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Sensors and Actuators B 181 (2013) 44– 49

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o me pa ge: www.elsev ier .com/ locate /snb

ighly sensitive ion sensor based on the MOSFET–BJT hybrid mode of a gatedateral BJT

eng Yuana, Hyurk-Choon Kwona, Byoung-Ho Kangb, In-Man Kangc, Dae-Hyuk Kwond, Shin-Won Kangc,∗

School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu 702-701, Republic of KoreaCenter for Functional Devices Fusion Platform, Kyunpook National University, Daegu 702-701, Republic of KoreaSchool of Electronics Engineering, College of IT Engineering, Kyungpook National University, Daegu 702-701, Republic of KoreaDepartment of Electronic Engineering, Kyungil University, Gyeongsang 712-701, Republic of Korea

r t i c l e i n f o

rticle history:eceived 1 October 2012eceived in revised form 9 January 2013ccepted 29 January 2013vailable online 7 February 2013

eywords:ated lateral bipolar junction transistor

a b s t r a c t

In this study, a highly sensitive ion sensor using a gated lateral bipolar junction transistor (BJT) wasproposed and evaluated. The proposed device was developed using a semiconductor technology thatcombined the MOSFET and the BJT structures. We found that the sensitivity of the conventionalsemiconductor-based sensors has a large dependence on the transconductance of the device. There-fore, a MOSFET–BJT hybrid-mode-operated gated lateral BJT structure ion sensor was developed thathas higher transconductance than MOSFET sensor devices. In order to confirm the characteristics of thesensor device, the VG–IE curve, transconductance, and current gain performance were evaluated and

OSFET–BJT hybridon sensorighly sensitiveigh transconductanceigh current gain

compared between the MOSFET mode and MOSFET–BJT hybrid mode. Then, ion detection experimentswere performed using pH buffer solutions. The evaluation results indicated that the sensitivity of theMOSFET–BJT hybrid mode can be controlled by altering the base current and is higher (21.77 �A/pH)than the sensitivity of the MOSFET mode, which is similar to the conventional MOSFET-structure-basedISFET (17.56 �A/pH). Further, this study proved that in the MOSFET–BJT hybrid mode, the proposed devicehas higher transconductance and current gain characteristics.

. Introduction

As a basic living process, cells absorb carbon sources (e.g., sugars,mino acids, and fatty acids) and produce energy using adenosineriphosphate (ATP) for normal physiological activities. In addition,ells secrete acidic waste products (lactic acid, CO2, H+) to maintainntracellular homeostasis [1]. Therefore, ion concentration detec-ion is an important area of research, as it can be used for pH valueetection and also as the basis of an electrical biosensor.

The ion-sensitive field-effect transistor (ISFET) was first pro-osed by Bergveld in 1970 [2,3]. In the last decade, the metal-oxide-emiconductor field-effect transistor (MOSFET) structure-basedSFET mainly occupied the field of ion sensor research because ofts advantages of small size, light weight, low cost, low power, andxcellent compatibility with other semiconductor-based functionalevices. In general, the research on ion sensors can be divided into

wo major topics: sensing membrane research and basic deviceesearch. Because the sensitivity of the MOSFET structure-basedSFET is limited by the performance of the sensing membrane,

∗ Corresponding author. Tel.: +82 53 950 6829.E-mail address: swkang@knu.ac.kr (S.-W. Kang).

925-4005/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rittp://dx.doi.org/10.1016/j.snb.2013.01.086

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

Ta2O5 and Al2O3 sensing membranes with good surface perfor-mance were studied for improvements in device sensitivity. Thesemembranes can additionally be deposited after standard com-plementary metal oxide semiconductors (CMOS) processes [4,5].Further, some reports show that a high quality sensing membranecan be fabricated by atomic layer deposition (ALD) for improve-ment in the sensitivity [5,6]. On the other hand, many researchershave developed a no-reference electrode ion sensor based on newstructures such as a carbon nanotube (CNT)–ISFET [7]. However, thesensitivity of this type of sensor has not improved and has less pro-duction capabilities. Meanwhile, many researchers have focused ondevices based on newer materials, such as a GaN FET, to developan ion sensor with high chemical stability and high transconduc-tance [8]. However, the sensitivity of this technology was limitedby the fabrication technology. Moreover, the GaN FETs lost manyof the advantages of Si-based ion sensors, such as the adaptabilityto mass production, low cost, and good reliability.

In our previous work, we reported the possibility of a gatedlateral BJT as an ion sensor, a gas sensor, and a biosensor [9–12].

These studies proved the gated lateral BJT has many advantages forsensor applications such as sensitivity adjustable and low powerdriving. In this study, the gated lateral BJT is proved that can obtaina higher sensitivity than the conventional MOSFET-based ISFET.

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H. Yuan et al. / Sensors an

Si3N4 sensing membrane was used because of its compatibilityith CMOS processes. The H+-ion concentration detection experi-ent was carried out using pH buffer solutions and the results were

hown at the last part of this paper. The results indicated that theroposed device can obtain higher transconductance, higher cur-ent gain, and higher sensitivity than a MOSFET in the MOSFET–BJTybrid mode. Further, other improved sensing membrane tech-ologies can be combined with the proposed ion sensor to improvehe sensitivity similar to the MOSFET-based ISFET, if necessary.

. Materials and methods

.1. Fabrication of the gated lateral BJT

The p-type gated lateral BJT shown in Fig. 1(a) was fabri-ated using a standard 0.35-�m CMOS logic process provided byagnachip-Hynix Co. Ltd. via the Integrated Circuit Design Educa-

ion Center Multi-Project Wafer (IDEC-MPW) project. The widthsf the n+ and p+ regions were designed to be 1.5 �m, and theate width was designed to be 0.5 �m. A floating gate was fab-icated on the top of the device that allows for a large sensingrea and simple fabrication. A Si3N4 layer was deposited on theop of the floating gate, which functions as a sensing membrane inhis study. The emitter and lateral collector of the lateral BJT can

unction as the source and drain of the MOSFET. In the gated lat-ral BJT, the emitter (source)–base–lateral collector (drain), emittersource)–base–substrate, and emitter (source)–base–lateral collec-or (drain)–gate form a lateral p–n–p BJT, vertical p–n–p BJT, and

Fig. 1. (a) Schematic and (b) layout of a gated lateral BJT.

ators B 181 (2013) 44– 49 45

p-type MOSFET, respectively. Therefore, the emitter (source) cur-rent consists of the lateral BJT current, vertical BJT current, andMOSFET channel current. Although a vertical p–n–p BJT structurecan be formed with the lateral collector (drain)–base–substrate,the BJT cannot function when the lateral collector (drain) and sub-strate are grounded. In order to obtain higher efficiency, the emitter(source) was designed to be in the center of the device with thecollector (drain) surrounding it [13,14]. Metal layers were used forconnecting the floating and the internal gates. The size of the fab-ricated device was approximately 16 �m × 16 �m, as shown thelayout in Fig. 1(b).

As seen in Fig. 1(b), the proposed device was designed as asymmetrical structure. The area within the dotted line “A” is theMOSFET. Considering the size, the gated lateral BJT can be reducedto the area within the solid line “B”. The small squares in this figureare metal contacts.

2.2. Operation of the gated lateral BJT

According to the structure of the gated lateral BJT, there arefive terminals: gate (G), emitter/source (E), lateral collector/drain(C), base (B), and substrate (Sub) that can supply the input biasunlike the conventional MOSFET-type ISFETs that have four termi-nals: gate (G), source (S), drain (D), and substrate (Sub). This specialstructure causes the gated lateral BJT to function as both a BJTand a MOSFET [15–18]. Consequently, the proposed device can beoperated under different modes: the BJT mode, MOSFET mode, andMOSFET–BJT hybrid mode under the appropriate emitter (source)bias, base current, and gate bias conditions described below.

The MOSFET mode of the device is driven using a gate biassmaller than the threshold voltage (p-type MOSFET) supplied tothe floating gate to operate the MOSFET channel. A positive basecurrent is supplied to the base region to shut down the lateral andvertical BJTs.

The BJT mode of the device is driven using a gate bias largerthan the threshold voltage supplied to the floating gate to shut theMOSFET channel down. A negative base current is supplied to thebase region to power the lateral and vertical BJTs.

The MOSFET–BJT hybrid mode of the device is driven using a gatebias smaller than the threshold voltage supplied to the floating gateto operate the MOSFET channel. A negative base current is suppliedto the base region to ensure lateral BJT and vertical BJT operation.

In these three modes, the emitter (source) is biased positively,and the lateral collector (drain) and vertical collector (p-sub) aregrounded. Although the vertical BJT functions together with thelateral BJT, we will not discuss this further because it is almost inde-pendent of the sensitivity of the gated lateral BJT. As a sensor, thesensing membrane is fabricated on the gate of the proposed device.When a gate bias is supplied by the reference electrode, the poten-tial of the gate changed depending on the interaction between thesensing target and sensing membrane. This change induces a con-centration of electrons between the lateral collector (drain) and theemitter (source). This concentration of electrons mainly induces theMOSFET channel current and the lateral BJT current changes.

2.3. Sensing system setup

In order to evaluate the characteristics of the gated lateral BJTion sensor for ion detection, an evaluation system was set up asshown in Fig. 2. This system consists of a gated lateral BJT embed-ded in a printed circuit board (PCB), a Ag/AgCl reference electrode(MF-2052, BASi, USA), a test fixture, a semiconductor parameter

analyzer, and a computer. The semiconductor parameter analyzerwas controlled by the computer, supplied all of the input biases,and detected the output signals. The test fixture is used to con-nect the semiconductor parameter analyzer and sensor device

46 H. Yuan et al. / Sensors and Actuators B 181 (2013) 44– 49

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Fig. 2. Schematic of the sensing system for a gated lateral BJT.

hile decreasing the detection noise. The reference electrode wasmmersed in the detection target solution and can be used to supply

gate bias to the sensor device.

.4. Detection of the VG–IE curve of the gated lateral BJT

The ion sensing mechanism can be simply explained by notinghat the ion concentration affects the Si3N4 floating gate to changehe potential of the floating gate and the internal gate. Hence,he device characteristics were first confirmed by constructing theG–IE curve by taking measurements at the device electrodes inrder to evaluate the sensing behavior of the proposed device. Theate bias was varied from −3 V to 1.5 V, the base current was variedrom −200 �A to 60 �A in steps of 20 �A, and an emitter (source)ias of 1 V was supplied. The lateral collector (drain) and verticalollector (substrate) were connected to the ground. Therefore, theated lateral BJT was operated from the MOSFET–BJT hybrid modeo the MOSFET mode and BJT mode.

.5. Detection of the transconductance properties of the gatedateral BJT

In order to confirm the sensitivity of the proposed device, theransconductance curve was evaluated. Although the gated lateralJT includes a MOSFET part and a BJT part, the sensitivity is mainlyooted in the MOSFET part owing to the floating gate. The transcon-uctance of the gated lateral BJT must follow the MOSFET equations a sensor device. The transconductance of the MOSFET is given byq. (1) [19].

m = ∂ID∂VGS

(1)

In the case of the gated lateral BJT, Eq. (1) can be rewritten asq. (2)

m = ∂IE∂VGC

(2)

In Eq. (2), IE is the emitter (source) current.In addition, sensitivity S of a sensor can be defined as

= ∂Ioutput

∂Vinput(3)

In this study, the emitter (source) current was detected and eval-ated while varying the pH. Therefore, Eq. (3) is equivalent to Eq.2). In other words, the transconductance performance indicateshe sensitivity of the proposed device.

.6. Detection of the current gain in a gated lateral BJT

In the development of nano-scale semiconductor technolo-ies, low power, low voltage, and high-speed logic is becoming

Fig. 3. VG–IE curve of the gated lateral BJT modes (VE = 1 V).

increasingly prevalent. Therefore, large transconductance and highcurrent gain are required for analog applications. In order to inves-tigate the current gain characteristics of the gated lateral BJT, weconducted the following experiment. The base current was var-ied from −100 �A to 100 �A, with a constant gate bias of −1 V.Therefore, the proposed device was operated from the MOSFET–BJThybrid mode to the MOSFET mode. Then, the common collector andcommon emitter conditions were evaluated and compared.

2.7. H+-ion detection experiment

After determining the characteristics of the gated lateral BJT,and H+-ion detection experiment was performed. pH buffer solu-tions (supplied by Orin Application Solution, Thermo Scientific, pH1.68, pH 4.00, pH 7.00, pH 10.01, and pH 12.46), which are mostlyused for H+-ion sensor research, were used in this study. The refer-ence electrode was immersed in the solution, as shown in Fig. 1(a).The experiment was performed and it compared the MOSFET–BJThybrid and MOSFET modes, where the base current was varied from−400 �A to 100 �A with an emitter bias of 1 V.

The sensing mechanism of the sensing membrane is similar tothe mechanism for a conventional Si-based ion sensor. A Si3N4floating gate was used as the pH sensing membrane according tothe site-binding model [20–22], which suggests that atoms in thesurface layer of the metal as well as semiconductor oxide act asamphometers that are in contact with an electrolyte. Hence, thepotential of the floating gate changes when varying the H+-ionconcentration. This change can affect the MOSFET channel to com-plete the sensing process. In contrast to a MOSFET mode ISFET,the MOSFET–BJT hybrid mode can affect the efficiency of the MOS-FET part and the threshold voltage of the device, even though thebipolar current has no sensitivity.

3. Results and discussion

3.1. Evaluation of the VG–IE curve and transconductance of thegated lateral BJT

The VG–IE curves of the gated lateral BJT as a function of thebase current variation are shown in Fig. 3. According to the opera-tion method of the gated lateral BJT discussed above, we know that

the BJT mode, MOSFET mode, and MOSFET–BJT hybrid mode areall illustrated in this figure. The right part of the emitter (source)current that has almost 0 A output indicates that the gated lateralBJT was shut off.

H. Yuan et al. / Sensors and Actuators B 181 (2013) 44– 49 47

Fig. 4. VG–sensitivity (transconductance) plot of a gated lateral BJT as the basecurrent is varied.

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Fig. 5. (a) Current gain results with VG = −1 V, (b) current gain comparison between

The current gain results are shown in Fig. 5. In Fig. 5(a), the cur-

Following Fig. 3, the emitter (source) current decreased as theate bias increased when the gate bias was varied from −3 Vo 1.5 V. This was because the gate bias can only affect the p-ype MOSFET channel current and not the BJT current whenn the MOSFET mode or in the MOSFET–BJT hybrid mode. Inhe MOSFET–BJT hybrid mode, the emitter current consists of

proportional MOSFET channel current and a bipolar current,hich was shown by Joarder using a simulation tool [23]. There-

ore, the p-type MOSFET channel current switches off gradually,nd the emitter (source) current decreases when the gate biasncreases.

Moreover, the shape of the curves corresponding to theOSFET–BJT hybrid mode changed as the base current was var-

ed. As the base current became more negative, a higher VG–IEurve was measured. However, the curves showed little variationhen the base current was varied from 20 �A to 60 �A in the MOS-

ET mode. This can be explained as follows. In the MOSFET–BJTybrid mode, the BJT effect of the gated lateral BJT becomes domi-ant as the gate bias increases. As the base current becomes moreegative, the BJT effect and the emitter (source) current increasedecause the BJT has a higher transconductance characteristic thanhe MOSFET. However, the BJT part of the device remained switchedff in the MOSFET mode. Any changes in the base current (neg-tive) cannot affect the MOSFET part of the device. Therefore,he gated lateral BJT in the MOSFET mode showed similar cur-ent and voltage characteristics even though the base currents areiffered.

In addition, the results indicate that the threshold voltages of theOSFET and MOSFET–BJT hybrid modes are different. The reason

or this difference is that when the MOSFET–BJT hybrid mode is sup-lied with a negative base current, electrons are injected into the-well to enhance the forward bias between the emitter (source)nd the base (body) of the gated lateral BJT. Therefore, the deple-ion width is smaller in the MOSFET–BJT hybrid mode than in the

OSFET mode, which causes variation in the threshold voltages24].

The sensitivity of a gated lateral BJT can be calculated with theransconductance in Eq. (2), and the results are shown in figure.n Fig. 4, the results indicated that the transconductance of the

OSFET–BJT hybrid mode can be controlled by the base current,hich means that the sensitivity can be varied. Further, the results

ndicated that the MOSFET–BJT hybrid mode can obtain a higherransconductance than that of the MOSFET mode.

the common collector and common emitter under hybrid mode conditions, and (c)current gain comparison between the MOSFET mode and the MOSFET–BJT hybridmode.

3.2. Evaluation of the current gain of the gated lateral BJT

rent gain values are detected from the MOSFET–BJT hybrid modeto the MOSFET mode, and the common collector (IE/IB) curves are

48 H. Yuan et al. / Sensors and Actu

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ig. 6. Comparison of the pH sensitivity between the MOSFET and the MOSFET–BJTybrid modes.

ompared with the common emitter (IC/IB) curves. According tohe operation methods discussed above, a negative base currentorresponds to a current gain under the MOSFET–BJT hybrid modeondition. In contrast, current gains with a positive base currentorrespond to the current gain in the MOSFET mode. Therefore,ig. 5(a) is re-plotted in Fig. 5(b) in order to further analyze theseesults. In the MOSFET–BJT hybrid mode, the absolute values of themitter (source) current results are compared. The common col-ector curves exhibited a higher value than the common emitterurves. This is because in the MOSFET–BJT hybrid mode, the emit-er (source) current consists of the collector current, base currentnd substrate current, which can be expressed by the followingquations.

CommonC = IEIB

= ICIB

+ ISS

IB+ IB

IB(4)

CommonE = ICIB

(5)

In Eqs. (4) and (5), ˇCommonC and ˇCommonE are the current gainsf the common collector and common emitter, respectively, and

ss is the substrate current. Hence, the emitter (source) current IEs greater than the collector (drain) current IC. Consequently, theurrent gain of the common collector is higher. From these results,he emitter (source) current is detected as the basic analysis target.

Fig. 5(a) was re-plotted as Fig. 5(c) to compare the current gainsetween the MOSFET–BJT hybrid mode and the MOSFET modesing the absolute values. According to the results, the current gainf the MOSFET–BJT hybrid mode was higher than the current gainf the MOSFET mode. This was because the emitter (source) cur-ent in the MOSFET–BJT hybrid mode was higher than the currentn the MOSFET mode, which can be observed in Fig. 3. The resultsndicated that, the gated lateral BJT with the MOSFET–BJT hybrid

ode operation has higher transconductance and higher currentain characteristics that show better performance for nanoscaleevice fabrication.

.3. H+-ion detection of the gated lateral BJT

The sensitivity with additional pH buffer solutions was detected

s the base current was varied, and the results are shown in Fig. 6.he results of the four groups shown on the left are those obtainedn the MOSFET–BJT hybrid mode, and the results of the group shownn the right are those obtained in the MOSFET mode.

ators B 181 (2013) 44– 49

In the MOSFET–BJT hybrid mode, the sensitivities of the pro-posed device were approximately 12.46, 14.27, 16.21, 19.40, and21.77 �A/pH for base current values of −400, −300, −200, −100,and −20 �A, respectively. On the other hand, the sensitivity wasapproximately 17.56 �A/pH in the MOSFET mode. The ion detectionresults show that the sensitivity of the MOSFET–BJT hybrid modecan be controlled by altering the base current, which can also resultin sensitivities higher than those for a conventional MOSFET-basedISFET. Additionally, the pH detection sensitivity increased with thebase current in the MOSFET–BJT hybrid mode. These results con-firmed the analysis results explained above.

According to Fig. 1(b), the size of the gated lateral BJT “B” isapproximately 74.5% greater than that of MOSFET “A”. When thebase current was −20 �A, the transconductance of the gated lat-eral BJT “B” was approximately 24% greater than that of MOSFET“A”. In addition, the sensors should be arrayed in order to expectbetter performance of the sensor in many cases [25–27]. In the caseof the gated lateral BJT arrangement, area “A” is repeated, and theother area (“B”–“A”) is shared. After calculation, the transconduc-tance/size ratio of the proposed gated lateral BJT should be greaterthan that of the MOSFET if the array count is greater than four.

Further, the MOSFET–BJT hybrid mode of the gated lateral BJTcan enhance the forward bias between the emitter (source) andthe base. Therefore, the proposed device can prevent short channeleffect better than the MOSFET structure. Following this reason, theproposed device has better adaptability for nanoscale fabricationprocess that decrease the size of the gated lateral BJT.

4. Conclusions

In this study, a highly sensitive Si-based ion sensor based ona gated lateral BJT was proposed. In order to evaluate the per-formance of the proposed device, the VG–IE, transconductance,and current gain characteristics were compared between theMOSFET–BJT hybrid mode and MOSFET mode before ion detec-tion. The results show that under the MOSFET–BJT hybrid mode,the proposed device has high transconductance and high currentgain, benefitting nanoscale fabrication, sensitivity variation, andhigh sensitivity properties. After the advantages of the gated lateralBJT were discussed in theory, an ion detection experiment was per-formed using a pH buffer solution and a Si3N4 sensing membrane.According to the ion sensing results, the sensitivity variation andhigh sensitivity characteristics were confirmed.

While this study focused on the device improvement of an ionsensor, other improved sensing membrane technologies (Ta2O5and Al2O3 sensing membranes, ALD process) can be combined withthe proposed ion sensor to improve sensitivity, if necessary.

Acknowledgments

This work was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) grantfunded by the Korea government (MEST) (no. 2012-0000632).

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Biographies

Heng Yuan received his master’s degree in School of Electrical Engineering andComputer Science, Kyungpook National University, Korea in 2007. He is currentlyPh.D. candidate in School of Electrical Engineering and Computer Science, Kyung-pook National University, Korea. His research activities include semiconductor gassensors, semiconductor biosensors and optical gas sensors.

Hyurk-Choon Kwon received his Ph.D. degree in School of Electrical Engineeringand Computer Science, Kyungpook National University, Korea in 2010. His researchactivities include semiconductor biosensors.

Byoung-Ho Kang received his Ph.D. degree in School of Electrical Engineeringand Computer Science, Kyungpook National University, Korea in 2012. He is cur-rently Post-Doctoral researcher at Center for Functional Device Fusion Platform,Kyungpook National University, Daegu, South Korea, in 2012. His research activitiesinclude QD LED, and optical sensors.

In-Man Kang is currently Professor at the School of Electrical Engineering and Com-puter Science, Kyungpook National University, South Korea. His research fields areCMOS RF modeling, silicon nanowire devices, tunneling transistor, low-power nanoCMOS, and light emitting diodes.

Dae-Hyuk Kwon is currently Professor at the Department of Electronic Engineer-ing, Kyungil University, South Korea. His research interests include semiconductormanufacturing process, semiconductor sensors, and display (TFT-LCD).

Shin-Won Kang is currently Professor at the School of Electrical Engineeringand Computer Science, Kyungpook National University, South Korea. Also, he isdirector of center for functional devices fusion platform. His research fields areopto-electronic functional device, OLED and optical nano devices.