ions reversibly bind to the active OH sites on the Al2O3 surface, generating the surface charge and potential. 1726

DOI: 10.1021/acssensors.9b00731 ACS Sens. 2019, 4, 1724−1729

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intentionally connected to the FET transducer (Figure 3c). A lower Cext is assumed for the case of a smaller sensing area. According to the voltage-divider model, the gate voltage (VG) is allocated to two components by a capacitive ratio, i.e., the external capacitor with Cext and FET transducer with CTR. The voltage applied to the FET transducer decreases with an increase in Cext. This can simply be expressed by the equation C VFG = C +extC VG . Therefore, it is expected that a higher

commercial FET for all the sensing areas (Figure 3b). The pH sensitivity of the commercial FET decreases with a decrease in the sensing area. This area dependence is consistent with the results of a previous study, in which a radiofrequency-sputtered tin oxide (SnO2) film on a glass substrate was used as a hydrogen-ion sensitive layer.22 However, the GAA FET exhibits a constant value of the pH sensitivity (∼50 mV/ pH), even when the sensing area is reduced to 0.09 mm2. As indicated by Table I, our GAA FET-based sensor can operate

ext

Table I. Comparison of the Sensor Performance with Those of Previously Reported Extended-Gate pH Sensors Transducer

Sensing material

pH sensitivity (mV/pH)

Sensing area (mm2)

Ref

GAA FET

Al2O3

49.9

0.09

Commercial FET (CD4007UB) Commercial FET (BSS159N) Commercial FET (CD4007UB) Commercial FET (CD4007UB) Commercial FET (NDP6060L)

SnO2/ ITO SixNy

57

0.8

This work 22

36.5

1

38

ITO

52.3

13

29

ZnO

38

100

30

TiO2

59.9

100

39

with a smaller sensing area of the extended-gate electrode, compared with other commercial FET-based pH sensors while having a comparable or higher pH sensitivity. The difference in the pH sensitivity according to the sensing area, which is observed in the experiment with the commercial FET, can be explained by the extended-gate capacitance (CEG) serially connected with a double-layer capacitance (CDL),34,35 which is formed when the extended gate is in contact with the pH solution (Figure 3a). As the sensing area decreases, both CDL and CEG also decrease. CDL is significantly higher than CEG for a high ionic concentration.36 The series capacitance of CDL and CEG is reduced to CEG. Thus, the effect of the ionic strength on CDL is not dominant for determining the sensitivity. The potential change (Δψ0) generated in the sensing region is not efficiently transferred to the FET transducer, as CEG decreases with a reduction in the sensing area compared with the transistor capacitance (CTR). This can C be expressed by ΔVFG = C +EGC Δψ0 , where ΔVFG is the EG

TR

voltage is needed to modulate the drain current at a lower Cext, which reduces the transconductance, gm = dID/dVG. In the electrical measurement, the maximum gm of the commercial FET connected with the external capacitor decreases with Cext, whereas the effect of Cext is negligible for the GAA FET (Figures 3d and S4). The minimum Cext for proper operation of the FET transducer is reduced in the GAA FET compared with that in the commercial FET, which confirms that the GAA FET has a lower CTR. The capacitive matching between the extended gate (CEG) and FET transducer (CTR) is needed to maintain the sensitivity; thus, CTR should be scaled down to achieve a sensitive signal from the compact sensing area having a low CEG. The use of the nanoscale FET as a sensitive transducer is a suitable approach for exploiting the extendedgate sensor. It is noted that the maximum gm of typical FETs is degraded with an increase in the source/drain series resistance.37 The slight dip of the gm ratio at Cext = 200 nF for the GAA FET may have originated from the source/drain series resistance when metal probe tips are placed in contact with the source/drain regions. However, the source/drain series resistance has a negligible impact on the transfer characteristics in the subthreshold region, where the threshold voltage (VT) is extracted. Thus, VT as the sensing signal is not affected by the source/drain series resistance. We investigated the effect of CTR on the pH sensitivity using GAA FETs with different nanowire widths (Figure 4a). CTR can be tuned by varying the width of the nanowire; CTR is reduced when the nanowire width is decreased. The area of the sensing region in the extended gate was chosen to be as small as 0.36 mm2 to maximize the effect of CTR compared with CEG by reducing CEG. A higher pH sensitivity is obtained for a smaller width of the GAA FETs with gate lengths of 1 and 2 μm (Figure 4b). At the reduced CTR with a small width, the change in the surface potential caused by the pH difference is well transferred to the FET transducer; thus, a higher pH sensitivity can be obtained. The experimental results for different nanowire widths confirm that CTR affects the pH sensitivity. By reducing CTR using nanoscale FET-based transducers, a limited amount of chemical and biological species at a low concentration can be detected with a small sensing area.

TR

change in the floating-gate voltage induced at the gate terminal of the FET transducer. Therefore, a reduction in the pH sensitivity at the reduced sensing area is observed for the commercial FET. However, the GAA FET has a lower CTR than the commercial FET because of the nanoscale GAA structure. The nanoscale FET can be operated as the transducer FET in a regime with a low CEG or a small sensing area, where the commercial FET-based sensor cannot operate. Figure 3b shows the fitting of the experimental data for the pH sensitivity in the commercial FET as a function of the sensing area based on a capacitive model in which the pH sensitivity is determined by the ratio of C TR to CEG (Supporting Information). With the decrease in CTR assigned in the modeling, the pH sensitivity increases in the smaller sensing area. To verify the hypothesis that the potential change due to the pH difference is not well transferred to the FET transducer as CEG decreases, CEG is modeled as an external capacitor (Cext)

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CONCLUSIONS We demonstrated a generic electrical transduction method for sensitive detection based on disposable sensing parts. We used nanoscale FETs as high-performance reusable transducers. The nanoscale FET provided a higher sensitivity in the scaled-down sensing area than a commercial FET. The capacitive matching between the FET transducer and sensing area was crucial for effectively transferring the signal to the transducer without loss. The scaling-down of the channel dimensions further reduced the input capacitance and enhanced the coupling to the extended gate for obtaining the sensitive signal. The generic method using nanoscale FET-based transduction can be 1727

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Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2018R1A6A1A03025242). This work was supported in part by the Center for Integrated Smart Sensors through the Global Frontier Project under Grant CISS-20110031848 and in part by the National Research Foundation of Republic of Korea under Grant 2018R1A2A3075302. This work was partially supported by a Research Grant from Kwangwoon University in 2018.

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Figure 4. (a) Schematic of the GAA FET showing the nanowire width. (b) Effects of the nanowire width on the pH sensitivity for gate lengths (L) of 1 and 2 μm. In both cases, a higher pH sensitivity is obtained at a smaller width. The sensing area is chosen to be as small as 0.36 mm2 to clearly show the structural effect of the GAA FET on the pH sensitivity.

applied to various chemical and biological sensing applications in which a low-cost analysis of a small sample volume is preferred.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.9b00731. Fabrication process of the GAA FETs, electrical measurement setup for the pH sensing, transfer characteristics as a function of the pH value in a log− linear scale, capacitive model of the extended-gate biosensor, and effects of the external capacitor on the operations of the FET transducers (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yang-Kyu Choi: 0000-0001-5480-7027 Jae-Hyuk Ahn: 0000-0001-7490-000X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP; Ministry of Science, ICT & Future Planning) (No. 2017R1C1B5017451). J.-H.A. was partially supported by the Basic Science Research Program through the National 1728

DOI: 10.1021/acssensors.9b00731 ACS Sens. 2019, 4, 1724−1729

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DOI: 10.1021/acssensors.9b00731 ACS Sens. 2019, 4, 1724−1729