1533
IEEE TRANSACTIONS ON ELECTRON DFV1CF.S. VOL 37. N O 6. J U N E I Y W
[7] M. Sakaue. T . Tamaina, and Y. Mizushima, "Unidirectional transfer
properties of plasma-coupled shift register." /EEE Trans. Electrotl
De1ice.s. vol. ED-29. no. 8, pp. 1276-1283, 1982.
[SI T . Tamarna and N. Kuji. "Auto-calibrated potential map drawing
equipment and its application to characterization of plasma-coupled devices." / E E E 7rcr)i.s. Electron Dc1,icxs. vol. ED-33, no. 2. pp. 192197. 1986.
Gallium Arsenide Photo-MESFET's
B . LAKSHMI, K. CHALAPATI. A. K . SRIVASTAVA.
B. M. ARORA, S . SUBRAMANIAN, A N D
D. K . SHARMA
Fig. 2. A radiation pattern of the plasma from a tired cell is shown, superimposed nn the usual optical pattern. The emitter injection current is
10 mA through 500-R load from 5 V . The plasma area is partly covered
by the metallized middle row. s o that the recombination radiation image
is observed as separated by this metallization line. The shift pulsewidth
and also the exposut-e time are 40 p s , and the integration is 60 s.
Abstract-Response of normall) off GaAs photo-MESFET's has been
investigated in two modes: i) the normal mode in which the photon
energy is greater than the bandgap and the light intensity is sufficient
to bias the device abme turn-on threshold, and ii) the wbthreshold
mode with subbandgap photon energy illumination. In the second
mode, the tranGstor operates b j internal photoemission from metal
gate to the semiconductor. In the normal mode, the square root of the
drain photocurrent \ a r k s as logarithm of the incident light intensity.
The device characteristics for subbandgap illumination have been analy7ed for the first time and we show that the photocurrent varies linearly with light intensity in this mode.
I. I N T R O D L ~ C I I O N
Fig. 3. Consecutive patterns of the emission by delaying the sampling signal. The delay is changed by 20-ps interval. A unit transfer occurs cvery
40 ps. A transient and stable plasma states are alternativcly displayed.
It implies that a timing analysis of digital circuit is po\sihlc.
[ l ] A. G . Chynoweth and K . G . McKay. "Photon emission from avalanche breakdown in silicon." P h ~ s Rei,.
.
. v o l . 102. no 2 , pp. 369376, 1956.
[2] C . Hu. S . C. Tam. F-C. Hsu, P-K. K O , T-Y. Chan. and K . W . Terril.
"Hot-electron-induced MOSFET degradation~model. monitor. and
improvenicnt." / € E € 7rwi.s. Elcctrori De\,ic.rs. vol. ED-32. no. 2. pp.
375-385, 1985.
131 T . Tsuchiya and S. Nakajiina. "Emission mechanism and h i a d e p e n dent emission cfhciency of photons induced by drain avalanche in Si
MOSFETs." l E E E Trtrris. Electroti Dci.ic(,.\. vol. ED-32. n o . 2 , pp.
405-412. 1985.
141 N.Khurana and C-L. Chang. "Analysis of product hot electron problems by gated emission microscope." i n 24th Ann. Pro(,. /EEE/RPS.
pp. 189-194. 1986.
[SI N. Khurana, "Pulsed infrared microscopy for debugging i n latch-up
on CMOS product\." in Ami. Proc. IEEEIIRPS. pp. 122-117. 1984.
161 S-C. Lim and E-G. Tan, "Detection of junction spiking and its induced latch-up by emission microscopy," i n A I I H . Proc,. /EEE//RPS.
pp. 119-125. 1988.
Recently, ultrafast photoresponse of GaAs M E S F E T ' s . also
called O P F E T ' s , has also been demonstrated [ I ] . These devices
are capable of detecting light pulses of 100-ps duration with a
2-GHz repetition rate [21, [31. Sugeta er al. [2] also explored the
mechanism of photodetection by M E S F E T ' s . They found that the
M E S F E T first acts as a photodiode, and then the F E T amplification
takes place. An alternative explanation, based on a direct optically
induced modulation of depletion layer, has been proposed by T .
Umeda et al. 141. Since Schottky-barrier structure is a majoritycarrier device. it can have turn-on and turn-off times as small as a
few picoseconds 151 and therefore M E S F E T ' s could be fabricated
to detect extremely short light pulses.
In this brief. photodetection response measurements were perfornied on GaAs M E S F E T devices. including their response to
photon energies smaller than the bandgap of GaAs. For the photon
energies between the Schottky-barrier height and the bandgap energy (0.85-1.4 e V ) it is the internal photoemission process which
gives rise to photocurrent. This current is, in turn, amplified by the
F E T . A self-consistent model using measured parameters is presented to explain the results. T h e spectral response in the subbandgap region shows the behavior predicted by the Fowler theory 161
and our results confirm the mechanism proposed by Sugeta et a / .
11. E X P E R I M E N T A L
Nomially off GaAs M E S F E T devices were fabricated on semiinsulating GaAs substrates ion-implanted with silicon using standard processing steps. T h e current-voltage behavior of the MESFET's was invcstigatcd under front illumination. For photon
Manuscript received May 23. 1989. revised December 15. 1989. The
review of this brief wa, arranged by Associate Editor G . Craford.
B. Lakshmi and K. Chalapati are with SAMEER, Indian Institute of
Technology Campus. Bombay 400076, India.
A. K . Srivastava, B. M. Arora, S . Subramanian, and D. K . Sharma are
with the Tata Institute of Fundamental Research, Bombay 400005. India.
IEEE Log Number 9034392.
0018-9383/90/0600-1533$01.OO O 1990 I E E E
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It.F.F, TKANSACTIONS O N F.I.ECIROK DF..VICk.:S. VOL.
1534
37. N O . 6. J U N t I940
energies hv > E,, a 2-mW He-Ne laser was used. Thc intensity
of illumination was varied by a set of neutral density filters. For
spectral response measurements, light from a tungsten lamp was
passed through a monochromator and then focused onto the gate
region. For photon energies h u < E,, the light beam was chopped
and the photocurrent was measured by using a lock-in amplifier.
Current-voltage behavior in dark was also measured.
111. RESULTSA N D DISCLISSION
A . Normal Operation
Fig. 1 shows typical l,),5-VDs characteristics of a large-area
(90-pm-long g a t e ) device operating in dark. The device shows
normally off behavior and operates a s an enhancement-mode MESF E T , with a threshold voltage V , = 100 m V . Similar characteristics were observed with 2-pm-long gate M E S F E T ' s .
F i g . 2 shows typical ID,s-VD,s characteristics of a large-area
MESFET, which is operated with a floating gate and light excitation from a He-Ne laser. Different IDs-VLls curves were obtaincd
by reducing the incident light with calibrated neutral density filters.
Characteristics in Fig. 2 resemble those in F i g . I . with the gate
becoming forward-biased by light, like in Schottky-barrier solar
cell with
VDS ( V )
Fig. I , /il,-V,)5 characteristics ot GaAs MESFET operating
in
dark
He - N e Loser Illurninallon
where I,yc is the short-circuit current proportional to the incident
light intensity, lois the dark saturation current. and n is the ideality
factor of the Schottky barrier. The value of n was measured to be
1.27 from dark I-V characteristics. By varying the intensity of illumination, the open-circuit voltage developed on the gate changes
and hence a family of IDS-VDscurves were obtained. In the square
law regime, with VGs >> V , . the drain current depends on the
incident light power P a s follows:
where q is the quantum efficiency and hv is the photon energy. Fig.
3 shows a plot of
versus log of normalized incident power.
The data are in satisfactory agreement with the above relationship
(2). T h e spectral response of the device is shown in Fig. 4. A sharp
I .4-eV photon energy indicates the onset
rise in responsivity at
of the band-to-band absorption. In addition to the high responsivity
at photon energies hv > 1.4 e V , the device shows significant response at subbandgap energies.
-
-.?
::k
01
0 2 0.3 0 4 0 5 0 6 07 0 8 09
V D ~(VOLTS)
Fig. 2 . Opcn-gate MESFET characteristics with He-Ne laser illumination
B. Subthreshold Subbundgap Operution
T h e inset in Fig. 4 shows the device response at photon energies
Illumination
hu < 1 . 4 eV extending down to -0.8 e V . G a A s does not have
significant absorption at these photon energies (0.8-1.3 e V ) to give
L G= 90prn
-
the observed photoresponse from a 0.4-pin-thick active layer in
the device. T h e photoresponse can be caused by the internal photoemission of electrons from the gate metal into the channel. T o
investigate the mechanism of the subbandgap photorcsponse. the
relationship between the incident light intensity and the drain current needs to be established. Under weak illumination, the Vc;sdeveloped is much smaller than V , and the device operates in the
subthreshold condition. A plot of I,,\ versus Vc;,5characteristics of
the device operating in dark at low gate bias ( Vc;,5< 100 m V ) on
a semilog scale gives a straight line indicating that in the subthreshold region IDS varies exponentially with Vc;,s
L
6
7
(3)
where Vo IS found to be 4 0 m V . Under illumination with hv < E,,
the open-circuit voltage at the gate can still be expressed by ( I )
From ( I ) and (3) we get
Fig. 3 . /i,.5-light intensity characteristics of MESFET for He-Ne illumination.
For ISc << I,,, w e obtain
,111 q , ,
(4)
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153s
where
f,,
is related t o incident chopped light power f a s
-
I,$(
I@
=
-.
11 v
Fig. 5 shows a plot of
versus i5(at I-Km wavelength. A s anticipated from (6). a linear relationship is observed between i/),s
and
is(..
T h e ratio f/),s/[,(. is constant and expresses the current gain in
the subthreshold region. T h e value of current gain is found to be 7
in our devices.
In order to substantiate the internal ohotoemission nature of the
photoresponse in the subbandgap regime. we have plotted the
square root of the drain current per incident photon o r the yield Y
a s a function o f t h e photon energy. T h e experimental curve is shown
in Fig. 6. A linear relationship is seen over a wide energy range h u
> 48. W e obtain a value of G8 0.87 cV. which is in good agreement with the Ti-GaAs Schottky-barrier height 161.
PHOTON ENERGY (eV)
-
Fig. 4. Spectral response of GaAs MESFET.
I V . CONCLUSION
A normally off M E S F E T can be used for detecting light of energy i) above the bandgap of the semiconductor by hand-to-hand
excitations and ii) below the semiconductor bandgap by internal
photoemission. T h e lower limit of the photon energy that can be
detected in the latter case is given by the gate Schottky-barrier
height. In both cases, the photoresponses can be explained by a
model in which the M E S F E T gate Schottky barrier acts like a photodiode and develops a photovoltage under illumination, which, in
turn. changes the drain current. T h e response current
is amplitied by the F E T transistor action. In the nomial mode of operation,
(
varies logarithmically with the light intensity. W e have
shown that in the subthreshold-mode operation the drain current
varies linearly with the light intensity. T h e photo-MESFET thus
provides means for light detection over a wide energy range with
amplification. Sincc M E S F E T ' s are majority-carrier devices. they
also otfer the advantage of high speed.
'
0
1
1
IO
20
Fig. 5 . AC photocurrent
1
30
,
40
50
1
8
1
'
60
70
80
90
f,,,,-f,, characteristics
wavelength of I
1
o f GaAs MESFET at a
ACKNOWL.~,DGMEN~'
pili.
T h e authors wish t o thank P. P. Suratkar and V . T . Karulkar for
assistance in the fabrication of M E S F E T ' s .
[ I ] J . C. Campbell. "Photo-transistora."
/ w r t r / s . vol.
22D. W . T. Tsang. Ed
in Ser,iic.orit/rc[.,ors t r r i d SemiNew York. NY: Academic Press.
1985.
121 T . Sugeta and Y . Mirushima. "High speed photoresponse mechanism
of a GaAs MESFET." J t r p t r ~ iJ . A ~ / J /Ph\s..
.
vol. 19. p. L27. 1980.
131 J . C. Gamniel and J . M. Ballantyne. "The OPFET: A new high speed
optical detector." in lEDM Proc... pp. 120-123, 1980.
141 T. Umeda and Y . Cho. "Etfect oi incident light illumination shape o n
responsivity o f GaAs MESFET photodetector," Jtrptrti J . Appl. P/i!.\.
pp. L367TL364. 1985.
I S ] A . McCowen. S . B . N. Shaari. and K . Board, "Transient analykis o f
Schottky harrier diode." / / I . s I . Elrc.. E r ~ ~ ypt.
. . 1. vol. 135. p. 71. 1988.
161 S. M. Sze. Ph!.sic..\ (!f S~,r,iic.oirt/rcc.ro,. DoVcrs. New York. NY:
Wiley. 1969. ch. 8 .
.
PHOTON
ENERGY (eV)
On the Calculation of Specific Contact Resistivity on
( 100) Si
KWOK K . NG
Fig. 6. (Yield)'
versus photon energy characteristics for GaA\ MESFET
in the subbandgap region.
T h e a c photocurrent at the drain is therefore given by
ANI)
RUICHEN LIU
Abstract-In order to design suhmicrometer Si MOSFET's properly,
the specific contact resistivity p , has to he controlled. The p, is known
Manuscript received April 24. 1989: revised January 4. 1990. The review of this brief was arranged by Associate Editor K . C. Saraswat.
The authors are with AT&T Bell Laboratories. Murray Hill, NJ 07974.
IEEE Log Number 903.5 180.
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