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NASA Technical Memorandum 88980 



Microwave Performance of an Optically 
Controlled AlGaAs/GaAs High Electron 
Mobility Transistor and GaAs MESFET 

(SASA-TM-88980) HICECKAVE EZEiOBKANCE OF AM N87-17993 

CEIICALLY CCNTBCllED 8l6aAs/GaAs HIGH 

ELECTECIi MOBILllX IfiAfiSISTCfi thD GaAs MESFET 
(NASA) 12 p CSCL 09A Uiiclas 

* ' G3/J3 ^4 3529 



Rainee N. Simons and Kul B. Bhasin 
Lewis Research Center 
Cleveland, Ohio 



Prepared for the 

1987 IEEE MTT-S International Microwave Symposium and Exhibition 

Las Vegas, Nevada, June 9-11, 1987 



1VI/>SA 



MICROWAVE PERFORMANCE OF AN OPTICALLY CONTROLLED AIGaAs/GaAs HIGH ELECTRON MOBILITY TRANSISTOR 

AND GaAs MESFET 



Rainee N. Simons* and Kul B. Bhasin 



National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 



Abstract 

Direct current and also the microwave 
characteristics of optically illuminated 
AlGaAs/GaAs HEMT are experimentally measured 
for the first time and compared with that of 
GaAs MESFET. The results showed that the 
average increase in the gain is 2.8 dB under 
1.7 mW/cm^ optical intensity at 0.83 pm. 
Further, the effect of illumination on 
S-parameters is more pronounced when the 
devices are biased close to pinch-off. Novel 
applications of optically illuminated HEMT as 
a variable gain amplifier, high-speed high- 
frequency photo detector, and mixer are 
demonstrated. 



Introduction 



Use of direct optical control of microwave 
semiconductor devices for optical injection lock- 
ing, phase shifting, and signal distribution has 
the potential to enhance the performance of future 
space borne phased array antenna systems. ^»^ 
Previously, several authors have experimentally 
investigated -the effect of light on the dc as well 
as the microwave characteristics of IMPATT diodes 
and GaAs MESFETs.-'"^ Their investigations show 
that these changes in the characteristics are due 
to photoconductivity and photovoltaic effects. 
Further, an analytical study, by the authors taking 
into consideration material properties of hetro- 
structures showed that the hetrostructures have a 
higher sensitivity to optical illumination. When 
this investigation was extended to microwave device 
structures, it was observed that the dc character- 
istics of an AlGaAs/GaAs High Electron Mobility 
Transistor (HEMT), when compared to that of a GaAs 
MESFET, are more sensitive to optical illumination 
greater than the semiconductor band gap.^ 

In this paper, we present for the first time 
extensive experimental results which show the sen- 
sitivity to optical illumination, that is, the 
light induced voltage and as a consequence the 
changes in the drain to source current, the intrin- 
sic transconductance, the scattering parameters. 



♦National Research Council 
Associate. 



NASA Research 



and the gain of an AlGaAs/GaAs HEMT. Further, 
fromthe de-embedded HEMT scattering parameters, 
the changes in the equivalent circuit element 
values due to optical illumination are also 
computed. In order, to compare and contrast the 
performance of a HEMT with a MESFET, experiments 
are also carried out on two different GaAs MESFETs 
and these results are also presented here. 
Finally, three novel applications of optically 
illuminated HEMT as a variable gain amplifier, 
high-speed high-frequency photo detector, and 
mixer are demonstrated. 

Experimental Setup 

A low noise AlGaAs/GaAs High Electron Mobility 
Transistor (MPD-H503, Gould Inc.) with recessed 
Pi-gate of length 0.5 nm and width 280 ym,^ a 
low" noise, low power GaAs MESFET (DXL 0503A, Gould 
Inc.) with recessed Pi-gate of length 0.3 ym and 
width 280 pm, and a medium power GaAs MESFET (RPX 
2322, Raytheon Co.) with T-gate of length of 0.5 urn 
and width 500 ym are used for investigation. For 
optical illumination an AlGaAs/GaAs Laser diode 
(SL-620 S, Ortel Corp.) with a fiber pigtail, which 
operates at a wavelength of 0.83 um and has a 
direct modulation bandwidth of 6 GHz is used. The 
optical power emitted from the 50 ym multimode 
graded index optical fiber pigtail as measured 
using a calibrated digital power meter and a photo- 
sensor (815, Newport Corp.) is 1.7 mW/cm^. The 
tip of the fiber is held at a distance of 1 mm from 
the device. 

These devices are mounted on a 0.375 by 
0.375 in., 25 mil thick alumina carrier. The alu- 
mina carrier also accommodates a pair of 50 a 
coplanar waveguides (CPW) which serve as the sig- 
nal input and also output ports. The device gate 
and drain pads and the source pad are wire bonded 
to the CPW center strip conductors and the ground 
plane respectively. The carrier is then mounted 
in a test fixture (Design Techniques, Inc.) which 
has two 3.5 mm coaxial connectors for external 
connection. The test fixture also has provision 
for ensuring repeatable pressure contact between 
the terminals of the CPWs on the carrier and the 
two 3.5 mm coaxial connectors on the fixture. A 
CPW calibration kit consisting of a 50 <5 through, 
two short circuits, and an open circuit on similar 
alumina carriers are used for calibrating the 
HP8510 automatic network analyzer and de-embedding 



the device S-parameters. Ablock schematic of the 
entire experimental setup is shown in Fig. 1, 

dc Device Characteristics Under niumination 

Light Induced Voltage 

The light generated voltage V-jit is 
obtained by plotting the measured gate current 
Iq as a function of the reverse biased gate to 
source voltage Vgj, and extrapolating the graph 
till it intersects the X-axis. The intersection 
point is read as the light generated voltage, which 
from Fig. 2(a) and (b) for a AlGaAs/GaAs HEMT and 
a GaAs MESFET are 0.57 and 0.24 V respectively. 

Drain to Source Current, Transconductance, and Ga in 

The measured drain to source current Ifjg 
as a function of the drain to source voltage V^^ 
with and without optical illumination for an 
AlGaAs/GaAs HEMT and a GaAs MESFET are presented 
in Fig. 3(a) and (b) respectively. 

Figure 4 presents the measured dc transcon- 
ductance g^ for a GaAs MESFET. The g^, is 
considered almost insensitive to optical illumina- 
tion since the maximum change observed is less 
than 2 mmhos. 

The measured gain with and without optical 
illumination as a function of Vgs for an 
AlGaAs/GaAs HEMT and a GaAs MESFET are presented 
in Fig. 5(a) and (b) respectively. As an example, 
for the case of a AlGaAs/GaAs HEMT, the gain 
increases by 2.5 dB at Vgr = -0.95 V and fre- 
quency equal to 26.5 GHz when the illumination is 
1.7 mW/cm2. 

Microwave Characteristics Under Illumination 

The S-parameters, namely S^, S22, and S]^2 
are measured as a function of the frequency and 
illumination (1.7 mW/cm^) with the devices biased 
close to saturation and also pinch-off. These 
bias points or operating points are labeled as 
points A and B respectively in Fig. 5(a) for the 
AlGaAs/GaAs HEMT and Fig. 5(b) for the GaAs MESFET. 

The measured Sn over the frequency range 
0.045 to 26.5 GHz for AlGaAs/GaAs HEMT in pinch- 
off (Vg5 = -0.95 V) condition is illustrated on 
a Smith Chart plot in Fig. 6(a). Similarly, S22 
is illustrated in Fig. 6(b). Figure 6(c) illus- 
trates S12 on a linear magnitude polar plot. 
In these figures L and D denotes that the 
measurements are carried out with or without illu- 
mination. A similar set of measurements have also 
been carried out for GaAs MESFET. 

Thus from Fig. 6 it is observed that illumi- 
nation does affect Sn, S22. and Si2. Besides, 
this effect is more pronounced when the devices 
are biased close to pinch-off. 

Using the CPW calibration kit and the through, 
short, delay (TSD) technique the influence of the 
small length of coplanar waveguide on either sides 
of the chip devices and the test fixture coaxial 



connectors are effectively removed. The small 
signal device equivalent circuit element values 
are next obtained from the de-embeddded device 
S-parameters using the models in Refs. 9 and 10. 
As an example. Fig. 7 shows an increase in the 
gate and the source capacitances and a decrease in 
the gate to drain feedback capacitance, with opti- 
cal illumination. In addition, the model also 
shows that the gate charging resistance Ri and 
the channel resistance Rg both decrease with 
optical illumination. The effect of these on the 
ft, fm^x' 3"<^ noise figure are being further 
investigated. 

Optically Controlled HEMT as a Variable 
Gain Amplifier 

The feasibility of using an AlGaAs/GaAs HEMT 
as an optically controlled variable gain amplifier 
is clearly evident from the measured S21 mag- 
nitude and phase characteristics shown in 
Figs. 8(a) to (d). Figure 8(a) and (b) when com- 
pared, show that the gain increases with illumina- 
tion. Further it is interesting to observe that 
the phase of S21 is insensitive to optical 
illumination as evident from Figs. 8(c) and (d). 

High Frequency HEMT Photodetector 

An experiment was conducted by illuminating 
the HEMT device with an optical signal which had 
been modulated with a 6 GHz RF signal and observ- 
ing the output on a spectrum analyzer, which is 
shown in Fig. 9(a). 

HEMT Oscinator and HEMT Mixer with 
Optical ly Coupled CP 

The capacitance variation of HEMT with illu- 
mination is shown in Fig. 7. This can be success- 
fully exploited in the design of an injection 
locked oscillator. 

Preliminary experiments with a HEMT as a 
mixer show that it is possible to optically couple 
the local oscillator signal. This is achieved by 
directly modulating a laser diode at the local 
oscillator frequency (6 GHz). The laser diode 
output is then made to illuminate the gate region 
of the HEMT. The RF signal (9 GHz) is electri- 
cally coupled to the gate terminal. The resulting 
IF signal (3 GHz) as seen on a spectrum analyzer 
is shown in Fig. 9(b) . 

Conclusions 

The paper presents for the first time exten- 
sive experimental results which show the sensitiv- 
ity to optical illumination. The light induced 
voltage and as a consequence the changes in, the 
drain to source current, the intrinsic transcon- 
ductance, the scattering parameters, and the gain 
of an AlGaAs/GaAs HEMT have been measured. The 
light induced voltage for a HEMT is observed to be 
0.57 V and 0.24 V for MESFET at 0.83 m wavelength. 
The higher V^^^ for HEMT is attributed to the 
higher increase in hole concentration Ap mainly 
due to the absorption thickness d, (see Eq. 1).' 
Further, from the de-embeddded HEMT scattering 



parameters the changes in the equivalent circuit 
element values due to optical illumination are also 
computed. These computations show an increase in 
the gate and also the drain capacitances and a 
decrease in the gate charging and also the channel 
resistances. The effect of these changes on the 
ft, f(n^x 3nd the noise figure is being further 
investigated. In addition to the above, experi- 
mental results or GaAs MESFETs have also been pre- 
sented for comparison. 

In these experiments the HEMT is optically 
illuminated by an AlGaAs/GaAs laser diode. This 
feature further enhances the attractiveness of the 
above experiments, since it leads to the possibil- 
ity of integrating a HEMT and a laser diode on a 
single MMIC chip to perform multiple circuit func- 
tions optically, such as, switching, amplifier 
gain control, phase shifting, and mixing. Such an 
integration, when fully accomplished not only 
promises improved MMIC circuit performance, but 
also vastly simplifies the signal distribution and 
beam steering in future phased array antenna. 

Finally, three novel applications of an opti- 
cally illuminated HEMT as a variable gain anipli- 
fier, high frequency photo detector, and mixer are 
demonstrated. 

Acknowledgment 

The authors wish to acknowledge Robert 
Romanofsky of NASA Lewis Research Center and 
Vladimir Sokolov of Honeywell Physical Sciences 
Center for assistance in bonding the MESFETs and 
HEMTs used in the experiments. 

References 



2. J. Austin and J.R. Forrest, "Design Concepts 
for Active Phased-Array Modules," IEEE Proc, 



1. R.G. Hunsperger, "Optical control of Microwave 
Devices," in Integrated Optical Circuit Engi - 
neering II , SPIE vol. 578, S. Sriram, ^d., 
Bellingham: SPIE, 1985, pp. 40-45. 



Part F; Communications, Radar and Signal 
Processing : vol. 127, pp. 290-300, 1980. 

3. A. A. A. DeSalles, "Optical Control of GaAs 
MESFET's," IEEE Trans. Microwave Theory Tech. , 
vol. MTT-3r, pp. 812-820, 1983. 

4. J.L. Gautier, D. Pasquet, and P. Pouvil, 
"Optical Effects on the Static and Dynamic 
Characteristics of a GaAs MESFET," IEEE Trans. 
Microwave Theory Tech., vol. MTT-33, 

pp. 819-822, 1985. 

5. H. Mizuno, "Microwave Characteristics of an 
Optically Controlled GaAs MESFET," IEEE Trans. 
Microwave Theory Tech., vol. MTT-31, 

pp. 596-600, 1583. 

6. W. Chen, N.E. Byer, M.P. Bendett, and 

R.G. Hunsperger, "Optical Control of IMPATT 
Diodes," in Optical Technology for Microwave 
Applications , SPIE vol. 477, S.-K. Yao, Ed., 
Be11ingham:SPIE, 1984, pp. 105-108, 

7. R.N. Simons and K.B. Bhasin, "Analysis of 
Optically Controlled Microwave/Millimeter Wave 
Device Structures," IEEE Trans. Microwave 
Theory Tech. , vol. MTT-34, pp. 1349-1355,' 1986. 

8. A. Swanson, J. Herb, and M. Yung, "First 
Commercial HEMT Challenges GaAs FETs," Micro- 
waves & RF , vol. 24, no. 12, pp. 107-11'5^ 

9. R.A. Minasian, "Simplified GaAs MESFET model 
to 10 GHz," Electron. Lett. , vol. 13, 

pp. 549-551, 1977. 

10. Touchstone User Manual . Version 1-4, EEsof 
Inc., pp. EL-15 to 18, June 1986. 



OF Fi 



QUALITY 



HP 9872 B 
PLOTIFR 



DlSPLAY/PROCESSOfi 



NETUODK ANALYZER 



HP 8515 A 

S-PARAftTEfi TEST SET 
_ 15 NHz - ?6.5 5Hi 



HP 8620 C 
SWEEP OSCILLATOR 



* 86290 B-H08 
HF PLUG-IN 
2-22.0 GHI 



HP 11612 A 
BIAS NErWOfiK 



DC VMIABIE 
POtCR SUPPLY 



DC VARIABLE 
POWER SUPPLY 




FIGURE 1. - BLOCK SCHEMATIC OF THE EXPERIMENTAL SETUP. 






1.5 


Pop, =1.7mW/cm2 






/ 




1.0 


/ 


a. 


/ 


0.5/ 

1 


■ .5 

I'' 


//^^ 



WITHOUT ILLUMINATION 
WITH ILLUMINATION 
Pgpt = 1.5 mwW 



1, 



0.21 V— s^ 



J ^^ 



:/ 



+1.0 



-1.0 



-2.0 



-3.0 

Vgs<V) 



-0.5 -1.0 



(A) DEVICE AlGaAs/GaAs HEMT 
(MODEL NO. nPD-H503); 



'ds 



V. 



(B) DEVICE GaAs MESFET (MODEL 
NO. RPX 2322); V^j = V. 



FIGURE 2. - MEASURED Ig VERSUS V()s FROM WHICH V^|, IS OBTAINED. 
DISTANCE BETWEEN END OF FIBER AND DEVICE, 1 MM; A = 0.83 pM; 
Pg , = 1.5 mwW. 



WITHOUT ILLUMINATION 
WITH ILLUMINATION 



m POOR 



.vjfc, iii 




DxL 0503A 

A = 0.83 [in 

Pgpt =1.7 hwW 

Vgs- V 

-.05 


,,---""" 


-0.8 


'-''^^^■^ 


"1.2 

-:-:i;i.5 



-JL2 
-1.5 



FIGURE 3. - MEASURED I^ij VERSUS Vh» WITH AND WITHOUT OPTICAL 
ILLUMINATION. 



^dS 



10 


RPX 


2322 


A 


= 0.83 MM 




Popt 


=1.5 mW/cm^ 




Vds 


= 1.0 V 

/X "^ 

// 
// 


30 




ll 
ll 
ll 
1 


20 


/ 


1 
1 
1 
1 
1 

It 
1 
I 
1 


10 


/ 






// 


^WITHOUT ILLUMINATION 




//- 






/ / 


^WITH ILLUMINATION 




// 






// 






1 


1 1 1 1 1 



-1 



V , V 
"gs' " 



MEASURED g„ VERSUS Vgj FOR GaAs MESFET 



FIGURE 1 

WITH AND WITHOUT OPTICAL ILLUMINATION 





I I dsi I I 



+7.5 
+5.- 
+2.5 


-2.5 
-5.0 
-7.5 
-10.0 



-0.5 



-0.9 -0.7 -0.5 -2.5 -1.5 

Vgs (V) 

(A) oeviCE algaas/gaas hemt (B) device, gaAs mesfet (hodel no. 

(HODEL NO. HPD-H503); 



Vds = ^'O V- 



NO. DxL 0503 A); V^j = 3.5 V. 



FIGURE 5. - HEASURED GAIN VERSUS Vgs WITH AND WITHOUT 
ILLUMINATION. DISTANCE BETWEEN END OF FIBER AND DE- 
VICE, 1 MM; ^= 0.83 MM; P(,p, = 1.7 mW/cm^. 



OF. POOR 



=11 z 

REF 1.0 Un i -t » 
Bj 200.0 mUni it/ 

V 15.352 O -162.72 n 

MPD H503 Srt3S=3V ID; 



(L) VGS=--95V 




S22 Z 

REF 1.0 Un i t« 
£ 200.0 mUni t»/ 

V 9.4375 n -G9.504. n 

M=D H503 VDS=3V ID; 



MARKER 

18.0^4-4- 



L) VGS=--95V 




Si 2 

REF 300.0 mUntts 
3 60.0 mUni tj/ 

V 221.39 mU. 25.332 



PD H 

MAR 

1 


503 VDS=3V ID5s*-^TOfT 


yrrrTtH^a.) VGs=- 


•95V 


KER 3/ ,'/' 

2.M>B87S yGHz 

/ ' -^X ' - 


^ 


\ 




' ;' / X 


^^^W^ 


11 






-^ 


1 




\ ^ ' ' ^ ^ 


~ "" "^ ~^ X 


/ 



FIGURE 6. - MEASURED S-PARAMETERS FOR AlGaAs/ 

gaAs hemt with and without illumination when 
biased close to pinch-off. start 0.045 ghz, 

STOP 26.5 GHz. 



. .2 



MPD 


- H503 


X 


= 0.83 (JM 


Popt 


= 1.7 mW/ch 


Vds 


= 3.0 V 








— WITHOUT ILLUMINATION 

... UTTU Tl 1 MMTUATTnU 


1 


1 1 


1 1 


.0 


-0.9 


-0.8 -0.7 


-0.6 -0.5 



.08 



.07 



.06 C 



.05 <-> 



.Ot 



.03 



FIGURE 7. - DE-EHBEDED GATE, SOURCE, AND 
GATE TO DRAIN CAPACITANCES FROM THE 
MEASURED S-PARAHETES WITH AND WITHOUT 
OPTICAL ILLUMINATION FOR AlGaAs/GaAs 
HEMT. 



8 



ORIGINAL PAGE fS 
Of POOR QUALITY 



S21 



LOG MAG 



REF. 0.0 dB 
3.0 dB/ 




(A) S2-, MAGNITUDE WITHOUT 
OPTICAL ILLUMINATION. 

$21 ANGLE 



(B) $21 MAGNITUDE WITH 
OPTICAL ILLUMINATION. 



REF. 0.0" 



100.0° / 





■"'[•■■ 


U»J. 


■ V 
















^ . ,., , 




' 






























































































1 


h~ 






















i= 


















8 






\ 




T 


■ — 1 


-1 








¥ 




~r 




-A 

























r 




















- 




— 


























r 


— 










.95 

















































































































(C) $21 PHASE WITHOUT 
OPTICAL ILLUMINATION. 



START 0.015 GHz 
STOP 26.500 GHz 
(D) S 



'21 



PHASE WITH OPTICAL 



ILLUMINATION. 
FIGURE 8. - MEASURED Sj-, FOR AlGaAs/GaAs HEMT. 



10 dB 




CENTER 6.000 GHz CENTER 3.000 GHz 

SPAN 10.0 MHz SPAN 10 MHz 

(A) DETECTED 6 GHZ SIGNAL. (B) 3 GHz IF SIGNAL. 
FIGURE 9. 



1, Report No. 

NASA TM- 88980 



2. Government Accession No. 



3. Recipient's Catalog No. 



4. Title and Subtitle 



5. Report Date 



Microwave Performance of an Optically Controlled 
A16aAs/6aAs High Electron Mobility Transistor 
and GaAs MtSFtl 



6. Performing Organization Code 

506-44 21 



7. Author(s) 

Rainec N. Simons and Kul. B. Bhasin 



8. Performing Organization Report No. 

E-3333 



10. Work Unit No. 



9. Performing Organization Name and Address 

National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 



11. Contract or Grant No. 



12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 
Washington, D.C. 20546 



13. Type of Report and Period Covered 

Technical Memorandum 



14. Sponsoring Agency Code 



15. Supplementary Notes 

Prepared for the 1987 lEtt Mll-S International Microwave Symposium and 
Exhibition, Las Vegas, Nevada, June 9-11, 198/. Rainee N. Simons, National 
Research Council - NASA Research Associate. 



16. Abstract 

Direct current and also the microwave characteristics of optically illuminated 
AlGaAs/GaAs HEMl are experimentally measured for the first time and compared 
with that of GaAs MtSFEl. The results showed that the average increase in the 
gain is 2.8 dB under 1.7 mW/cm^ optical intensity at 0.83 ym. Further, the 
effect of illumination on S-parameters is more pronounced when the devices are 
biased close to pinch-off. Novel applications of optically illuminated HEMl as 
a variable gain amplifier, high-speed high frequency photo detector, and mixer 
are demonstrated. 



17. Key Words (Suggested by Atithor(s)) 

AlGaAs/GaAs HEMl; GaAs MESFEl; 
Optical control; Microwave 
integrated circuits 



18. Distribution Statement 



Unclassified - unlimited 
STAR Category 33 



19. Security Classif. (of this report) 

Unclassified 



20. Security Classif. (of tfiis page) 

Unclassified 



21. No. of pages 



22. Price* 



A02 



01 -aie by the National Technical Information Service, Springfield, Virginia 22161