1056 IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 9, SEPTEMBER 2010

High-Sensitivity Ion-Selective Field-EffectTransistors Using Nanoporous Silicon

Nina Zehfroosh, Mehran Shahmohammadi, and Shams Mohajerzadeh, Senior Member, IEEE

Abstract—We report the realization of high-sensitivity ion-selective field-effect transistors (ISFETs) using nanoporous polysi-licon on the gate region. Owing to the presence of a nanoporousfilm, the effective area of the exposed surface becomes larger thanthat of the channel area of a regular transistor. The response ofsuch transistors to pH has been measured for a wide range fromfour to nine, showing a different behavior from regular ISFETswhere a change in the threshold voltage is recorded. A relativecurrent-based sensitivity can be adapted for such devices. A highsensitivity on the order of 300 mV/pH is reported, owing to thepresence of 3-D nanostructures.

Index Terms—Ion-selective field-effect transistors (ISFETs),nanoporous, pH, sensitivity.

I. INTRODUCTION

ION-SELECTIVE field-effect transistors (ISFETs) are mi-croelectronic devices that have an important role in the de-

velopment of chemical sensors [1]–[3]. ISFETs are analogousto MOSFET transistors, where the metallic gate is replaced bya sensitive membrane and a reference electrode [4]. The smallsize of their sensitive area, rapid response, high sensitivity, lowsample volumes, and potential for on-chip circuit integrationmake them desirable for biosensor applications [5]. The mostimportant use of ISFETs is as pH meters. The pH sensitivityof ISFETs, which is shown as the variation in the transistorthreshold voltage, is limited to values around 59 mV/pH dueto thermodynamic constrains [6].

On the other hand, the use of nano- or microporous structuresis found to be suitable for various sensors as humidity trans-ducers, liquid-phase ion detectors, and gas sensors [7]–[9]. Inthis letter, we take advantage of the formation of nanoporousstructures of polysilicon films to realize high-sensitivity pHtransistors. As a result, not only a slight threshold voltage shiftis detected but also a much more remarkable slope variationis observed in the electrical drain-voltage characteristics. Ananoporous structure increases the effective adsorption surfaceon the channel area, which, in turn, transmits the ion effect to

Manuscript received April 22, 2010; revised May 16, 2010; acceptedMay 23, 2010. Date of publication July 23, 2010; date of current versionAugust 25, 2010. This work was supported in part by a grant from the ResearchCouncil of the University of Tehran and in part by the Nano-Electronic Centerof Excellence. The review of this letter was arranged by Editor C.-P. Chang.

The authors are with the Nano-Electronic Laboratory, School of Electricaland Computer Engineering, University of Tehran, Tehran 14395-515,Iran (e-mail: n.zehfroosh@gmail.com; m.shahmohammadi@ece.ut.ac.ir;mohajer@ut.ac.ir).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2010.2052344

Fig. 1. Fabrication process of pH meter and the evolution of the nanoporouslayer on the gate oxide. (a) SiO2 growth on the p-type silicon substrate andpolysilicon deposition. (b) Transistor fabrication. (c) Passivation of source anddrain regions. (d) Nanoporous silicon formation.

the underlying insulator surface (SiO2). The accumulation ofthe positive ions on the porous layer exerts an electric field onthe insulator and subsequently on the channel region, increasingthe inversion charge, which, in turn, raises the source/draincurrent.

II. FABRICATION PROCESS

The fabrication steps are shown in Fig. 1. pH-meters werefabricated using N-MOS transistors on p-type 〈100〉-orientedsilicon wafers with channel length and width of 50 and 400 μm,respectively. A thermally grown silicon dioxide with a thicknessof 120 nm is used as the gate dielectric material. A 1.5-μm-thick polysilicon layer is used for the gate of the transistor, andit is achieved using a low-pressure chemical-vapor-deposition(LPCVD) method. The source and drain regions were definedusing a diffusion process at a temperature of 950 ◦C. Finally, atrilayer of SiO2, silicon-oxy-nitride, and SiO2 with a total thick-ness of 1.3 μm passivates the whole structure. This depositionstep is done using an RF plasma-enhanced CVD unit at 300 ◦Cand 100-W RF power. The passivation layer on the gate regionis etched away using buffered HF to expose the polysilicon film.

The key feature of this transistor is the formation ofnanoporous structures right on the gate region, which is accom-plished using reactive ion etching. To convert the polysiliconfilm into a nanoporous structure, we used a sequential reactiveion etching process with two subcycles. In this method, a mix-ture of H2/O2 gases with a trace value of SF6 is used during thefirst subcycle (passivation), while the second subcycle (etching)

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ZEHFROOSH et al.: HIGH-SENSITIVITY ISFETs USING NANOPOROUS SILICON 1057

Fig. 2. (a) and (b) SEM images of nanoporous structure at different mag-nifications, evidencing a porous structure where the pores are on the order of50 nm. (c) and (d) Cross-sectional views of the nanoporous surfaces, evidencingthe porosity down to the gate oxide. The arrow points at the bottom of thenanoporous silicon layer on oxide layer.

step is achieved using mere SF6 as the inlet gas. Details aboutthis process are found elsewhere [10]. A three-step processhelps to convert the polysilicon film into desired nanostructures.In the first step, a mixture of H2 and SF6 gases with flows of100 and 10 sccm are used for 90 s, while the plasma poweris set at 250 W. The next subcycle uses O2 with a flow of100 sccm, a power of 200 W, and a duration of 10 s, while inthe last subcycle, 100 sccm of SF6 with a power of 130 W anda time of 7 s is used. Depending on the thickness of the polysil-icon layer, these steps are repeated as many times as neededto ensure that the whole layer converts into a porous film. TheSiO2 underneath the polysilicon gate acts as an etch stop in thisprocess. For 1.5-μm-thick polysilicon, ten steps are needed toachieve the desired nanoporous structure.

III. RESULTS AND DISCUSSIONS

Fig. 2 shows the several SEM images of the nanoporouspolysilicon films on an oxide layer at different magnifications,indicating the formation of porous features, which have reachedthe bottom of the polysilicon layer. Parts (a) and (b) of thisfigure confirm the presence of a highly porous layer withpores on the order of 50 nm. The cross-sectional views ofthe nanoporous structure are also shown in parts (c) and (d).These parts depict how the pores extend down to the gate oxide.Interface with the underlying oxide is shown with an arrow. Theelectrical characteristics and the response of such transistors tovarious ion concentrations have been obtained using a Keithley2361 parameter analyzer unit. For this experiment, the solutionsunder test are the standard buffer solutions and are biased viaa Ag/AgCl reference electrode with its voltage set at 6 V.A typical result of the output characteristics (Id–Vds) of ISFETpH meters with nanoporous polysilicon gate is shown in Fig. 3,where the pH values are 4, 7, and 9, evidencing a well-behaving transistor action. It is worth mentioning that, in all

Fig. 3. Output characteristics of nanoporous structure, for three pH val-ues with an electrode reference voltage of 6 V, evidencing a well-behavingtransistor.

Fig. 4. Result of the measurement of porous pH sensor with polysiliconthicknesses about 1.5 μm. The slope variation evidencing a significant increasein the transistor current as a result of high hydrogen concentrations.

measurements, the surface of the pH meter was cleaned by DIwater after each test.

The transfer characteristic of the device is obtained byplotting the source/drain current with respect to the referenceelectrode voltage at various pH values and shown in Fig. 4.The inset in this figure shows the setup for the electricalmeasurement of the nanoporous devices. For these tests, thedrain voltage is constant, while the gate voltage is swept. If aconstant current source of 40 μA is applied onto the drain side,a remarkable shift in the voltage on the order of 300 mV/pH isobserved. This is shown with a straight line drawn in this figure.

The results presented in this letter differ from the reportson regular planner pH meters with a gate of SiO2 layer [11].The pH sensitivity of regular ISFETs is shown as the variationin the transistor threshold voltage. Whereas in our fabricatedpH meter with porous gate, not only a slight threshold voltageshift is detected but also a more remarkable slope variationis observed in the electrical drain-voltage characteristics. Thisslope variation causes a high sensitivity which depends on thebiasing current of the transistor. Fig. 5 shows the variation inthe sensitivity with respect to the biasing current of nanoporousISFETs.

1058 IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 9, SEPTEMBER 2010

Fig. 5. Constant current sensitivity of fabricated ISFET under different cur-rent bias conditions. As a result of changes in the slope of ID–VGS curves,higher current biases lead to higher sensitivities. The top view shows an opticalimage of the fabricated devices, where various regions are shown by arrows.

The rise in the current for higher ion concentrations isbelieved to be due to the high porosity of the polysilicon film,as opposed to regular samples, which could enhance the effectof the accumulated charge on the transistor gate. It is commonto model regular ISFETs as a MOSFET, where its thresholdvoltage varies by the value of pH. We propose a simple modelfor the fabricated ISFETs by including a charge insertion factorto fit with the experimental data. This factor could be a functionof charge, as well as the level of porosity of the layer

ID = 0.5 μ(W/L)(εox/tox)f(Q)(VGS − Vth)2 (1)

where “μ” is the electron mobility; “W ” and “L” are theconventional channel width and length, respectively; “εox” and“tox” are the oxide permittivity and thickness, respectively;“Vth” is the threshold voltage; and “VGS” is the gate–sourcevoltage. The charge insertion factor, represented by f(Q), is adimensionless factor. For a regular ISFET, this insertion factorwill be “unity” and only the threshold voltage changes, whereasfor the nanoporous silicon ISFETs, the value of this parameter

is strongly affected by the concentration of the hydrogen ions(pH) and by the porosity of the layer. The value of this insertionfactor can be much higher than unity for high concentrationsand highly porous films.

IV. CONCLUSION

We have fabricated and tested high-sensitivity ISFETs byadding a nanoporous polysilicon layer on the gate region.Since the gate region has a 3-D structure, the porous structureincreases the effective adsorption surface on the channel area,which, in turn, transmits the ion effect to the underlying insula-tor surface (SiO2). The response of these structures to pH is notlimited to threshold shifts, but the slope of the current–voltagecharacteristics dramatically increases, leading to high values ofsensitivity. The use of such devices in biological applicationsand particularly in the realization of DNA sensors is beingpursued.

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