IEEE MTT-V042-I03 (1994-03)

180 Pages·1994·93.61 MB·English

by  FANTOMASPING

~IEEE
TRAN SACTI 0 NS ON
MICROWAVE THEORY
AND TECHNIQUES
A PUBLICATION OF THE IEEE MICROWAVE THEORY AND TECHNIQUES SOCIETY
MARCH 1994 VOLUME 42 NUMBER 3 IETMAB (ISSN 0018-9480)
PAPERS
Switching Characteristics of an Optically Controlled GaAs-MESFEf .................................................... .
. . . . . . . . . . . . .. . . . . . . . .. . . . . .. . . . . .. . . .. . . . . . . . .. . . P. Chakrabarti, S. K. Shrestha, A. Srivastava, and D. Saxena 365
Improved Analysis Method for Multiport Microstrip Annulat-Ring Power Dividers ............ F. Tefiku and E. Yamashita 376
A Self-Diplexing Quasi-Optical Magic Slot Balanced Mixer .................................. C.-Y. Tong and R. Blundell 383
Microwave Noncontact Examination of Disband and Thickness Variation in Stratified Composite Media .............. .
.. .. . .. . .. .. .. .. .. .. . .. . .. . .. .. . . .. . . .. .. .. .. .. .. .. .. S. Bakhtiari, N. Qaddoumi, S. I. Ganchev, and R. Zoughi 389
Coplanar Transmission Lines on Thin Substrates for High-Speed Low-Loss Propagation ................................ .
. . .. . .. . .. . . . . . .. .. . . .. .. . . .. .. . . .. . . .. .. .. .. .. . . .. .. .. .. . .. .. M. Y. Frankel, R. H. Voelker, and J. N. Hilfiker 396
Physics-Based Expressions for the Nonlinear Capacitances of the MESFET :Equivalent Circuit ......................... .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. D' Agostino and A. Betti-Berutto 403
Nonuniform Transmission Line Codirectional Couplers for Hybrid Mimic and Superconductive Applications ......... .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. Uysal, C. W. Turner, and J. Watkins 407
A Multicomposite, Multilayered Cylindrical Dielectric Resonator for Application in MMIC's ... W. K. Hui and I. Wolff 415
Analysis of a Microstrip Crossover Embedded in a Multilayered Anisotropic and Lossy Media ........................ .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Martel, R. R. Boix, and M. Horno 424
A Finite Element Cavity Resonance Method for Waveguide and Microstrip Line Discontinuity Problems .............. .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. S. Wang and R. Mittra 433
Finite Element Analysis of MMIC Structures and Electronic Packages Using Absorbing Boundary Conditions ........... .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. S. Wang and R. Mittra 441
Space Domain Approach for the Analysis of Printed Circuits ................. M. Kahriz', T. K. Sarkar, Z. A. Maricevic 450
Transient Responses of an Exponential Transmission Line and its Applications to High-Speed Backdriving in In-Circuit
Test ................................................................................... C.-W. Hsue and C. D. Hechtman 458
Scattering from an Infinite Cylinder of Small Radius Embedded Into a Dielectric One .................................. .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. A. Roumeliotis and N. B. Kakogiannos 463
Analysis of the Scattering by Dielectric Bodies Using the SIE Formulation ............. A. Jostingmeier and A. S. Omar 471
Scattering of IB-Polarized EM Wave by Discontinuity in Grounded Dielectric Layer ................................... .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Song, S. Ohnuki, D. P. Nyquist, K.-M. Chen, and E. J. Rothwell 481
Efficient Analysis of Planar Circulators by a New Boundary-Integral Technique..... G. G. Gentili and G. Macchiarella 489
(Continued on back cover)
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994 365
Switching Characteristics of an
Optically Controlled GaAs-MESFET
P. Chakrabarti, S. K. Shrestha, A. Srivastava, and D. Saxena
Abstract-The switching characteristics of an optically con ion-implanted GaAs-MESFET under illumination was first
trolled Metal Semiconductor Field Effect Transistor (MESFET), proposed in [7]. Unfortunately, the model fails to account
popularly known as Optical Field Effect Transistor (OPFET), for a number of important physical phenomena whi~h shape
have been derived analytically. The limitations of the existing
the device characteristics in the illuminated condition. An
model have been overcome in the present model. Calculations
improved de model of the device is very recently proposed
are being carried out to examine the effect of illumination on
the current-voltage characteristics, drain-to-source capacitance [8].
(Cdc), internal gate-to-source capacitance (C8.), drain-to-source The switching characteristics of a GaAs MESFET was first
resistance (Rds), the transconductance (gm), the input RC time
reported in [9]. The model was later extended in [10], for an
constant and the cutoff frequency (h) of a GaAs-MESFET.
optically controlled GaAs-MESFET . The model tlOJ suffers
The variations of these parameters with gate length L and the
doping concentration Nd have also been studied in 8d ark and from a number of major limitations. In their work the authors
illuminated conditions. The results of numerical calculations show appear to have ignored the photovoltage developed across
that there is an overall decrease in the input RC time constant of the Schottky gate of the MESFET under illumination. It is
the device in the illuminated condition arising from the internal
understood that in presence of a high gate bias resistance
gate-to-source capacitance and the transconductance. The results
the photovoltage developed across the Schottky gate is nearly
obtained on the basis of the model show a close agreement with
the reported experimental findings. The simple model presented equal to the open circuit voltage of an illuminated Schottky
here is fairly accurate and can be used as a basic tool for circuit contact and the voltage strongly influences the characteristics
simulation purposes. of the MESFET in the illuminated condition [8]. Furthermore,
in calculating the intemal gate-to-source capacitance in the
I. INTRODUCTION illuminated condition they have considered the excess photo
generated charge in the channel region [ 10) in place of the
I N recent years, considerable interest has been shown in
excess charge in the depletion region below the gate. In their
studying and modelling Optically Controlled Field Effect
work [10), the authors have also ignored the effect of illumina
Transistors (OPFET's) fabricated with Schottky gate config
tion on the minority carrier lifetime and various losses of the
uration.These OPFET's are expected to emerge as promis
incident radiation due to reflections at various interfaces. The
ing detectors for use in integrated optoelectronic circuits.
present paper makes an attempt to overcome the limitations of
A number of theoretical and experimental investigations on
the previous models and provide a simple but fairly accurate
the effect of illumination on MESFET structures have been
model suitable for use in integrated optoelectronic circuit
recently reported [1)-[8]. Preliminary investigations reveal
simulation purposes. The outline of the proposed model has
that photo response of illuminated MESFET is due to the
been described in the next section.
optically generated carriers which increase the conductivity
of the channel and the photovoltage developed across the
Schottky barrier which affects the applied reverse voltage II. FORMULATION OF THE MODEL
on the gate [5], [8]. Simple models have been developed The GaAs-MESFET structure under consideration is similar
by several workers [2)-[4] in order to explain the results of to one in [10) and is reproduced in Fig. l(a). In the present
experimental investigations on the effect of illumination on analysis the channel has been considered to be uniformly
commercially available GaAs MESFET's. The large signal doped. The radiation has been assumed to be incident on
characteristics of an illuminated GaAs MESFET has been the semitransparent metal gate in the vertical direction while
reported in [6]. Unfortunately, the models proposed so far the drain-to-sorce current flows in the horizontal fdirection.
are either too complicated or not adequate to be used for The incident radiation enters the underlying semiconductor
circuit simulation purposes. A simple yet fairly accurate model after encountering a loss at the gate metallization as well
which takes into account all the important physical phenomena as at the metal-semiconductor interface. This loss can be
involved in an illuminated MESFET need to be considered reduced substantially by suitably designing the structure [8].
for this purpose. A closed form de analytical model of an Absorption of the radiation in the semiconductor results in the
generation of excess electron-hole pairs. These excess carriers
Manuscript received May 14, 1992; revised April 22, 1993.
P. Chakrabarti is presently with the Department of Electronics Engineering, change the charge distribution below the gate and the channel
Institute of Technology, Bamaras Hindu University, Varamasi-221005, India. due to photoconductive effect. The excess photo generated
The authors are with the Department of Electronics and Communication
carriers also reduce the minority carrier life time. Further, the
Engineering, Birla Institute of Technology, Ranchi-835 215, India.
IEEE Log Nµmber 9215171. excess carriers also result in the development of a photovoltage
0018-9480/94$04.00 © 1994 IEEE
366 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994
V0p across the Schottky gate due to photovoltaic effect. In
presence of a high gate bias resistance the photovoltage is
nearly equal to the open circuit voltage of an illuminated .-----.v•
Schottky junction and it strongly influences the characteristics
of the device under illuminated conditions. In the analysis, it
has been assumed that in the illuminated condition also the
1
internal space charge region below the gate can be divided
into sections I, II, and III (in Fig. l(a)) similar to the one
a
cosidered in [10] with charge distributions Q10P, Q20P and l
Q3op• respectively. In the dark condition, these charge amounts
correspond to Q1, Q2 and Q3, respectively [9]. The MESFET
channel can thus be considered to be divided into three sub
channels. The centre subchannel below Section I is associated
with the gate metallization while the other two are due to
r----------------- --- - - ----,
inter electrode spaces [10]. Absorption of incident radiation in
· Intrinsic mod ti
the active region of the channel through the surface features
Gate Drain
of MESFET gate ·affects all the three subchannel regions and
causes a change in the I-V characteristics of the device in
the illuminated condition. The spacings between the gate and
source and that between the gate and drain will offer series
resistances. Since the gate of MESFET has been assumed to
I
be illuminated only, the two series resistances will remain I -------
~--
unaffected in the presence of illumination.
In the present analysis, the incident radiation has been
characterized in terms of incident optical power density
2
Popt(W/m ) which undergoes reflection at the metal surface Source Source
as well as at the metal semiconductor interface. Assuming
the optical power density to decrease exponentially with the Fig. I. (a) Internal space charge distribution in the photo-MESFET before
pinch off region. (b) Equivalent circuit of the MESFET.
depth of penetration in the semiconductor, the excess carriers
generated per unit volume within the semiconductor can be
written as [8] From (1) and (2), TL can be obtained as
112
uA n = Go p · TL = (1 - Rm)(al h-v Rs)PoptTL ( 1 - exp ( - aa )) TL = ~{ 1~ +~ 4(~l2-~(Ra1,n-.~,R.;)h(,.v~,l. )-(R~1,-)R~ Ps.o) ~ ppt ~T P ~((1l ~-_ ~eexxp~p((-~-aaa~a))~))} ~ ~--1
(1) an;hv opt
(3)
where G0p is the optical generation rate per unit volume, a is
the optical absorption coefficient, TL is the minority carrier
B. Photo-Induced Voltage
lifetime in the illuminated condition, Rm and R s are the
reflection coefficients of the metal and semiconductor surfaces Due to direct illumination of the gate, there is a development
respectively at the operating wavelength >. [11], a is the width of photovoltage V0P at the Schottky gate contact which effec
of the active region, h is the Planck's constant, and v is the tively reduces the applied reverse bias on the gate. However,
operating frequency. the photo-induced voltage is greatly influenced by the surface
recombination, the lifetime of the minority carriers in the
illuminated condition and the external resistance present in
A. Effect of Illumination on the Minority Carrier Life Time
the gate bias circuit. The previous models [2], [4], [6] fail to
The excess carriers generated in the GaAs region below the account for the effect of surface recombination and the change
gate affects the minority carrier lifetime. The lifetime TL of the in minority carrier life time on the photo-induced voltage. The
minority carriers in the illuminated condition can be obtained present model calculates the photo voltage by considering the
from surface recombination and the lifetime of minority carriers
in the illuminated condition (TL) which has been found to
be significantly different from the equilibrium minority carrier
TL n;
Tp = n; + t1n (2) lifetime (Tp). The electron hole pairs generated in the depletion
region are seperated out by the existing field in the region.
The electrons move towards the channel while the holes move
where n; is the intrinsic carrier concentration of the semicon towards the surface where they recombine via the surface traps.
ductor, Tp is the minority carrier lifetime in equilibrium and The surface recombination results in a leakage current that
t1n is the excess photo generated carriers in the semiconduc affects the photo-voltage which, in tum, gives a feed back
tor. effect either through the source or substrate [7]. The excess
CHAKRABARTI et al.: SWITCHING CHARACTERISTICS OF AN OPTICALLY CONTROLLED GaAs-MESFET 367
TABLE I
PARAMETER VALUES REFERENCES
Tn 1 x 10-6 s [7] 1611 X13
Tp 1 x 10-s s [7]
kn 3.1 x 10-15m3s [7] 12
k p 3.1x10-17m3s [7]
Nr 4 x 1017 m2s [7]
II
vb; 0.8V [II]
µn 0.85m2 /Vs [II]
µp 0.04m2/Vs [II] 10
Rm 0.1 [8]
a 106 Im [ll] 9
n; 1.79 x 1012/m3 [ll]
>. 0.83µm [5]
f
e
photo generated carriers also affect the minority carrier lifetime -;:; 7
....
in the illuminated condition under steady state. Both the effects ."....',
tend to reduce the magnitude of the photo-voltage. .J
!-' 6
It has been observed that for a very high external gate bias
resistance only, the photovoltaic effect is dominant [2]. In
5
our analysis, we have assumed a very high resistance to be
present in the gate circuit. This approximates the photovoltage
4
developed to be equal to the open circuit photo voltage
(Vp) that affects the applied gate bias voltage in presence
0
of illumination. The open circuit photo voltage generated at 3
the Schottky gate in presence of surface recombination can be
obtained as 2
I
0 2 3 4 5 6 7 8
OPTICAL POWER DENSITY, Popi (W/m2)-+
Fig. 2. Variation of minority carrier lifetime, TL with the optical power
where T is the absolute temperature, k is the Boltzmann
density, Popt·
constant, q is the electronic charge, 'T/ is the constant depending
on the semiconductor [11], µP is the hole mobility of GaAs and
the channel. This photovoltage tends to forward bias the
ls is the saturation current density of the metal-semiconductor
Schottky contact and reduces the width of the depletion region.
Schottky contact. For a high mobility semiconductor like
This causes an increase in the effective channel thickness. In
GaAs, the current density ls can be approximated by the
other words, the photovoltage modulates the effective channel
thermionic emission theory and can be expressed as [11]:
opening resulting in a change in the channel conductance. The
ls = A*T2 exp ( - q:;n) effective channel thickness at the drain end, H in presence of
optical illumination can be written as
q<f>an being the barrier height of the Schottky contact at
thermal equlibrium and A* is the effective Richardson constant
for thermionic emission.
The surface recombination rate R can be written as [7]
R = NrKnKp(nsns - ntPt) where vbi is the built-in potential at the metal semiconductor
Kn(ns + nt) + Kp(Ps +Pt) junction, Vd is the drain voltage, V is the gate voltage,
9
Nd is the doping concentration in the channel, and E is the
where Nr is the area density of the traps, Kn and Kp are the
permittivity of the semiconductor.
capture factors. The surface carrier concentraions ns and Ps
under illuminated condition are given by:
C. Internal Gate-to-Source Capacitance
ns = aPopt(l - Rm)(l - R )Tn/hv
8 Assuming the n channel to be doped uniformly with donor
Ps = aPopt(l - Rm)(l - Rs)TL/hv concentration Nd(m-3) and considering a forward voltage
which take the values nt and Pt. respectively, when the Fermi (V p) to be developed across the Schottky gate under illu
0
level lies in the traps. mination, the total charge below the gate in the space charge
The photovoltaic effect discussed previously arises from region can be written as
the collection of excess photo generated carriers in the high
electric field in the depletion region which is transverse to
368 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994
The internal gate-to-source capacitance Cgs in the illuminated
- - - ILWMINATED CONDITION (WITHOUT RECOMBINATION)
condition can be obtained as: - · - ILLUMINATED CONDITION (WITH RECOMBINATION)
290
C gs = ( 8Qavsso p)TvT g d = constant. (5)
2BO
The total space charge below the gate in the illuminated
condition can be obtained as
270 ..,.,-·
,, .,,...-·
/
/
260 / -·-
/ -
t /
/
250 / /
/
/
~
I / /
::a. 240 I / /
I . ·-r-·-·
.... I; · I ./
C~l 230 / ./ ......- ......
Nrz4XI017/m2
>f0? 2 20 I,' /. !/ /. / NT =3 X I01Jm 2
:0r //// NT •2X 1017/ m2
Q.
210 fl/ Nr=-0
fl.
where W is the width of the gate, L is the gate length, v;, 200
is the source voltage, and V,, is the p9i nch off voltage and is I
given by: 190
V. _ qa2Nd
P - 2c
The total gate to source capacitance in the illuminated condi OPTICAL POWER DENSITY,Popt CW/m~ -
tion has been obtained as Fig. 3. Variation of photovoltage, V0p with incident optical power density,
Popt (with and without recombination).
channel region under illumination. The excess electron hole
pair generated in the channel modulates the conductivity. The
photovoltage developed across the Schottky barrier reduces
the width of the depletion region. Thus, both the channel
conductivity as well as the channel conductance are affected
in the illuminated condition. This fact results in a large change
in a drain current under illumination. However, in GaAs
(7)
MESFET surface recombination tends to reduce this effect
to some extent.
Thus, the internal gate-to-source capacitance in the illuminated The channel charge due to doping and optically generated
condition is an increasing function of the gate-to-source volt carriers taking surface recombination effect into account can
age and depends on the value of V0P, which is determined by be obtained as
the incident optical power density. From (7), it is also clear
that the gate-to-source capacitance shows a very high value
qWL Nd
for a particular level of illumination when Vgs (= V9 - Vs) is Qcop(Vx) = aqW L9Nd - 2 9
in the neighborhood of (Vbi - V p). [(
0 q~d) 1/2 { (Vbi - Vg + V,, - Vop)1/2
D. The Current-Voltage Characteristics l
The drain-to-source resistance of the MESFET under illumi
nation (Rds) can be obtained from the current voltage relation + (Vbi - Vg + Vx - Vop)112}
of the device under illumination. In order to find the current
voltage relation of the device, both photovoltaic effect and + ~~t (1 - Rm)(l - Rs)
photoconductive effect should be taken into consideration. In
the previous model [10), the photovoltaic effect was not con (1- exp(-aa))TLWqL9 - qRWL9TL (8)
sidered. The current voltage characteristics of the device can
be obtained by integrating the total charge in the undepleted The carrier concentration per unit area in the channel in
CHAKRABARTI et al.: SWITCHING CHARACfERISTJCS OF AN OPTICALLY CONTROLLED GaAs-MESFET 369
- DARK CONDITION - DARK CONDITION
- - ILLUMINATED COM>lllON (WITHOUT llECOMelNATION) 8 - - ILLUMINATED CONDITION (WITHOUT RECOMBINATION)
- · -LLUMINATED COl\OITION (WITH RECOM!ltlAOON) 6 -·-ILLUMINATED CONDITION (WITH RECOMBINATION)
Vd = O·I V ././ vd = o.1v
Vg =-0·2V ,, ,, ./ Vg =·0·2V
NPTo pt=• 4S XWI0/m17l/ m1 ,,"',,,/" -/ NPoTp t == 4SXW 1/0m17/lm l
// 6·5
-:-/·;-·
t t
4 5 6 ..
.... ....
~ ~.6
.j. .· '.u-.~. '. 5.Su~
u"~" u(uz! u~z (u"z"!
uz < u< ~
.<... 4 Q~. <uQ. j
luu"<<Q"i . 3 .00:"u"a.>. ":'. :"0"u.oa.>. :''. 4 45.".0::0.>... '.,'
:>
.0"0.. '. "!"<" ""tc" 0;a<! :;
3.s
•z 3
3
a:
0
2~2-~4---:~-76---:~-~s---'9'---i,o_ __,,-, -,-2~x-,0~2,,-J2 25
DOPING CONCENTRATION, Nd (m·l)-
..__-~-~--..__-~ _ __._ __. .___~ _ _,2
-0·7 -0-6 -0.5 -OA -0·3 -0.2 -O~ 0 Fig. 5. Variation of gate-to-source capacitance, Cgs and drain-to-source
- GATE TO SOURCE VOLTAGE, Vgs (Vl capacitance Cdc with the doping concentration, Nd for dark and illuminated
condition (with and without recombination).
Fig. 4. Variation of gate -to-source capacitance, Cgs and drain-to-source
capacitance Cdc with the reverse gate to source voltage, Vgs for dark and
illuminated condition (with and without recombination).
+ qµn W Popt (1 - R,,.)(1 - Rs)
L hv
presence of illumination at the steady state can be obtained as 9
qµnRWTL
112 x ( 1 - exp(-aa) ) TL Vds - L Vds ( 11)
Pcop(Vx) = ~d [2a - (q~d) { (Vbi - V9 +Vs - V0p)112 g
l
E. The Drain-to-Source Resistance
+ (Vbi -V9 + Vx -V0p)112} + ~~tTL The drain-to-source resistance at a constant applied gate
voltage in presence of optical illumination can be obtained as:
x (l -Rm)(l -Rs))(l - exp(-aa)) - (RTL)
= --1 -
(9) Rds
~)
( Vg =COilSt.
The drain current under illumination can be obtained by 8
integrating the above from s1ouvrcde. to drain as follows: = qµLn9 W [2N d { 2a - ( q2NEd (Vbi - V9 + V, - V0p) ) 1/2
qµnW
Ids = -L- Pcop(Vx)dVx (10) 112
g 0 - (q~d (Vbi - V9 + Vd - Vop +Vi.)) } rl
Substituting the value of Pcop(Vx) from (9) and integrating,
we obtain + ~~t rL(l - Rm)(l - R.) (1 - exp(- aa)) - RTL
qWµnNd
Ids=----
2L9 (12)
The reciprocal of the drain-to-source resistance corresponds to
x [ { 2a -
the so-called drain conductance of the device in the illuminated
condition [4].
F. Drain-to-Source Capacitance
The drain-to-source capacitance of the device in the illumi
nated condtion can be obtained by taking the derivative of the
370 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994
- DARK CUIDITION 2·0
--- ILLUMINATED CONDITION (WITHOUT RECOMBINATION) - DARK CONDITION
- · - ILLUMINATED CONDITION (WITH RECOMBINATION) - - ILLlMNATED CONDIT~IN (WITHOUT RECOMBINATION)
9 - · - ILLUMINATED CONDITlllN CWlTH RECOMBINATION)
Vd = O·IV Vg "-0·2 V
VPgo pt== ·0S·W2V/m 2 NPoTp t == S4XWI/0 m172/ m2 ,. ,. ,
.. NT : 4Xl017/m2 ,. / ,.
,. /
II... 1·5 /
'"" /
/ /
;J 7 / / /
....' / . /
uz . / / ./
:~6 // / / . /
,
" /
.u<. , ,.:
~ 1-0
u a:
a: a:
:gJ s :uJ
....
0 ua:
.f.-.. 0:J
"'
~
"'4 0f-
z
: 0·5
2 0
3
2.__~....._~_.__~_.~~.__~....._~_.__~--1.~--''----'o
0 0.2 0.4 0.6 O.B 1.0 1.2 1.4 1.6 l.B
GATE LENGTH, Lg (µm)- o~~~::'-~~-='::,--~~L-~~...L..~~--:".,--~--'
Fig. 6. Variation of gate-to-source capacitance, C and drain-to-source 0 O~ O<! 0.3 o.4 0·5 0-6
98 DRAIN TO SOURCE VOCTAGE, Vd• (Vl-
c(wapitahc itaanndc ew, iCth doeu t wreitcho mgabtien alteionng)th. , L 9 for dark and illuminated condition Fig. 7. Variation of drain current, Ids with drain-to-source voltage, Vds for
dark and illuminated condition (with and without recombination).
ttoo tatlh ec hdarnaninel vcohlatraggee . inT hthee dilrlauimn-itnoa-steodu rccoen dciatpioanc itwanitche reins ptehcet + ( Vbi - V9 + Vs - Vop - Vd ) 1/2
illuminated condition can be obtained as - (Vbi - Vg +Vs - Vop)1l2] (15)
1/2
C de -_ ~4 WL g [ Vbi + V2dE -qNVd9 - V0p ] (13) defTinheed caust otfhfe ffrreeqquueennccyy iant twheh icilhl utmhei ncauterdre ncto nthdriotiuognh, fCTg,s iiss
equal to that of the current generator 9m Ve in the intrinsic
G. R-C Time Constant and Cut-Off Frequency
MESFET [Fig. l(b)] and is given by
The equivalent circuit of the MESFET has been shown in _ 9m
Fig. l(b). Under high-frequency operations, two factors limit f r---- (16)
27rCgs·
the frequency response of a GaAs MESFET, e.g., the transit
time and the RC time constant. The transit time effect is The values of various parameters can be obtained under dark
the result of a finite time being required for the carriers to condition, assuming Popt = 0.
travel from source to drain. This transit time is usually small
compared to the RC time constant resulting from the input Ill. RESULTS AND DISCUSSION
gate capacitance and the transconductance. The input RC time Numerical calculations have been carried out to exam
constant of the device in the illuminated condition is given by ine the effect of illumination on the internal gate-to-source
capacitance, drain-to-source capacitance, current-voltage char
RC= Cgs
(14) acteristics, drain-to-source resistance, transconductance, cutoff
9m
frequency, and input RC time constant of a GaAs MESFET at
where 9m is the transconductance and the Cgs is the input 300 K. The channel has been assumed to be uniformly doped
gate-to-source capacitance of the device in the illuminated with Nd= 5 x 1022/m3. The depth of the active channel has
condition. The transconductance in the illuminated condition been assumed to be 0.2 µm and the gate length to be 1 µm. The
can be obtained as width of the channel has been assumed to be 10 µm. Numerical
9m_- qµn W Nd (~) 112 ccoalncduiltaiotino nast sVhgosw = t h-a0t .t4he7 cVh afnonre lV dis =pi n0c.1h edV .o fFfo irn ththee sdaamrke
2L9 qNd
applied Vgs• the channel thickness in the illuminated condition
x [~s (Vbi - Vg +Vs - Vop)-1/2 has been found to be more than that in the dark condition.
As a result the pinch-off takes place at a higher voltage (for
CHAKRABARTI et al.: SWITCHING CHARACTERISTICS OF AN OPTICALLY CONTROLLED GaAs-MESFET 371
__;DARK CONDITION. :_ _ - ILLUMINATED CONDITION (WITHOUT RECOMBINATION)
-4 --ILLUMINATED CONDLTKlN tWITHOUT RECOMBINATION) - · - LLUMINATED CONDITKlN lWITH RECOMBINATION)
IOX4·5 ·-ILLUMINATED CONDITION tWITH RECOMBINATION)
V4 : O'IV
I 3.5 Vg =-0·2 V --
4·0 vPdo pt == 05 ·W1 v/m 2 I I I I '', //NcT= 4X:I01;7/m--2 -------- 5BO
Ny = 4 >CI017/ m2 I I I .
I .
3·5 \ I I 3-0 f
\ /I //
530~
b \ I I I / . I 2·5 t ~ 1\ f/ { / ~."...,',-
\ i / I I . / ~ ~~ I\,\ : I <z.)
~ 480~
\ / "'
I / . 2·0 ; ~ ~ ! .i.i,i
\ // / "..', .".',
\ \ / / ./ <z.) ~2-8 '\ / u
/ I ~ 5"'
"":"u0.0z<:.>. ' '' . 1 ·5 \ \ \/ \/\ ' . >\.~/ < /.. _x_ / ../1 /, . // / / . / 11··50 ."~.u5""..',,' ' :o~~z III'~ 1 !~i1 - ji 433800 g1<0z" ;-'
~ 1·0 / ' ." .0. . 2-l I,. , '. \
/ / / / . ..... '--~·---·::::-.:-- 0·5z: I•I. I '\ ' \ .
0·5 ........ 0 '~~·-·-·-·-·- 330
- - ·-
'\:r--- --------
-0·7 -0·6 -0·5 -0'4 -0·3 -0·2 -0·1 0
l~L--....L..----L. __. ___.......__---L._ __,..__-!:---~--'280
GATE TO SOURCE VOLTAGE ,Vgs (VJ 0 I 2 3 4 5 6 8
Fig. 8. Variation of drain current, Ids and drain-to-source resistance, Rds OPTICAL POWER DENSITY,Popt CWf<rJ-l-·
with reverse gate-to-source voltage, Vgs in dark and illuminated condition Fig. 9. Variation of drain current, Ids and drain-to-source resistance, Rds
(with and without recombination). with incident optical power density, Popt (with and without recombination).
example, at -0. 75 V) in the illuminated condition (Popt = 5
of the minority carriers decreases significantly which limits
W/m2). In presence of surface recombination, the pinch:-<>ff
the optical generation . rate and causes V p to saturate. The
has been found to occur at Vgs = -0. 71 V for. the assumed 0
previous models [2],.[4], [5] cannot predict the ultimate limit
values of recombination.p arameters. The numerical values of
various paraineters used iii the calcuiation are given in Table L of V0P (i.e., the built-in potential of the Schottky contact) as
they fail to account for the change in lifetime of the minority
The excess photo generated carriers in the active region
carriers under illumination. Fig. 3 also shows the variation
below the gate cause a change in the minority earner. lifetime
under illumination. The variation of the minority carrier life of the photovoltage, V0P, with incident optical power density
in presence of surface recombination for various values of
time under illumination (TL) With the incident Optical power
density has been shown in Fig. 2. It is seen that TL decreases ttora pa plionwg edr evnasliutye iNn Tth. eI tp creasne nbcee eoafs isluyr fsaeceen rtehcaotm Vb0ipn astaiotunr aatneds
with increase in the incident optical power density. This·
the value ts less for higher values of NT.
decrease in TL in presence of illumination is an important Fig. 4 shows the variations of gate-to-source capacitance
consideration in the present model. Results of our calculation
Cgs. as well as drain-to-source capacitance Cdc· below pinch
show the equilibrium mean lifetime of the minority. carriers
off with the gate-to-source voltage in the dark and illuminated
decreases from 10-S seconds to 3.93 X 10-ll seconds for an condition for Vds = 0.1 V. The incident optical power
iii.ciderit optical power density of 5 W/m2. This decrease in the density has been assumed to be Popt = 5 W/m2. It is seen
minority carrier lifetime, in turn, limits the value of the photo that, in both dark arid illuminated conditions, the Cgs and
voltage Vop· This means that V p cannot increase. indefinitely Ccic decrease with increasing reverse gate-to-source voltage.
0
with the increase in the optical generation rate caused by an However, for a fixed applied gaie-to-source reverse voltage,
increase in the incident optical power density. This is clear the capacitance increases significantly in the presence of
from Fig. 3, which shows the variation of the open circuit illumination in both the cases. This is due to the fact that, in
photo-voltage devefoped across the Schottky barrier with the presence of illumination, the photovoltage V0P developed
the incident optical power density (with and without surface across the Schottky contact reduces the gate-to-source reverse
recombination). From the graph, it is seen that V p increases voltage which in turn enhances the capacitances. This result
0
with the increase iri the incident optical power density in th~ is in accordence with the experimentally measured variations
beginning and finally tends to saturate for a higher value of reported in [5]. A comparison of the results of our theoretical
Popt even in the absence of surface recombination. This opt is calculations with those of experimentally measured values of
because, at higher values of incident optical power, the lifetime [5] reveal that the percentage change in the capacitance values
372 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994
·7·5X10-4
- DARK CONDITION 1·25
- - - ILLUMINATED CONDITION CWITHOUT RECOMBINATION>
- ILLUMINATED CONDITION (WITH RECOMBINATION) - DARK CONDITION
- -- ILLUMINATED CONDITION (WITHOUT RECOMBf.IATIONl
I - ·-ILLUMINATED CONDITION lWITH RECOMBINATION)
vd =0·1 v
HPor p t== 54 Wit/om127 /m2 VVdg :~CoH·2VV --
1-0 Popi-= 5W/m2
Nr = :o 101'lm2
6·5
i >
6 0·75
..5
.l
w'
~ uz
~
5'5 ~z !u)
t3 80
;:) 0·5
0z "z'
5 0~ ,a<_:t
z
,a<_:t
0·25
4.5
-0·5 -0-4 -0·3 -0·2 -0·1
GATE TO SOURCE VOLTAGc:, Vg, (Vl
4 5 6 B 9 10 11 12x1022
Fig. I0 . Variation of transconductance, 9m with reverse gate-to-source
DOPING CONCENTRATION;-Nd (m-1)-
voltage in dark and illuminated condition (with and without recombination).
Fig. 11. Variation of transconductance, 9m with the doping concentration,
Nd in dark and illuminated condition (with and without recombination).
is more in our case. This is because, in our analysis, we have
assumed the gate to be semitransparent, which is not true for ing concentration, both capacitances increase in the presence
the commercially available MESFET used by Simons. On the of illuminatfon. However, surface recombination reduces the
other hand, a quantitative comparison of our results with those capacitance values in the illuminated condition as shown in the
obtained by [4 ] on the basis of their theoretical model indicates figure~ The variations of Cgs and Cdc at constant gate-to-source
that the percentage change in the corresponding capacitance voltage have been plotted in Fig. 6 under dark and illuminated
values under illumination is less in our case. This is perhaps conditions (with and without recombination) by taking Lg as a
because their model ignores the reflection of incident optical parameter. In all the cases, both capacitances increase with an
power at the entrance as well as at the metal semiconductor increase in Lg. This is due to the fact that, at constant applied
interface. Their model also neglects the decrease of minority voltages, an increase in Lg causes an increase in the depletion
carrier lifetime in the presence of illumination, which ~ends to region charge below the gate, which results in an increase in
reduce the V0P and hence the capacitance. The effect of surface the capacitance value. The surface recombination has a similar
recombination on the variations of Cgs and Cdc with the effect on the capacitace values as already discussed.
reverse gate-to-source voltage has also been depicted in Fig. Fig. 7 shows the current voltage characteristics of the device
4. It is seen that due to surface recombination the capacitance in the dark and illuminated condition up to pinch off. It is
values slightly decrease in presence of illmT\ination. seen that, at pinch off, there is a significant change in the
The gate-to-source capacitance Cgs and the drain-to-source drain current Ids in the illuminated condition as compared
capacitance Cdc variations with the doping concentration have with that in the dark condition. The huge change in the drain
been shown in Fig. 5 for dark and illuminated conditions current in the illuminated condition is due to both photovoltaic
(with and without recombination) at a constant gate-to-source and photoconductive effect. These characteristics are similar to
voJtage. It is clear that both the capacitances increase with the those of the experimental results reported in [5]. However, the
increasing doping concentration in dark as well as illuminated illumination effect ·on the drain current somewhat decreases
conditions. This may be accounted for by the fact that, at when the surface recombination effect is taken into account.
a constant applied gate-to-source and drain-to-source voltage, A quantitative comparison of our results with those of [4],
the width of the depletion region below the gate decreases with [5] reveals that the percentage change in the dark current at
an increase in Nd. This decrease in the width of the depletion saturation under illumination in our case is more than that
region, in tum, results in a corresponding increase in the of [5] and less than that of [4]. This discrepancy is due
capacitance values. It is further seen that, for a particular dop- to the reason already discussed. The variation of Ids and