High temperature (300 C) diodes for geothermal and other energy applications were developed. A comparison of reverse leakage currents of Si, GaAs, and GaP was made. Diodes made from GaP should be usable to 500 C. A Liquid Phase Epitaxy (LPE) process for producing high quality, grown junction GaP diodes is described. This process uses low vapor pressure Mg as a dopant which allows multiple boat growth in the same LPE run. These LPE wafers were cut into die and metallized to make the diodes. These diodes produce leakage currents below ten to the -9th power A/sq cm at 400 C while exhibiting good high temperature rectification characteristics. High temperature life test data is presented which shows exceptional stability of the V-I characteristics

Chaffin, R. J.

Dawson, L. R.

English

NASA Technical Reports Server

GALLIUM PHOSPHIDE HIGH TEMPERATURE DIODES* 
R. J. Chaffin, Division 5133 
L. R. Dawso:r, Division 5154 
Sandia National Laboratories 
Albuquerque, New Mexico 87185 
SUMMARY 
The purpose of t h i s  work i s t o  develop 
high temperature (>300°c) diodes f o r  geothermal and 
other  energy applications. A comparisorr of reverse 
leakage currents  of S i ,  Ws and Gap is made. 
Diodes made from Gap should be usable t o  > 5 0 0 ~ ~ .  An 
LPE process f o r  ~ r ~ c i ~ c i n g  high qua l i ty ,  grown junction 
Gap diodes is  described. This process uses low vapor 
pressure M? a s  a dopant which allows multiple boat 
growth i n  the same LPE run. These LPE wafers have 
been cu t  i n t o  d i e  and metallized t o  make the diodes. 
These diodes produce leilltage currents  below 
~/cm' a t  4 0 0 ~ ~  while exhibiting good high temperature 
rec t i f i ca t ion  c h a r a c L e r i s t i ~ s .  High temperature l i f e  
t e s t  data  i s  presented which shows exceptional s t a b i l -  
i t y  of the V - I  charac te r i s t i cs .  
I THEORY 
The figure shows t h a t  f o r  temperatures i n  the 
20-300'~ range GaAs and S i  have s imilar  leakage 
currents. (The high depletion region generation- 
recombination current  i n  GaAs o f f s e t s  its wider 
bandgap.) Calculatians show t h a t  Gap diode reverse 
leakage should be dominated by generation-retombina- 
t ion current up t o  650'~ and i s  a t  l e a s t  5 orders  of 
magnitude lower than Si. Hence it can be seen t h a t  
Gap should be an excel lent  choice f o r  high temperature 
semiconductor devices. 
This fac t  is demonstrated i n  Figure 2.  This 
f igure shows a V-I charac te r i s t i c  of a Sandia-made 
P+ -N Gap diode a t  4 0 0 ~ ~ .  The leakage of the Gap 
diode is not discernable on the  figure. The measured 
current density a t  -3V and 4 0 0 ~ ~  was 7 x lo-" amp/'cm2 
for  t\e Gap diode. 
The choice of semiconductor mater ial  used t o  
RUWUPT Pb-N UIODEz- fabr ica te  diodes is dominated by the reverse leakage 
charac te r i s t i cs  desired. The reverse leakage current : 1------ 
3 a t - 3 v o r t ~  
7 
density of an abrupt P+ -N junction is  given by:l w Z  
- 2 .  5' /. 3 
: I I  S t  Ueasured -n ,/ / b 
, -.-./ /--- j :I ,.----- Gap. 
n2  en.^ 4,- 
c -7 / J R = e  (1) -../.-q 
1' 
- -3  I , Cap Weasured-)l - 
: -, 1
,.* , 
_./-- 
- I 
. 
, 
where D = hole diffusion coef f ic ien t  ,' ,* 
P , ,.--' I 
T = hole l i fe t ime  ( i n  the n region) 
P 
T~ = depletion region c a r r i e r  l i fe t ime  
ni = i n t r i n s i c  c a r r i e r  concentration 
e = e lec t ron ic  charge 
ND = don$-r concentration 
W = ,:epletion layer  width 
The f i r s t  term on the r i g h t  s ide  of Eq. (1) 
represents the  diffusion of minority c a r r i e r s  within 
a diffusion length of the junction which produces a 
reverse leakage current.  This component is  independ- 
en t  of bias. The second term on the  r i g h t  of Eq. (1) 
represents generation-recombinathn currer.t i n  the 
depletion region and is dependent on b ias  through the 
depletion width. Generally the recombination current 
term w i l l  dominate a t  lowtemperaturesand the diffusion 
current  term w i l l  dominate a t  higher temperatures. 
The crossover point  is  primarily dependent. on the 
semiconductor band gaF and c a r r i e r  l i fe t ime.  The 
appropriate parameters for Sili:.on (Si  ) , Gallium 
Arsenide (GaAs) and Gallium Phosphide (Gap) were used 
t o  evaluate Eq. (1) a s  a function of temperature f c r  
the th ree  materials.  The r e s u l t s  f o r  reverse leakage 
current  derrsity a t  -3V a re  shown i n  Figure 1. Thc 
arrrws on t h e  curves mark t h e  crossover temperaiure 
Figure 1. C3mparison of reversL leakage current  
densi ty vs. temperature t o r  GaAs, S i  and Gap. 
of t1.e tvo components of leakage current.  For 
temperatures t o  the l e f t  of the arr.jrs generation- Figure 2 .  Cap grown junction diode character is t -  
recombination current  dominates and diffusion current  i c  a t  4 0 0 ~ ~ .  (Horizontal = 5V/div, v e r t i c a l  = 1 mA/ 
dominates a t  temperatur3s t o  the  r ight .  div.) 
11 DIODE FABRICATION 
To r e a l i z e  a device whose operation w i l l  not br. 
*This work dponsored by the U. S. Department of Energy degraded by the  high density of chemical impuri t ies  ( ~ . o . & . )  under Contract DE-ACFC-16-9P00789 and s t r u c t u r a l  defec t s  present i n  typ ica l  bulk Gap 
subs t ra te  mater ial ,  t h e  a l i -ep i tax ia l  s t ruc ture  of 
''A. U. S. bepartment of Energy f a c i l i t y .  Figure 3 i s  used. The K s ide  of the junction is  
https://ntrs.nasa.gov/search.jsp?R=19820007449 2020-03-21T15:51:54+00:00Z
l i g h t l y  doped t o  provide a s  high reverse breakdown 
voltage a s  possible. The P s ide  can the.1 be re la t ive ly  
highly doped t o  f a : i l i t a t e  ohmic contactinq of the  top 
surf ace. 
This s t ructure -$as prepared usins  l iqu id  phase 
epitaxy f o r  the grovth of both layers  during a s ing le  
growth cycle. 
/ Or/Au CONTACT 
L- f i  mil - p u a a  conrrcr 
Figure 3. Grown Junction GIP Diode. 
The growth apparatus shown i n  Figure 4 i s  a 
s l id ing  boat assembly constructtd from high pur i ty  
graphite. Thc body of the assembly contalns wells 
f o r  tw qowth  solutions, orie f a r  thc qrowth of the 
v layer ,  a second f o r  the  growti1 of the  FJ layer. To 
maintain background (no intent ional  doping) c a r r i e r  
co..centration a s  low as  the  1 x 1016 leve l  desired 
f o r  the N laycr ,  the growth temperature used -d?s 850"~. 
A t  t h i s  relat ivel t .  l o w  growth temperature, S i  contamin- 
at ion of the  g row~s  solut ions from the quartz  walls of 
t h e  system 1s minimal. To ensure Lhat no c r x s  
-mntamination of the N solution occurs from the 
heavily doped P solut iun,  re la t ive ly  non-volatile Mg 
is used a s  the P dopant i n  place of the  highly v o l a t i l e  
Zn normally used t o  dope Gap P type. Since Mg 
possosses a s tab le  oxide, provision is made f o r  adding 
t h i s  dcpant a f t e r  a pre- bake cycle removes zesidual 
oxygen from the growth solut ions,  a s  i n  Figure 4A. 
The syztem is then permitted t o  equ i l ib ra te  a t  the 
growth temperature ( 8 5 0 ~ ~ )  fo r 2 hours, a s  i n  4 8 ,  
a f t e r  which the s l i d e r  is translated t o  bring the 
Gap substrate  ink0 contact with the f i r s t  grohlh 
solutiorr, a s  i n  4C. Cooling then causes the solution 
t~ become super-saturated and e p i t a x i a l  growth occurs 
on the substrate. When the  N- layer  is sufficient1,r 
thick, the s l i d e r  is  again t ranslated t o  brinr, the 
substrate  i n  contact with the second melt Zor growth 
of the P layer ,  a f t e r  which fr?..'t:rer t rans la t ion  of the 
s l i d e r  separates the substrate  from the  l iquid.  
The diode metallization system used was: 
P+ contact - Be/Au (~OOOA % 1% Be by weight) 
(7 m i l  dot) followed by 3000i of pure Au 
+ (vacum evaporated) 
N contact  - The contact was sput tered ( f u l l  
surface) i n  the following sequence: 
Au/Ge (8p/12) %lo00 A, AU % XOA, 
N i  % 600 A, AU % 4000 A, 
Contacts annealed a t  4.50'~ (10 minutes! in  H 
2 
The f i r s t  l o t  of diodes tested were mounted i n  ceramic 
f l a t  pack headers (N+ s ide  down) with a high tempera- 
tu re ,  polymide s i l v e r  loaded adhesive. Gold wires 
(1.5 m i l )  were used t o  make contact t o  the  top of. the 
die. kiowaver, t h i s  conflguratibn was found to be 
unsatisfactory due t o  deter iorat ion of the  a3hesive a t  
high temperatures. The scheme f i n a l l y  chosen was a 
s tress-free configuration using 1.5 m i l  diamet.er gold 
wires bonded t o  each s ide  of the chip. .his was done 
t o  eliminate anv d i e  a t t ach  s t resses  o r  mater ial  
interact ions due t o  the header attachment scheme. 
These devices a r e  shown i n  Flgure 5. I t  should be 
emp)*asized thaL the mountinq confiquration ahown in 
Figure! 5 is not proposr'  fo r  fielded Jevices, but 
rather  it is a scheme used t o  remove any contr ibut ion 
of header s t r e s s  o r  bonding agent react ions fo r  
t c s t i r ~ q  devica charac te r i s t i cs .  
Gap SUBSTRATE 
( 4 1 BAKE OUT 
Figure 4. Liquid Phase Epit;.xial Growth System. 
Figure 5. Stress-Free Diode Mounr. 
I11 DIODE CHARACTERISTICS 
A typ ica l  I -V  c h a r a c t e r i s t i c  of a Gap diode was 
shown i n  Figure 2. The breakdown voltage was measured 
t o  be 90V a t  4 0 0 ~ ~ ;  t h e  breakdown c h a r a c t e r i s t i c  re-  
mains f a i r l y  sharp even a t  t h i s  elevated temperature. 
The f a c t  t h a t  the leakage current  is l a r g e r  than t h e  
value predicted (see Figure 1) means tha t  there  is  some 
leakage aL the  sawed edges of the  die .  
The zero bias 'capaci tance of the 15 m i l  square 
chips was measured t o  be 22 pP. This corresponds t o  
a 0.56 u zerc b i a s  deplet ion width. 
I V  ENVIRONMENTAL TESTS 
The Gap diodes  were subjected t o  a l i f e  t e s t  
under b ias .  Three b i a s  condi t ions  were used; forward 
b i a s  (5  IDA), reverse  b i a s  (-10 V! and open c i r c u i t  
( ze ro  b i a s ) .  The diodes  were placed i n  ovens a t  
3 ~ 0 ~ ~ .  The devices were not  sealed and t h e  oven 
contained room a i r .  The p4:-ameters o f  t h e  d iodes  
were checked as a function of  ti-.. i n  t h e  oven. 
The r e s u l t s  , ~ f  t h i s  t e s t  a r e  summarized i n  
Figure 6. This f igu re  shows t h a t  no d e t e c t a b l e  
degradation i v  s e r i e s  r e s l s t a z ~ c e  occurred i n  any of  
the  t h r e e  b i a s  s t a t e s .  The room temperature reverse  
leakage d i d  st,ov a s l i g h t  increase ,  increas ing from 
nominally lo-'? amps t o  lo-' amps a r t e r  991 hours a t  
300°L. There was not  a s t rong  w r r e l a t i o n  between 
b i a s  s t a t e  an6 ieakage increase.  The reverse  leak- 
age a t  3 0 0 ~ ~  ahowed a s l i g h t  decrease  a f t e r  991 
hours. Th i s  s t a b i l i t y  i n  diode parameters is i n t e r -  
pre ted  a s  aeaning t h a t  t h e  diode me ta l l i za t ions  and 
junction dopants a r e  s t a b l e  a t  3000C with b i a s  f o r  at- 
l e a s t  1000 hours. 
RB*REVERS -10 V 
FB= FORWARD 50 mA 
OB= OPEN QRCST 
I P CONTACT &lAu I 
N CONTACT AulGe 
- 
0 
0 
a 
STRESS FREE 
g 3 -  
I. 
> 
2 - 08. FB. R8 
0 I- 
0 2 0 0  4 0 0  6 0 0  800 KHX) 1200 
TIME - HOL!S (01 300%) 
Figure 6 .  ~ 0 0 ~ ~  L i fe  Test  Data on Stre:; Free  Gap 
Diodes. 
Th i s  paper has  presented b a t a  on gall ium- 
phosphide, grown junction diodes  f o r  high temperature 
a :p l i ca t ions .  Information on f a b r i c a t i o n  methods 
were presented.  Evaluation d a t a  shows: good low 
learage r e c t i f i c a t r c n  c h a r a c t e r i s t i c s  a t  4 0 0 ~ ~  and 
s t a b l e  junct ion anE meta l l i za t ion  parameters a t  3 0 0 ~ ~  
f o r  a t  l e a s t  i O O O  hours. The only  problem encountered 
was t h e  "high temperature" polyimide adhesive used t o  
bond t h e  diode &ips  t o  the  headers. A new e u t e t i c  
ch ip  bonding procedure 1s p r e ~ e r ~ t l y  being developed 
t o  so lve  t h i s  problem. 
'I ACKNOWLEDGEMENTS 
This work was suppJrted in  p a r t  by the  Dept. of 
Eqergy, Division of  Geothermal Energy. The au thor s  
would a l s o  l i k e  t o  thank J .  B. Snel l ing and T. A. Plut  
f o r  t ! ~ e i r  a s s i s t a n c e  in t a b r i c a t i n g  and t e s t i n g  the  
devices.  
REFERENCES 
--
1. .I. S. Grove, Physics and Technology of Semi- 
conductor r)evices, John Uiley 6 Sons, KT 1967* 
p. 176. 
2. S. ?f .  Sze, Physics of Seniconductor D e v e .  
John Uiley 6 Sons. NY. 1969. p. 103. 
2. L. R.  Dawson. "iloping Control in t h e  LPE Growth 
of GaP". papcr presented a t  E lec t ron ic s  Mate r i a l s  
Cqnf erence, Cornel l  Univ., 6/24/80. 


Gallium phosphide high temperature diodes

The purpose of this work is to develop high temperature (> 300/sup 0/C) diodes for geothermal and other energy applications. A comparison of reverse leakage currents of Si, GaAs and GaP is made. Diodes made from GaP should be usable to > 500/sup 0/C. An LPE process for producing high quality, grown junction GaP diodes is described. This process uses low vapor pressure Mg as a dopant which allows multiple boat growth in the same LPE run. These LPE wafers have been cut into die and metallized to make the diodes. These diodes produce leakage currents below 10/sup -3/ A/cm/sup 2/ at 400/sup 0/C while exhibiting good high temperature rectification characteristics. High temperature life test data is presented which shows exceptional stability of the V-I characteristics

Gallium phosphide high temperature diodes

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