International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Characterization of Optical MISFET
Gayatri Phade#1, Dr B K Mishra*2
#1 PhD Scholor,
SNDT University Mumbai, India,
*2
Principal,
Thakur College of Engineering and Technologycollege, Mumbai, India
Resent research shows the tremendous
potential for the development of optical devices viz.
photodetector, optical sources, connectors and
applications etc. This is mainly because of the success of
optical communication in the recent for gigabit
transmission and is intended for terabits transmission in
future. It needs the parallel advancement in
optodetection with higher efficiency and reliability. In
this paper, mathematical model for the optical
dependence of I-V, C-V characteristics of MISFET
structure (to be used as photodetector) is reported.
Model is based on solution of current continuity
equation. Proposed structure of MISFET includes,
In0.53Ga0.47As used as substrate material and InP as
insulator. Light is made to incident perpendicular to the
surface. Drain current and gate capacitance can be
controlled optically by means of varying light intensity
of incident radiations. There is significant effect of
intensity modulation on IV and CV characteristics of
MISFET. To control these characteristics optically,
optical power is varied from 0.25mW to 25mW. As a
result of intensity modulation, drain current and
transconductance increases significantly in presence of
illumination mainly due to change in carrier
concentration of channel results from photogenerated
carriers. Simulation of mathematical model is carried
out in MATLAB
Abstract—
operation. Schottky contact gate structure of GaAs
MESFET produces large gate leakage current, when used on
InP and InGaAs. MIS structures offers very low leakage
currents and reduces gate capacitance. Ternary
semiconductor compound InxGa1-x,As is a promising
material for advanced optoelectronic devices. In 0.53Ga0.47As
is a good substrate for such circuits. In0.53Ga0.47As lattice
matched to semi-insulating InP has higher low field
mobility, high drift mobility and peak electron velocity[3,4].
Therefore, InGaAs MISFETs have potential for better
microwave performance.
Primary goal of the model is to provide necessary
information related to optical interaction in the device to
reader. Basic MISFET model is based on charge control and
change in charge concentration, due to effect of
illumination, is incorporated in the model. Optical
modelling
gives
focus
on
minority
carrier
concentration[7,8].
I. MODELLING OF OPTICAL INTERACTION IN
MISFET
Figure 1 shows device under consideration. Under
illumination, due to optical absorption of incident
radiations, photo generation of carriers takes place.
Keywords— MISFET, modelling, Optical
INTRODUCTION
Insensitivity to the electromagnetic noise is the advantages
of the classical optics. Miniaturization, better reliability and
low cost are some of the advantages of integration.
Optoelectronic and photonic integrated circuits brings both
the advantages of classical optics and integration. Field
effect transistors (FET) devices are sensitive to light, having
high package density and are suitable for microwave
applications. Lots of efforts are carried out in the
development of microwave transistors from III-V compound
semiconductor material systems[1]. Presently the GaAs
Schottky-gate FET (MESFET) is the only such device
commercially available. Irrespective of remarkable
microwave gain and noise performance, this device suffers
from the limitation of a restricted range of enhancement
ISSN: 2231-5381
Figure 1. Schematic Diagram of MISFET Structure under Illumination
Photo generation rate with space per unit volume is given
by, G= ∅ ∝
------1
where, ∅= photon flux density per unit area
∝ = photon absorption coefficient per unit length
y = direction perpendicular to the surface.
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
The optically generated carriers are generated by the current
continuity equation( , )
( , )
=
+
( , )
−
−
-----2a
for electrons and for holes,
( , )
( , )
=
+
−
( , )
−
-----2b
where
is surface recombination velocities for
electrons and holes respectively and ,
is the minority
carrier life time of electron and hole. The current density
equation consists of both drift and diffusion current and are
given by( , )
( , )=
( , )+
----- 3a
( , )=
( , )
( , )+
----- 3b
where
- Drift velocity in y direction, which is constant
and independent of the field.
and
are diffusion
coefficient of electron and hole respectively.
Incident flux density modulated by the signal frequency `ω’,
=
+
------ 4a
= +
------ 4b
= +
------ 4c
Substituting eq. 4a in eq. 1, carrier generation becomes
frequency dependent and is computed as,
)
= ( + ------ 5
Carrier generation depends on flux density of incident
radiation and frequency of the signal. In depletion region,
the carriers flows due to the drift and recombination. In
channel, the carriers moves due to the diffusion and
recombination, hence electron moves to channel and holes
towards substrate[9,10].
2.1 Calculation of photo voltage
Calculation in the section is based on the structure shown in
figure 1.
Putting 4b in 3b we get,
( , )=
+
+
- 2.1.1 which
results into
( , )=
+
+
.
- 2.1.2
Substituting 2.2.2 and 5 in eqn 2b results into,
( , )
=
−
.
−−
+
+
- 2.1.3
eqn 2.1.3 results into first order differential equation for dc
and ac hole concentration. Where ‘0’ indicates the dc value
and ‘1’ indicates ac value. Term Rs in eqn 2.1.3 is given by
[9]
=
0
+
1
The boundary condition for evaluating the constant is given
as, at = , = +
here
is
width of gate depletion region and
- minority carrier
concentration at dark
i.e. dc condition and
=
i.e. ac condition.
ISSN: 2231-5381
Solving equation 2.1.3 under ac and dc condition gives hole
density as ( )=
−
+
-- 2.1.4
is life time of minority carries under ac condition.
Sidewalls of depletion region are assumed to be quarter
arcs. Let
be the radius of arc at source and drain
side respectively, therefore
=
( ) = 0
),
=
---- 2.1.5
( ) = (
( ) is the channel potential at that point.
The number of hole crossing the junction at y = 0 is given
by
(0) = (
)
+
---- 2.1.6
Where
=
−
=
−
---- 2.1.7
Generated optical voltage [8, 9] is given as =
( )
From equation ,
---- 2.1.8
( )
=
---- 2.1.9
From 2.1.5, equation 2.1.8 becomes
.
=
---- 2.1.10
Optical voltage is a function of flux density of incident
radiations , frequency of incident radiations and absorption
coefficient.
3. CHARGE BASED MODEL FOR I-V
CHARACTERISTICS OF MISFET
To study the basic characteristics of the device, charge
control model is used which is described as follows.
Optically generated voltage increases the potential across
the insulator. Let
is the potential across the insulator
considering optical effect is given by,
=
and =
----3.1
−Ψ
where =(
+
)
Total induced charge in the semiconductor per unit area
under illumination is given by=( . )=
----- 3.2
– Ψ
,
where, Ψ is the surface potential
(∈
∗∈ )
=
, the gate capacitance per unit area
∈
∈
: Permittivity of the air
: Permittivity of the insulator material, InP
: Thickness of insulator
The charge within the surface depletion region is given by
=− 2
∈ (Ψ )
----- 3.3
Total charge in the inversion layer is given by:
( ) = −( ( ) +
( ))
----- 3.4
From equation 3.2 and 3.3, equation 3.4 becomes
( ) − 2Ψ
(x) = −
+ 2
∈ Ψs ----- 3.5
−
Let
be the channel resistance of an elemental section
‘dx’ is given as
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
=
′
Voltage drop across this elemental section is given by
=
=
---- 3.7
′
(x)
Drain current at elemental section ‘dx’ is given by | ′( )|
=
---- 3.8
Substitute equation 3.5 in equation 3.8 and integrating from
the source (x = 0, V= 0) to the drain (x = L, V = )[5,6],
we get, ′ ∫
=
+ 2
∈ (Ψs ) -∫
– Ψs
-- 3.9
Solving equation 3.7 with Tailor’s series,
′
=
–
Ψs −
−
+
∈
/
--3.10
For smaller values of drain voltages, drain current is given
as
′=
---- 3.11
−
where is the threshold voltage.
The drain voltage when increased to a point, such that the
charge in the inversion layer Q(x) at x = L becomes zero,
this point is called as pinch-off point. The drain voltage and
drain current at this point designated as
and
respectively. Beyond the pinch-off point it is saturation
region.
(
)
=
– 2Ψ
(
)
+K
∈ (
where K =
1− 1+
)
4.
2
---3.12
K
−
CAPACITANCE MODELLING
=
Value
0.8 x10-6
W
20x10
-6
Unit
Description
m
Channel length
m
1−4
−
----- 4.3
Channel width
2
µ
0.800
εi
1.11x10-10
m /V
s
F/m
εs
0.97x10-12
F/m
ti
600
µm
17
Low field mobility
Dielectric constant
of InP
Dielectric constant
of InGaAs
Insulator layer Thickness
m-3
ni
2.47x10
Na
1 x1021
m3
Intrinsic
doping
concentration
Channel doping level
T
300
K
Temperature
Vt
0.5
V
Threshold Voltage
Vgs
1
V
Gate Voltage
Vds
1.5
V
Drain Voltage
TABLE 2
Optical Parameters
--- 3.13
In linear region, gate to source and gate to drain capacitance
are gibe by,
=
+ 2/3
---- 4.1
=
----- 4.2
where,
Sym
bol
L
and
∈
=
TABLE 1
Design Parameters
---- 3.6
(x)
Symbol
α
λ
Value
106
1.65
Unit
/cm
µm
P
Ø
0.25
1015
mW
Wb/m2
Description
Absorption coefficient
Wavelength of incident
radiations
Optical power
Flux density
f
1
GHz
Operating frequency
In saturation region, gate capacitance are given as,
2
x 10
-3
Id- Vd C hara cte ri s tics of a MISFET
Vgs = 0.8 V
=
( .
+ 2/3.
+ .
5.
)
( .
(
)
)( .
( .
)
)
---- 4.4
---- 4.5
RESULTS AND DISCUSSION
1. 5
D r ai n Id re n t,A
=
1
Vgs = 0.7 V
0. 5
Vgs = 0.6 V
Design Parameters of the device are given in Table 1.
Table 2 gives optical parameters. Under dark condition
Drain current vs. Drain Voltage characteristics is plotted
with charge control model as shown in figure 2. With
increase in gate voltage there is increase in Drain current.
Under illumination there is increase in charge concentration
in the inversion layer which increase drain current as
compared to dark current.
ISSN: 2231-5381
0
0
0.2
0.4
0. 6
0. 8
1
Vds , V
Figure 2. Drain current Vs drain voltage plotted with charge control model.
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
2.5
x 10
-3
-4
x 10 photo current - Drain Voltage Characteristics of a MISFET
I- V Ch ar ac teris tic s o f a MISFET w ith v op
6
Vgs-0.2
Vgs-0.3
5
2
Vgs-0.4
Vgs-0.5
Vgs-0.6
P hoto Current, A
Dr ain Cur r ent, A
Vgs = 0.8 V
1.5
1
Vgs = 0.7 V
0.5
4
Vgs-0.7
Vgs-0.8
3
2
1
Vgs = 0.6 V
0
0
0.2
0.4
0.6
0.8
0
1
0
0.2
0.4
Vd s, V
0.6
0.8
1
Vds, V
Figure 5. Photo Current Versus Vds with increasing gate bias.
Figure 3. Effect of incident radiation on drain current of MISFET with
varying Vgs.Optical power 0.25mW; λ = 1.65 μm
-3
4
x 10
6
I-V Characteristics of a MISFET with vop
Photo C urrent,A
3
Drain Current,A
-4
Photo current Vs Gate Bias of a MISFET
5
3.5
2.5
4
3
2
2
1.5
1
Dark
Pop1
Pop2
Pop3
1
0.5
0
x 10
0
0.2
0.4
0.6
0.8
0
0.2
0.3
0.4
0.5
Vgs V
0.6
0.7
0.8
Figure 6. Photo current Vs Gate Bias of MISFET
1
Vds, V
Transconductance Vs Gate Voltage for MISFET
Figure 4. I-V characteristics of InGaAs MISFETwith increasing optical
power. Pop1 = 0.25mW; Pop2 = 2.5mW; Pop3 = 25mW
0.07
dark
Pop1
0.06
Figure 4 shows effect of intensity modulation on the drain
current at Vgs = 0.8 V where optical power is varied from
0.25 mW to 25 mW. Increase in optical power increases
magnitude of drain current.
Pop2
Transconductance
Figure 3 shows increase in drain current due to incidence
of optical power of 0.25 mW which is the interpretation of
equation 3.9 and 3.11.
Pop3
0.05
0.04
0.03
0.02
0.01
0
0
0.5
Generated photo current, due to optical illumination,
increases with increase in gate bias as shown in figure 5.
Increase in drain current due to illumination increases
transconductance of MISFET as shown in figure 7.
Figure 8 gives gate capacitance versus gate voltage
characteristics for varying drain voltage under dark
condition. For smaller values of gate voltage there is no
effect of drain voltage on gate capacitances. For greater
values of gate voltages, Cgd (gate to drain capacitance)
decreases and Cgs (gate to source capacitance) increases.
ISSN: 2231-5381
1.5
2
Figure 7. Transconductance of MISFET
Gate Capacitances vs Vgs Wth varying Vds
1
Normalized Gate Capacitances C/Cix
Figure 6 shows photo current versus gate bias
characteristics which increases exponentially after threshold
voltage.
1
Vgs [V]
0.9
0.8
Cgs
0.7
0.6
0.5
0.4
0.3
Cgd
0.2
0.1
0
0
1
2
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3
4
5
6
7
8
Vgs [V]
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Figure 8. Gate Capacitances Vs Vgs with increase in Drain Voltage
Drain Voltage Vs. Gate Capacitances Under Dark Condition
Normalized Gate Capacitances
0.7
Cgs
0.6
0.5
0.4
0.3
0.2
0.1
Cgd
0
0
0.5
1
1.5
2
Vds, [V]
2.5
3
3.5
4
Figure 9. Gate Capacitances Vs drain voltages with increase in Gate
Voltage
Normalized Gate capacitance CGS, CGD
Normalized Gate Capacitances Vs Gate Voltage for different Optical Power
0.7
0.6
0.5
Cgs
0.4
0.3
0.2
Cgd
0.1
0
0
0.5
1
1.5
Vgs, [V]
2
2.5
3
Figure 10. Gate Capacitances Vs Vgs with increase in optical power
Pop1=0.25mW; Pop2=2.5mW
Table 3
Effect of Different Parameters on Gate Capacitance
Sr.No
1
2
3
Parameter
Drain ( )
Voltage
Gate ( )
Voltage
Optical ( )
Power
Cgs
Increase
Cgd
Decrease
Decrease
Increase
Decrease
Increase
Variations shown in figure (9), (10), are tabulated in table
3 for the increase in the gate and drain voltage along with
the optical power[11,12].
6.
CONCLUSIONS
Drain current and gate capacitance of MISFET can be
controlled optically. Under illumination, inversion level in
the channel increases which increases drain current.
Transcoductance of the device increases due to increase in
drain current.
Increase in optical power decreases gate to source
capacitance and increases drain to source capacitance.
Current gain is the function of transcondutance and gate to
ISSN: 2231-5381
source capacitance. Current gain of the MISFET can be
increased due to optical effect.
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