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Numerical Model of Gate-All-Around MOSFET With Vacuum Gate Dielectric

for Biomolecule Detection

Article in IEEE Electron Device Letters · December 2012

DOI: 10.1109/LED.2012.2216247

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1756 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 12, DECEMBER 2012

Numerical Model of Gate-All-Around MOSFET With

Vacuum Gate Dielectric for Biomolecule Detection
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 letter, a dielectric-modulated GAA MOSFET

with vacuum gate dielectric is proposed for enhanced sensitivity
for label-free detection of neutral and charged biomolecules. We
developed an analytical model to model the response of GAA
MOSFET in the presence of biomolecules. The model is verified
with simulation results of ATLAS-3-D. Results indicate that GAA
MOSFET biosensor with vacuum gate dielectric is able to serve
as a highly sensitive low-power label-free biosensor along with
advantages of robustness, reliability, and CMOS compatibility.
Index Terms—ATLAS-3-D, biosensor, dielectric-modulated Fig. 1. Schematic structure of GAA MOSFET (a) with nanogap cavities
field-effect transistor (FET) (DMFET), gate-all-around (GAA) at source and drain ends and (b) with air gap/vacuum dielectric. Device
MOSFET, vacuum gate dielectric. parameters are the following: Channel length L = 100 nm, length of each
cavity (L1 ) = 20 nm, air gap thickness td = 9 nm, radius of silicon pillar
I. I NTRODUCTION R = 10 nm, source/drain doping ND = 1 × 1026 m−3 , and substrate doping
NA = 1 × 1021 m−3 . (c) Calibration with experimental results.

S ILICON nanowire FET biosensors have recently been

demonstrated experimentally [1] for direct label-free real-
time detection of DNA and proteins with high sensitivity and Fig. 1(b) shows the schematic structure of GAA MOSFET
selectivity. To further increase the sensitivity of the ISFET biosensor with vacuum/air gap dielectric. In simulations, the
biosensor, a concept of dielectric-modulated FET (DMFET) presence of a biomolecule is considered by replacing the air
[2], [3] with nanogap cavity at source and drain ends was pro- gap with a dielectric material having k > 1, and the value
posed. The streptavidin binding changes the dielectric constant of k depends upon the type of biomolecule (i.e., k = 2.1 for
(and capacitance) of the gate, resulting in a large shift in the biotin–streptavidin [2], k = 5 for low-hydrated protein powders
threshold voltage and current of the device. In this letter, a new [6], and k = 1–64 for DNA [3]). In particular, when charged
damage-immune GAA MOSFET biosensor with vacuum gate biomolecules such as DNA are introduced, the DMFET oper-
dielectric is proposed for the first time for enhanced sensitivity ation can be changed by both the dielectric constant and the
of label-free detection of neutral and charged biomolecules. strength of the charges in the gate dielectric layer. A single
FET with vacuum gate dielectric has been recently proposed strand of DNA which is nonhybridized possesses both the
in [4] where a vacuum gate dielectric is formed by a sacrifi- dielectric constant and charge. Thus, the effect of a charged
cial layer deposition and removal process and is found to be biomolecule is emulated in simulation by introducing fixed
resistant to radiation and stress damage [4]. In this letter, an oxide charges in the dielectric layer. According to Kim et al.
analytical model is developed for dielectric-modulated GAA [7], a comparative study was carried out in watery and dry
MOSFET biosensor with nanogap cavities at source and drain environments, where it was shown that dry environment has
ends, and the analytical results are validated by the simulated inherent advantage of overcoming limited sensitivity due to
results using ATLAS device simulator [5]. Impact of the radius high ion concentration of the fluidic solutions; therefore, in
of the Si body and vacuum dielectric thickness on the sensitivity this work, simulation has been carried out for DMFET-based
is also investigated. biosensor under dry environment [2], [3], [7]. Calibration of
model parameters used in simulation has been performed ac-
II. S IMULATION A PPROACH cording to the experimental results [8]. Since the radius of Si
pillar is greater than 5 nm, thus, quantum effects are not taken
Fig. 1(a) shows the schematic structure of GAA MOSFET into account [9]. Closed proximity of simulated results with the
biosensor with nanogap cavities at the source and drain ends. experimental results as shown in Fig. 1(c) validates the choice
of parameters taken in modeling and simulation.
Manuscript received March 20, 2012; revised August 25, 2012; accepted
August 26, 2012. Date of publication September 19, 2012; date of current
version November 22, 2012. This work was supported in part by UGC, III. A NALYTICAL M ODEL F ORMULATION
Government of India, and in part by DRDO, Government of India. The review
of this letter was arranged by Editor E. A. Gutierrez-D.
To derive potential distribution in the Si film, channel region
R. Gautam and M. Gupta are with the Department of Electronic Sci- is divided into three regions. Regions 1 and 3 correspond to
ence, University of Delhi, New Delhi 110021, India (e-mail: mridula@south. nanogap cavity, and region 2 corresponds to region without
du.ac.in). cavity. Assuming parabolic profile in the radial direction and
M. Saxena is with the Department of Electronics and the DDU College,
University of Delhi, New Delhi 110021, India.
applying appropriate potential and electric field boundary con-
R. S. Gupta is with the Department of Electronics and Communication ditions, surface potential is expressed as [10]
Engineering, MAIT, New Delhi 10086, India.
Digital Object Identifier 10.1109/LED.2012.2216247 φs,i (z) = Ai eki z + Bi e−ki z + φi (1)

0741-3106/$31.00 © 2012 IEEE

GAUTAM et al.: MODEL OF GAA MOSFET WITH VACUUM GATE DIELECTRIC FOR BIOMOLECULE DETECTION 1757

Fig. 2. Surface potential as a function of position along the channel (a) for Fig. 3. Ids versus Vgs in the presence of biomolecules (a) for GAA MOSFET
GAA MOSFET with nanogap cavities and (b) for GAA MOSFET with vacuum with cavities and (b) for GAA MOSFET with vacuum dielectric. Vds = 0.05 V.
gate dielectric at Vgs = 0 V and Vds = 0 V. (Line) Analytical and (symbol) simulated.

where i = 1 for 0 ≤ z ≤ L1 , i = 2 for L1 ≤ z ≤ L1 + L2 , and

i = 3 for L1 + L2 ≤ z ≤ L. ki is given by
 
ki2 = (2εdi )/ εsi R2 ln(1 + td /R) (2)
and Φi is given by
Φi = Vgs − Vfb − qNA /εsi ki2 + qNf /Cdi . (3)
Vgs is gate-to-source voltage, Vfb is flatband voltage, and Cdi
is capacitance per unit area of the gate dielectric of the GAA
represented by εdi /((R) ln(1 + td /R)). Here εdi is the relative
permittivity of the gate dielectric layer. εd1 = εd3 = 1 and
εd2 = 3.9, whereas εdi = 1 in all the regions for vacuum di-
electric. tsi is the Si film thickness, R is the Si pillar radius, and
td is the gate dielectric layer thickness. Nf is the interface fixed-
charge density for charged biomolecules. Coefficients A and B
are calculated using continuity equations of electric potential
and field at the interface. Complete 2-D potential is given by
φi (r, z) = φs,i (z)+Cdi (Vgs −Vfb −φs,i (z)) (r2 −R2 )/2εsi R. (4)
Subthreshold current is given by
Vd
2πRμqni e−qV (z)/kT dV (z)
Vs
Isub =   . (5)
L R
dz/ eqφi (r,z)/kT dr
0 0

IV. R ESULT AND D ISCUSSION

For GAA MOSFET (L = 100 nm) with vacuum dielec-
tric (without high-k filling, i.e., εd = 1), threshold voltage is
low due to weaker gate control; however, when the dielec-
tric is changed from 1 to 5 or 10 (i.e., in the presence of Fig. 4. (a) Impact of channel length on sensitivity for R = 25 nm. (Line)
biomolecules), gate control is enhanced enormously and short- Analytical and (symbol) simulated. () εd = 1, (x) εd = 2, (◦) εd = 5, and
channel effects are also lowered, thus leading to an increase (Δ) εd = 10. (b) Impact of channel radius on sensitivity for L = 100 nm. (c)
in threshold voltage and a decrease in subthreshold leakage Comparison between different architectures for change in Ioff . (d) Change in
Ion . (e) Change in Ioff for nonuniform distribution. (f) Type of profile: Nf 1 =
current. The analytical results for GAA DMFET with L = −6 × 1011 cm−2 , Nf 2 = −1 × 1011 cm−2 , and Nf 3 = −1 × 1010 cm−2 .
100 nm are in close proximity of the simulated results which
are also in accordance with previously reported work [11]. This region in the absence of Fermi level pinning. Furthermore,
type of behavior is also demonstrated experimentally in [12] for the impact of dielectric change on subthreshold characteristics
multichannel MOSFET where HfO2 -based device has greater is less visible for long-channel GAA MOSFET; however, for
threshold voltage in comparison to SiO2 dielectric. In the case short-channel GAA MOSFET, the effect of dielectric change
of charged biomolecules, threshold voltage is further increased, is much more pronounced as shown in Fig. 4(a). Fig. 4(b)
and subthreshold current decreases due to increase in flatband shows that sensitivity is higher for thicker silicon body (i.e.,
voltage because of negative interfacial charges as illustrated higher radius) because when dielectric constant is changed for
in [11]. When biomolecules are present in the nanogap cavity higher values of radius, the effect is translated in greater change
(vacuum gate dielectric), change in surface potential as shown in off current due to greater change in effective gate control.
in Fig. 2 and change in subthreshold current as shown in Interestingly, this is opposite to the traditional results that a
Fig. 3 are much greater for GAA MOSFET with vacuum smaller radius shows enhanced sensitivity due to an increase
gate dielectric as compared to GAA MOSFET with cavity. in surface–volume ratio. The advantage of large surface-to-
Recently, [13] reported 500 times improvement in protein de- volume ratio applies to surface detection sensors [1].
tection limit by operating NW FET in subthreshold regime. In GAA DMFET, larger radius leads to greater change in
This can be attributed to the additional band bending due to Ioff which is in accordance with [14]. Also, in the case of
charges possessed by the charged biomolecules in subthreshold charged biomolecules, change in band bending is equal to
1758 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 12, DECEMBER 2012

qNf /Cdi , and for larger radius (R), Cdi is small which means formity has been considered by choosing different dielectric
greater change in flatband voltage and current due to fixed thicknesses and charge densities along the channel as shown
interface charges and, thus, greater sensitivity. Fig. 4(c) and in Fig. 4(f)(i) when biomolecules are attached to the gate
(d) shows the comparison between bulk DMFET with cavity, electrode and Fig. 4(f)(ii) when biomolecules are attached to
bulk DMFET with vacuum gate dielectric, GAA DMFET with the channel. As can be seen from Fig. 4(e) when nonuniformity
cavity, and GAA DMFET with vacuum gate dielectric. All is considered in radial and longitudinal directions, sensitivity is
devices are optimized for the same threshold voltage (i.e., reduced; however, GAA MOSFET with vacuum dielectric still
Vth = 0.38 V). As can be seen from Fig. 4(c), GAA MOSFET has the highest sensitivity among all the architectures and, thus,
with vacuum gate dielectric shows maximum change in Ioff is the most suitable architecture for biosensing applications.
because of large surface area occupied by the biomolecules.
Furthermore, the change in surface potential (ΔΦs ) for Vgs =
0 V, εd = 5, and Nf = −1 × 1011 cm−2 is 10 (20 mV) and V. C ONCLUSION
55 mV (215 mV) for DMFET with cavity and DMFET with GAA MOSFET with vacuum dielectric has shown high
vacuum gate for bulk MOSFET (GAA MOSFET), respec- sensitivity toward detection of biomolecules (both neutral and
tively. The smaller (larger) difference in Ioff for bulk (GAA) charged) in subthreshold region, and the sensitivity of the
MOSFET as one moves from cavity to vacuum architecture device can be further increased by increasing the radius of the
[see Fig. 4(c)] is attributed to the smaller (larger) value of silicon nanowire. Effective gate control and ideal subthreshold
ΔΦs,vacuum /ΔΦs,cavity , i.e., 5.5 (10.7) times, due to smaller characteristics make GAA MOSFET with vacuum gate dielec-
(larger) area occupied by the charged biomolecules. Fig. 4(d) tric a promising candidate for ultrasensitive, small, low-power,
shows that change in on current (Ion ) due to biomolecules robust, damage-immune, and reliable CMOS biosensor.
is very small for all devices as compared to change in off
current [Fig. 4(c)] which further decreases in the case of
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