Interdigitated photoconductive detectors were fabricated on microwave device structures, making them easily integratable with Monolithic Microwave Integrated Circuits (MMIC). Detector responsivity as high as 2.5 A/W and an external quantum efficiency of 3.81 were measured. Response speed was nearly independent of electrode geometry, and all detectors had usable response at frequencies to 6 GHz. A small signal model of the detectors based on microwave measurements was also developed

Bhasin, Kul B.

Claspy, Paul C.

Hill, Scott M.

English

NASA Technical Reports Server

I - 
I 
NASA Technical Memorandum 101485 I. 
t 
i. , -  
Microwave Characteristics of 
GaAs MMIC Integratable 
Optical Detectors 
Paul C. Claspy 
Lewis Research Center 
Cleveland, Ohio 
Scott M. Hill 
Case Western Reserve University 
Cleveland, Ohio 
and 
Kul B. Bhasin 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
1989 IEEE MTT-S International Microwave Symposium 
Long Beach, California, June 13-15, 1989 
( N A S A - T H - 1 0 1 4 8 5 )  HICBOWAVE CBARACTERIST I C s  
OF GaAs IlHIC INTEGRATABLE OPTICAL DETECTORS 
(NASA. Levis Research Center) 6 p CSCL 20# 
N 89- 2 45 20 
Unclas  
G3/32 0217728 
/ 
https://ntrs.nasa.gov/search.jsp?R=19890015149 2020-03-20T03:02:50+00:00Z
MICROWAVE CHARACTERISTICS OF GaAs MMIC INTEGRATABLE OPTICAL DETECTORS 
Paul 17. Claspy' 
NASA Lewis Research Center 
Cleveland. Ohio 44135 
Scott M. Hill 
Case Western Reserve University 
Cle*/elaid. Ohio a4106 
and 
Kul B. Bhasin 
NASA Lewis Research Center 
Cleveland. Qhio 44135 
E 
Summary 
Inyerdigitated photoconductive detectors have 
been fabricated on microwave device structures, 
making them easily integratable with MMIC's. 
Detector responsivity as high as 2.5 A/W and an 
external quantum efficiency of 3.81 were measured. 
Response speed was nearly independent of electrode 
geometry and all detectors had usable response at 
frequenc.es to 6 GHz. A small signal model of the 
detector based on microwave measurements is also 
developec. 
Introduction 
Over the past few years technology advances 
have occurred which have increased the possibility 
that in the near future phased array antennas com- 
posed of monolithic microwave integrated circuits 
tMMIC's) will become a practical rea1ity.l 
maximum advantage of the potential size reduction 
that this advance represents it will be essential 
to effect a similar size reduction in the chip-to- 
chip high frequency interconnects that the system 
will require. Various authors have suggested that 
fiber optic links. which are small and lightweight, 
may be a viable alternative to coaxial cables and 
waveguides for this application.2 If these links 
are to be practical, however, high frequency opti- 
cal transmitters and receivers must be available, 
and it would be particularly desirable that opti- 
cal components be process- and material-compatible 
with GaAs heterostructure MMIC's so that they can 
be integrated onto the same chip. 
In this paper we report the results of a study 
of the optical and electrical characteristics of 
interdigitated photoconductive detectors of various 
geometries that were fabricated on a HEMT-type het- 
erostructure material. The operating wavelength 
was chosen to be 820 nm. 
To take 
'P.C. Claspy is a National Research Council - 
NASA Senior Research Associate. Permanent address: 
Department of Electrical Engineering and Applied 
Physics, Case Western Reserve University, 
Cleveland, Ohio 44106. 
Detector Fabricaticn 
The detector; were fa3ricated on the ME!- 
grown GaAlAsKaAs heterost-ucture marerial j:own 
szhematically in Fig. 1 .  ,A typical 3etectoi- i s  
shown in Fig. 2. Interdigitated electrode geome- 
tries, with finger spacings ranging cram 1 to J pm, 
were used because they increase the effective 
active area of the detector while keeping the tran- 
sit distance small. Since the GaAlAs window ihdt 
remains after fabrication i s  essentially transpar- 
ent to the incident 820 nm radiation, almost all of 
the photon absorption, and therefore the electron- 
hole pair generation, occurs within the undoped 
GaAs layer. For testing, the detectors were mounted 
directly on the Teflon insulation of a specially 
prepared length of semi-rigid coaxial cable, witn 
short wire-bonded leads to the center conductor 
and the shield. 
Ootical Response Measurements 
Frequency domain response measurements were 
made over the range 0.01 to 10 GHz using the syscem 
shown schematically in Fig. 3, and normalized 
responses of detectors of rhree different geometries 
are shown in Fig. 4. The detectors exhibit nearly 
identical response dispersion, with a 3 dB curoff 
frequency at approximately 185 MHz. 
decays at about 12 dB/decade in the decade between 
100 MHz and 1 GHz, with a small plateau at 510 MHz. 
then falls off at 20 dB/decade after a larger pla- 
teau at 1 GHz. The shape of the response suggests 
that device characteristics are limited by trapping 
effects. 3 
The gain 
t 
Detector responsivities at 500 MHz, R(500 MP:), 
were measured using an Ortel SL620 diode laser, aid 
lie in the range 0.13 to 0.31 A/W. On the basis of 
measurements at 500 MHz, the low frequency respon- 
sivity of the 1 by 1 pin detector was calculated to 
be 2 . 5  A/W. The (internal) quantum efficiency o f  
this detector is 5.44, and the external quantum 
efficiency is 3.81, d11 of which are comparable to 
results reported for GaAs detectors. 
M i c r o w a v e  Impedance  Measurements and  Model 
DC and  AC impedance  c h a r a c t e r i s t i c s  o f  t h e  
d e t e c t o r s  were s t u d i e d  u n d e r  v a r i o u s  l e v e l s  o f  
i l l u m i n a t i o n .  DC I - V  c u r d e s  f o r  o u r  d e t e c t o r s  were 
t a k e n  u s i n g  w h i t e  l i g h t  i l l u m i n a t i o n ,  and  a t y p i c a l  
r e s u l t  i s  g i v e n  i n  F i g .  5 .  The DC r e s i s t a n c e  o f  
t h e  d e t e c t o r s  i s  low and  c o n s t a n t  f o r  s m a l l  b i a s ,  
s m o o t h l y  i n c r e a s e s  as  t h e  b i a s  i s  i n c r e a s e d ,  becom- 
i n g  v e r y  l a r g e  a t  h i g h  b i a s  as  a r e s u l t  o f  c a r r i e r  
v e l o c i t y  s a t u r a t i o n .  
w i t h  i n c r e a s i n g  o p t i c a l  power  i s  a l s o  shown i n  
F i g .  5 .  
The r e f l e c t i o n  c o e f f i c i e n t ,  S11,  was measured  
o v e r  t h e  f r e q u e n c y  r a n g e  0 . 5  t o  5 . 5  GHz a t  v a r i o u s  
b i a s  l e v e l s  and  i n c i d e n t  w h i t e  l i g h t  i n t e n s i t i e s  
u s i n g  a n  HP-4910 N e t w o r k  A n a l y z e r .  Some r e s u l t s  o f  
t h e s e  measuremen ts  a r e  shown i n  F i g s .  6 to  8 .  F i g -  
u r e  6 snoiws t h a t  S 1 1  d e c r e a s e s  w i t h  d e t e c t o r  f e a -  
t u r e  s i l e ,  F i g .  7 shows t h a t  i t  i n c r e a s e s  w i t h  b i a s  
v o l t a g e  a t  m o d e r a t e  i l l u m i n a t i o n ,  and F i g .  8 shows 
t h a t  i t  d e c r e a s e s  w i t h  i n c r e a s i n g  i l l u m i n a t i o n ,  a t  
m o d e r a t e  b i a s .  
The d e c r e a s e  o f  r e s i s t a n c e  
A s m a l l - s i g n a l ,  h i g h  f r e q u e n c y  m o d e l ,  c o n s i s t -  
i n g  of a p a r a l l e l  RC c o m b i n a t i o n  w i t h  a n  i n d u c t a n c e  
i n  s e r i e s  w i t h  each  node of t h e  RC n e t w o r k ,  as 
shown i n  F i g .  9 ( a ) ,  was d e v e l o p e d  from t h e  measured  
S11 r e s u l t s .  The m o d e l ,  w h i c h  i s  s i m i l a r  t o  t h a t  
d e v e l o p e d  b y  W o j t c z u k  a n d  B a l l a n t y n e 4  l e a d s  t o  a 
comp lex  impedance  g i v e n  b y  
2 R ( V . @ )  + j w  { ( L 1  t L 2 )  - CR ( w , @ ) }  
( 1 )  2 Z =  1 + wCR (V,O) 
where  t h e  r e s i s t a n c e ,  R ( V , 9 ) ,  i s  a f u n c t i o n  o f  b o t h  
t h e  DC b i a s  and  t h e  i l l u m i n a t i o n .  
The l i g h t  i n t e n s i t y  p r i m a r i l y  a f f e c t s  R ,  
w h i c h  a t  low b i a :  d r o p s  from w e l l  o v e r  100 R a t  
a m b i e n t  t o  o n l y  a f e w  t e n s  of ohms a t  h i g h  i n t e n s i t y  
T h e r e f o r e ,  a t  low b i a s  and  h i g h  l i g h t  l e v e l  t h e  low 
r e s i s t a n c e  e f f e c t i v e l y  s h o r t - c i r c u i t s  t h e  d e v i c e  
c a p a c i t a n c e ,  r e d u c i n g  t h e  impedance  t o  
Z = R t j w ( L 1  t L 2 ) ,  ( 2 )  
as  shown i n  F i g .  9 ( b ) .  The AC r e s i s t a n c e  was 
o b s e r v e d  t o  i n c r e a s e  s i g n i f i c a n t l y  w i t h  b i a s ,  so 
t h a t  a t  h i g h  b i a s  t h e  comp lex  impedance e v e n t u a l l y  
s i m p l i f i e s  t o  
z = - -  R i i  ( 3 )  2 w c  ' wCR 
as shown i n  F i g .  9 ( c ) .  T h i s  change from i n d u c t i v e  
t o  c a p a c i t i v e  b e h a v i o r  s u g g e s t s  t h e  p o s s i b i l i t y  of 
a z e r o  r e a c t a n c e  o p e r a t i n g  p o i n t  b y  t h e  e l i m i n a -  
t i o n  o f  a n y  RC or R / L  t i m e  c o n s t a n t s  i f  t h e  
d e t e c t o r  i s  t i m e  c o n s t a n t  l i m i t e d .  The impedance 
o f  t h e  ze 
t i n g  t h e  
Then, i f  
w h i c h  for  
l e s s  t h a n  
o r e a c t a n c e  p o i n t  i s  d e t e r m i n e d  b y  s e t -  
m a g i n a r y  p a r t  of i q .  ( 1 )  e q u a l  t o  z e r o .  
wCR2) < <  1 ,  t h e  impedance r e d u c e s  t o  
( 4 )  
o u r  d e v i c e s  i s  a b o u t  250  Q a t  somewhat 
3 V b i a s .  
Summary and  C o n c l u s i o n  
The h i g h  f r e q u e n c y  c h a r a c t e r i s t i c s  o f  i n t e r -  
d i g i t a t e d  p h o t o c o n d u c t i v e  d e t e c t o r s  f a b r i c a t e d  o n  
a HEMT s t r u c t u r e  have  been p r e s e n t e d .  The f a b r i -  
c a t i o n  p r o c e s s  !das c o m p l e t e l y  c o m p a t i b l e  w i t h  t h a t  
used  t o  f a b r i c a t e  MODFET's m a k i n g  t h e s e  d e t e c t o r s  
e a s i l y  i n t e g r a b l e  f o r  MMIC u s a g e .  I t  s h o u l d  De 
n o t e d  t h a t  ( J h i l e  t h e  d e t e c t o r s  d i d  n o t  e y r l i b i t  an 
e x t r e m e l y  h i g h  b a n d w i d t h ,  t h e y  d i d  p o s s e s j  u s a b l e  
r e s p o n s e  w e l l  i n t o  t h e  GHz r a n g e ,  and  t h e  change 
f rom i n d u c t i v e  t o  c a p a c i t i v e  r e a c t a n c e  w i t h  b i a s  
s u g g e s t s  t h e  p o s s i b i l i t y  o f  a z e r o  r e a c t a n c e  o p e r -  
a t i n g  p o i n t .  I n  c o n c l u s i o n ,  t h e n ,  t h e  c o m b i n a t i o n  
of f a b r i c a t i o n  c o m p a t i b i l i t y  a n d  p e r f o r m a n c e  c h a r -  
a c t e r i s t i c s  makes t h e s e  d e v i c e s  i n t e r e s t i n g  f o r  
i n t e r c o n n e c t i o n  a p p l i c a t i o n s .  
R e f e r e n c e s  
1 .  J .  F r e y ,  and  K . B .  B h a s i n ,  e d s . ,  M i c r o w a v e  
I n t e g r a t e d  C i r c u i t s ,  2nd  Ed.  Dedham, MA: 
A r t e c h  House,  1985.  
2 .  K.B.  B h a s i n ,  G. A n z i c .  R . R .  K u n a t h ,  and  D . J .  
C o n n o l l y ,  " O p t i c a l  T e c h n i q u e s  t o  Feed and  
C o n t r o l  GaAs MMIC M o d u l e s  f o r  Phased A r r a y  
A n t e n n a  A p p l i c a t i o n s , "  i n  AIAA 1 1 t h  Annua l  
C o m m u n i c a t i o n s  S a t e l l i t e  Systems C o n f e r e n c e ,  
New York: A IAA,  p p .  506-514.  1986  ( N A S A  
TM-87218). 
3 .  G.J. P a p a i o n a n n o u ,  and J.R. F o r r e s t ,  "On t h e  
P h o t o r e s p o n s e  o f  GaAs MESFET's: B a c k g a t i n g  
and Deep T r a p s  E f f e c t , "  I E E E  T r a n s .  E l e c t r o n  
D e v i c e s ,  v o l .  ED-33, n o .  3, p p .  373-378,  
Mar .  1986 .  
4 .  S.J. W o j t c z u k ,  and J.M. B a l l a n t y n e ,  " Imped-  
ance  P r o p e r t i e s  a n d  Broad-Band O p e r a t i o n  o f  
GaAs P h o t o c o n d u c t i v e  D e t e c t o r s , "  J .  L i q h t -  
wave T e c h . ,  v o l .  LT-5, no. 3, p p .  320-324,  
Mar .  1987.  
2 
F i g .  1 .  Schematic cross s e c t i o n  of 
photodetector  m a t e r i a l .  
400 A 
3000A 
40 A 
10 000 i 
F i g .  2 .  1 pm x 1 pin d e t e c t o r .  
LASER 
TRANSMITER 7 
\FIBER OPTIC LINK 1 
"* W1I"D SPECTRlM 
AWLIFIER ANALYZER 
X.Y.Z STAGE BIAS 
DETECTOR-' TEE 
F i g .  3 .  Frequency domain measurement system 
ORIGINAL' PAGE 
BLACK AND WHITE PHOTOGRAPH 
FREQUENCY, GHz 
F l g .  4 .  Normalized frequency response o f  
t y p i c a l  d e t e c t o r s .  
(a)  1 p x 1 prn; ( b )  2 prn x 2 prn; 
( c )  2 pm x 2 pin. 
F i g .  5 .  I - V  c h a r a c t e r i s t i c  curve for 
1 pm x 1 pm d e t e c t o r .  
3 
w 
0 2 x 3  
0 2 x 2  
0 1 x 1  i n  
V 
0 0  
0 1  
0 3  i n  
F i g .  7 .  E f f e c t  o f  b i a s  v o l t a g e  o n  S11 for 
2 prn x 3 prn d e t e c t o r .  
0 MEIENT LIGHT 
0 LEVEL 1 
0 LEVEL 2 1.0 
F i g .  8 .  E f f e c t  of i l l u m i n a t i o n  on S 1 1  f o r  
2 pm x 2 pm d e t e c t o r .  
F i g .  9 D e t e c t o r  e q u i v a l e n t  c i r c u i t  models. 
( a )  G e n e r a l  e q u i v a l e n t  c i r c u i t ;  ( b )  Low 
b i a s  e q u i v a l e n t  c i r c u i t ;  ( c )  H i g h  b i a s  
e q u i v a l e n t  c i r c u i t .  
4 
Msn National Aero autics and Report Documentation Page 
Space Administration 
1. Report No. 1 2. Government Accession No. 
NASA TM-101485 
4. Title and Subtitle 
Microwave Characteristics of GaAs MMIC Integratable Optical Detectors 
7. Author(s) 
Paul C. Claspy, Scott M. Hill, and Kul B. Bhasin 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135-3191 
12. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, D.C. 20546-0001 
3. Recipient’s Catalog No. 
5. Report Date 
6. Performing Organization Code 
8. Performing Organization Report No. 
E-462 1 
10. Work Unit No. 
506-44-20 
11. Contract or Grant No 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
15. Supplementary Notes 
Prepared for the 1989 IEEE MTT-S International Microwave Symposium, Long Beach, California, 
June 13- 15, 1989. Paul C. Claspy, National Research Council-NASA Senior Research Associate; permanent 
address: Dept. of Electrical Engineering and Applied Physics, Case Western Reserve University, Cleveland, 
Ohio 44106. Scott M. Hill, Case Western Reserve University. Kul B. Bhasin, NASA Lewis Research Center. 
6. Abstract 
Interdigitated photoconductive detectors have been fabricated on microwave device structures, making them easily 
integratable with MMIC’s. Detector responsivity as high as 2.5 A/W and an external quantum efficiency of 3.81 
were measured. Response speed was nearly independent of electrode geometry, and all detectors had usable 
response at frequencies to 6 GHz. A small signal model of the detectors based on microwave measurements is 
also developed. 
7. Key Words (Suggested by Author@)) 
GaAs; Optical detectors; MMIC; 
Microwave charateristics 
18. Distribution Statement 
Unclassified - Unlimited 
Subject Category 32 
9. Security Classif. (of this report) 20. Security Classif. (of this page) 121. No ofgpages 22. Price’ 
Unclassified Unclassified 1 A02 
*For sale by the National Technical Information Service, Springfield, Virginia 221 61 NASA FORM 1626 OCT 86 


Microwave characteristics of GaAs MMIC integratable optical detectors

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