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Nov 6, 2013 - Abstract—In this paper, gate-all-around (GAA) MOSFET with catalytic metal gate is proposed for enhanced sensitivity of gas sen- sor.

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 6, NOVEMBER 2013

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Gate-All-Around Nanowire MOSFET With Catalytic Metal Gate For Gas Sensing Applications Rajni Gautam, Member, IEEE, Manoj Saxena, Senior Member, IEEE, R. S. Gupta, Life Senior Member, IEEE, and Mridula Gupta, Senior Member, IEEE

Abstract—In this paper, gate-all-around (GAA) MOSFET with catalytic metal gate is proposed for enhanced sensitivity of gas sensor. P-channel GAA MOSFET with palladium (Pd) metal gate is used for hydrogen sensing and n-channel GAA MOSFET with silver (Ag) metal gate is used for Oxygen sensing. The GAA nanowire MOSFET has already been demonstrated experimentally for biosensing and chemical sensing applications. However, cylindrical GAA MOSFET with catalytic metal gate for gas sensing applications is proposed for the first time in this paper. An analytical model is developed for both p-channel and n-channel GAA MOSFET to calculate the sensitivity of the device in the presence of gas molecules and analytical model is verified with the simulation results of ATLAS-3D. Sensitivity of the GAA MOSFET gas sensor is compared with the conventional bulk MOSFET gas sensor and impact of the radius of the silicon pillar on the sensitivity of the GAA MOSFET is also studied. Index Terms—ATLAS-3D, gas sensor, gate-all-around (GAA) MOSFET, sensitivity, work function.

I. INTRODUCTION O meet ever increasing demand of gas detection sensors for environmental monitoring, automotive and medical industries, MOSFET-based gas sensors [1]–[3] is a good choice as it offers low cost, low power, small size, and high sensitivity along with CMOS compatibility. In recent years, a MOSFET gas sensor has been developed for detection of gas species by measuring the induced shift of work function at the surface of a sensitive film. Research and development in this field is undergoing in two directions: 1) exploration of suitable sensitive films such as catalytic metals [1]–[7], metal compounds [8]–[15], hydrated salts [16], [17], polymers, and other organic compounds [18], [19]; and 2) device engineering including design and optimization of the field-effect device to enhance the sensitivity. In the area of device engineering, floating gate MOSFET [20], SOI MOSFET [21], dual-gate MOSFET [3], and now nanowire MOSFET [14], [15] devices have been taken into con-

T

Manuscript received March 17, 2013; revised July 21, 2013; accepted July 29, 2013. Date of publication August 2, 2013; date of current version November 6, 2013. The review of this paper was arranged by Associate Editor A. Bhalerao. R. Gautam and M. Gupta are with the Semiconductor Device Research Laboratory, Department of Electronic Science, University of Delhi, South Campus, New Delhi 110021, India (e-mail: [email protected]). M. Saxena is with the Department of Electronics, Deen Dayal Upadhyaya College, University of Delhi, New Delhi 110015, India. R. S. Gupta is with the Department of Electronics and Communication Engineering, Maharaja Agrasen Institute of Technology, New Delhi 110086, India. 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/TNANO.2013.2276394

sideration. To increase the sensitivity by increasing the opportunities for surface reactions, a high ratio of surface area to volume is needed. High surface-to-volume ratio, low leakage current, better gate control, and nearly ideal subthreshold characteristics [22], [23] make gate-all-around (GAA) nanowire MOSFET promising device architecture for developing a low-power highly sensitive, and nanoscale CMOS-compatible gas sensor. Gas sensors based on metal oxide NWs such as ZnO, In2 O3 , and SnO2 field-effect transistors (FETs) are already demonstrated [13]–[15]. Another class of FET-based gas sensors using catalytic metal gate is reported earlier [1]–[3]. Reaction of gas molecules at the catalytic metal gate surface results in change in the work function of the gate metal which further causes change in flat-band voltage, threshold voltage, and drain current of the MOSFET device. This change in conductivity is directly related to the amount of a specific gas present in the environment, resulting in a quantitative determination of gas presence and concentration. Recently, an FET-based oxygen sensor using Ag catalytic metal gate has been reported in [2]. Dual-gate FET hydrogen gas sensor using Pt metal gate was studied by Tsukada et al. [3]. However, cylindrical GAA MOSFET with catalytic metal gate has not been explored for gas sensing applications. In this paper, a GAA p-channel MOSFET with catalytic metal gate is proposed for the first time for improving the sensitivity applications. Conventionally, FET-based gas sensors use threshold voltage as the sensitivity parameter; however, in this study, subthreshold current instead of threshold voltage is used to calculate sensitivity of the gas sensor, which enables low-power operation along with high sensitivity. A much higher sensitivity for gas detection is observed in this study when the device is operated in the subthreshold regime. This type of behavior was also reported by Gao et al. [24] for nanowire biosensors. They have reported 500 times improvement in protein detection limit by operating NW FET in the subthreshold regime. This high sensitivity in the subthreshold regime is attributed to the additional band bending taking place in absence of Fermi level pinning due to change in metal semiconductor work function after surface reaction. Fabrication of GAA MOSFET has already been demonstrated experimentally [25], [26] and its device physics is explained through an analytical model [27], [28]. In this study, GAA MOSFET with Ag metal gate for Oxygen sensing [5] is modeled and validated by the simulated results using the ATLAS device simulator [29]. The impact of the radius of the silicon body on the sensitivity of the gas sensor is also investigated. The sensitivity of the GAA MOSFET is compared with the Bulk MOSFET which is a conventionally used architecture for the FET-based gas sensor.

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Fig. 2. Validation of simulation results with experimental results [25] for a cylindrical GAA MOSFET. L = 60 nm, R = 5 nm, and tox = 3.5 nm.

Fig. 1. Schematic structure of GAA MOSFET with air gap dielectric. Device parameters: channel length (L) = 50 nm, radius (R) = 5 nm, oxide thickness (tox ) = 2 nm, and channel doping (N A ) = 1 × 102 1 m−3 . (b) Simulated structure of GAA MOSFET. TABLE I LIST OF TECHNOLOGY PARAMETERS

are not taken into account [28]. Various models used in simulation are as follows: drift diffusion, concentration-dependent mobility, field-dependent mobility, and Shockley–Read–Hall recombination model. Closed proximity of analytical and simulated results with the experimental results [25] as shown in Fig. 2 validates the choice of parameters taken in modeling and simulation. Two catalytic metal gates (i.e., Pd for hydrogen sensor and Ag for oxygen sensor) are used. Chemical reaction of gas molecules at the surface of the catalytic metal gate changes the work function of the metal. Thus, gas sensitivity of the device is manifested as a change of the threshold voltage and drain current. This work is valid at room temperature only as work function principle of gas sensing is dedicated for room temperature or slightly elevated temperatures of operation as suggested by Eisele et al. [30]. III. ANALYTICAL MODEL FORMULATION Assuming parabolic profile in the radial direction and applying appropriate potential and electric field boundary conditions, surface potential can be expressed as φs (z) = Aek z + Be−k z + Φ φs (z) = −Ae

kz

II. SIMULATION AND CALIBRATION Fig. 1(a) shows the schematic structure and Fig. 1(b) shows the simulated structure of GAA MOSFET gas sensor with catalytic metal gate. N-channel GAA MOSFET with Ag metal gate is used for oxygen detection and p-channel GAA MOSFET with Pd gate is used for hydrogen sensing. Ag with p-channel and Pd with n-channel can also be used but it results in very low off currents (i.e., in the range of femtoamperes) which are under measurable limits and also vulnerable to noise. Also, sensing circuitry for such low currents adds to the cost of the sensor which is a major concern. Table I shows all the technology parameters used in the simulation and model. Calibration of model parameters used in the simulation has been performed according to the experimental results [25]. Since radius of silicon pillar is greater than 5 nm, thus quantum effects

−k z

− Be

for an n-channel

−Φ

for a p-channel

(1)

where k is given by k 2 = 2εox /(εsi R2 ln(1 + tox /R))

(2)

and Φ is given by Φ = Vgs − Vfb − qNsi /εsi k 2 for n-channel Φ = Vfb − Vgs − qNsi /εsi k 2 for p-channel.

(3)

Boundary conditions of electric potential and field are given as follows. 1) The center potential is a function of z only: φ (r = 0, z)

=

φc (z).

(4)

2) The electric field at the center of silicon film is zero:  dφ (r, z)  = 0. (5) dr  r =0

GAUTAM et al.: GATE-ALL-AROUND NANOWIRE MOSFET WITH CATALYTIC METAL GATE FOR GAS SENSING APPLICATIONS

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3) The electric field at the silicon oxide interface is given by     Cox tsi dφ (r, z)  t si = , z − V − φ r = V gs fb dr  r = 2 εsi 2  dφ (r, z)  dr 

r=

t si 2

=

Cox εsi

for n-channel     tsi φ r = , z − Vgs + Vfb 2 for p-channel.

(6)

Here, Cox is the oxide capacitance per unit area of the GAA MOSFET represented by Cox = εox / ((R/2) ln(1 + tox /R)) .

(7)

tsi is the silicon film thickness, R is the silicon pillar radius, and tox is the gate dielectric layer thickness. εsi is the permittivity of the silicon and εox is the permittivity of the oxide layer. Vgs is the gate-to-source voltage and Vfb is the flat-band voltage. ΔΦm is the change in the catalytic metal work function induced by the gas molecules reaction at the metal surface. Thus, effective Vfb is given by Vfb = φm − φs ± Δφm

(8)

where φs is the silicon work function given by Eg + χ + qφf p for n-channel 2 Eg + χ − qφf n for p-channel φs = 2

φs =

(9)

Δφm depends upon the gate metal and the gas to be detected and is given by   RT Δφm = cont − ln P (10) 4F where F is Faraday’s constant, R is the gas constant, T is the absolute temperature, and P is the gas partial pressure. Since partial pressure depends upon concentration of gas molecules, thus the gas sensors can be calibrated to read out in mole fraction of the gas in air, or units of moles of gas per mole of air. The coefficients A and B are calculated using boundary conditions at the source and drain and are given by   (Vbi + φ) 1 − e−k L + V ds (11) A= 2 sinh (kL)   (Vbi + φ) ek L − 1 − V ds B= . (12) 2 sinh (kL)

Fig. 3. (a) Effect of work function change on surface potential of n-channel GAA MOSFET with Ag metal gate. Device parameters: channel length (L) = 50 nm, radius (R) = 5 nm, oxide thickness (tox ) = 2 nm, channel doping (NA) = 102 1 m−3 . (b) Effect of work function change on surface potential of p-channel GAA MOSFET with Pd metal gate. Device parameters: channel length (L) = 50 nm, radius (R) = 5 nm, oxide thickness (tox ) = 2 nm, channel doping (N A )=1 × 102 1 m−3 .

Based on 2-D potential, subthreshold current is given by  V d −q V (z )/k T e dV (z) Isub = 2πRμq ni Vs L . (14) 0

R 0

dz

eq

φ (r,z )/ k T

dr

Mobility reduction effects are incorporated into the model as follows: μ μeff = (15) 1 − θ (Vgs − Vth ) where θ is a fitting constant whose value used in this study is θ = 0.04.

Complete 2-D potential is given by φ (r, z) = φs (z) +

Cox (Vgs − Vfb − φs (z)) (r2 − R2 ), 2εsi R for n-channel

φ (r, z) = φs (z) +

Cox (φs (z) − Vgs + Vfb ) (r2 − R2 ), 2εsi R for p-channel .

(13)

IV. RESULT AND DISCUSSION The FET is used as a transducer which transforms the shift of work function at the surface of the sensitive film/catalytic metal into a corresponding electrical signal: a change in the drain-source current. Fig. 3(a) and (b) shows the effect of work function change on surface potential of n-channel GAA MOSFET with Ag metal gate and p-channel GAA MOSFET with Pd

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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 6, NOVEMBER 2013

Fig. 5. Id s versus V g s with and without gas molecules for p-channel bulk MOSFET with Pd metal gate. Device parameters: channel length (L) = 50 nm, silicon film thickness (tsi ) = 20 nm, oxide thickness (tox ) = 2 nm, channel doping (N A )=102 1 m−3 , drain-to-source voltage (V d s ) = 0.05 V. TABLE II SENSITIVITY COMPARISON OF p-CHANNEL GAA MOSFET WITH Pd GATE AND BULK MOSFET WITH Pd GATE

Fig. 4. (a) Id s versus V g s with and without gas molecules for n-channel GAA MOSFET with Ag metal gate. Device parameters: channel length (L) = 50 nm, radius (R) = 5 nm, oxide thickness (tox ) = 2 nm, channel doping (N A ) = 102 1 m−3 , drain-to-source voltage (V d s ) = 0.05 V. (b) Id s versus V g s with and without gas molecules for p-channel GAA MOSFET with Pd metal gate. Device parameters: channel length (L) = 50 nm, radius (R) = 5 nm, oxide thickness (tox ) = 2 nm, channel doping (N A ) = 102 1 m−3 , and drain-to-source voltage (V d s ) = 0.05 V.

metal gate, respectively. Reaction of gas molecules at the catalytic metal gate surface results in change in the work function of the gate metal which further causes change in flat-band voltage due to additional band bending. Flat-band voltage change results in shift in the surface potential, threshold voltage Vth , and drain current. Thus, by monitoring the change in Vth , Ioff , and Ion , it is possible to detect the presence of gas molecules, for example, oxygen, hydrogen, ammonia, and other hydrocarbons using suitable catalytic metal gates. Fig. 4(a) and (b) shows the effect of work function change on the drain current for n-channel and p-channel GAA MOSFET respectively. As can be seen, Ioff is changed exponentially when the work function is changed in millivolts and it clearly shows that effect of gas molecules is much higher on Ioff as compared to Ion ; thus, subthreshold region offers much higher sensitivity along with low-power operation, thus providing low-cost gas sensor device. This high sensitivity in the subthreshold regime is attributed to the ad-

ditional band bending in absence of Fermi level pinning due to change in metal semiconductor work function after surface reaction of gas molecules. This type of behavior has also been reported by Gao et al. [24] for nanowire biosensors. 100-meV change in work function of Pd metal gate in p-channel GAA MOSFET results in 43 times change in Ioff . Close proximity of the analytical results with the simulated results for both nchannel and p-channel GAA MOSFETs validates the analytical model. Fig. 5 shows the effect of work function change on drain current characteristics of the bulk MOSFET. The order of change in Ioff is only five times for bulk MOSFET, whereas it is 43 times for GAA MOSFET for 100-meV change in the work function; thus, GAA MOSFET shows much higher sensitivity for gas detection as compared to bulk MOSFET. Table II demonstrates the sensitivity comparison in terms of order of change in Ioff , i.e., ratio of Ioff before and after gas reaction at the gate metal. The effect of work function change

GAUTAM et al.: GATE-ALL-AROUND NANOWIRE MOSFET WITH CATALYTIC METAL GATE FOR GAS SENSING APPLICATIONS

induced by gas molecules on Ioff is compared for two architectures, i.e., bulk MOSFET and GAA MOSFET. Both devices are optimized for the same threshold voltage. Enhanced sensitivity in case of GAA structure is due to surrounding gate structure having higher surface-to-volume ratio which means exposing the channel to a more effective control of gate which appears in greater change in subthreshold current when the work function of the gate metal is changed due to reaction of gas molecules with catalytic metal gate. Table II also illustrates the effect of silicon pillar radius on the sensitivity of the GAA MOSFET gas sensor. For a GAA MOSFET, higher radius gives higher current driving capability and higher gain but thinner radius leads to better subthreshold characteristics. Since in this study, change in subthreshold current is used as the sensitivity parameter for gas sensing, therefore, subthreshold characteristics are of major concern. It can be seen from Table II that sensitivity is enhanced for thinner silicon body. Higher sensitivity in case of thinner silicon body is attributed to the higher surface to volume ratio, enhanced gate control, and low subthreshold leakage current.

[4] [5]

[6] [7] [8] [9]

[10] [11]

V. CONCLUSION GAA MOSFET with catalytic metal gate shows high sensitivity toward detection of gas molecules over conventional bulk MOSFET due to its surrounding gate structure and higher surface-to-volume ratio. N-channel GAA MOSFET with Ag gate is used for oxygen sensing and p-channel GAA MOSFET with Pd gate is used for hydrogen sensing. Change in subthreshold current induced by work function change of gate metal due to reaction of gas molecules at the surface of gate catalytic metal is used as the sensitivity parameter which provides very high sensitivity as compared to the case when sensitivity is calculated in terms of threshold voltage change or change in on current. Operating device in the subthreshold region is also advantageous because it provides low-power, low-cost gas sensor. The sensitivity of the device can be further increased by decreasing the radius of the silicon nanowire. The effective gate control, large surface-to-volume ratio, and better short-channel characteristics of GAA MOSFET make it a promising candidate for ultrasensitive, small, low-power, low-cost, CMOS-based gas sensors. ACKNOWLEDGMENT The authors are grateful to the Defence Research and Development Organization, Government of India. R. Gautam is thankful to the University Grants Commission, Government of India for providing the necessary financial assistance to carry out this research work.

[12] [13] [14] [15] [16] [17] [18] [19] [20]

[21]

[22]

[23]

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Manoj Saxena (SM’08) received the B.Sc.(Hons.), M.Sc., and Ph.D. degrees in electronics from the University of Delhi, New Delhi, India. He is currently an Associate Professor with Deen Dayal Upadhyaya College, University of Delhi, India.

Rajni Gautam (M’10) is currently working toward the Ph.D. degree in the Department of Electronic Science, University of Delhi South Campus, New Delhi, India.

Mridula Gupta (SM’09) received the Ph.D. degree from the University of Delhi, New Delhi, India, in 1998. She is currently an Associate Professor with the Department of Electronic Science, University of Delhi South Campus, New Delhi.

R. S. Gupta (LSM’10) received the Ph.D. degree from Banaras Hindu University, Varanasi, India, in 1970. He is currently a Professor and a Department Head with the Maharaja Agrasen Institute of Technology, New Delhi, India.

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