Development of high reverse breakdown voltage and low forward voltage drop stability in high power Schottky GaP rectifier for high temperature application is reported

Craford, M. G.

Keune, D. L.

English

NASA Technical Reports Server

December 1972	 B72-10673 
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
High Temperature Gallium Phosphide Rectifiers 
The problem: 
The potential of gallium phosphide (GaP) rectifiers 
for high-temperature applications has been known for 
some time,and rectifiers have been produced both with 
diffused junctions and with Schottky barriers. However, 
the development of stable high-power Schottky rectifiers 
has been complicated both by the stringent requirement 
of uniform surface preparation prior to metallization 
and by the tendency of the barrier metal to alloy 
with the GaP during high-temperature operation. Also, 
studies of large-area diffused-junction rectifiers revealed 
not only problems in fabricating a stable high-temperature 
device, but also a difficulty in achieving both high 
reverse breakdown voltages and low forward voltage 
drops in the same device.
The solution: 
Refinement of GaP vapor phase epitaxial (WE) 
growth techniques and the utilization of n/n+ GaP 
structures has resulted in the fabrication of Zn-diffused 
GaP p/n/n+ rectifiers which exhibit reverse avalanche 
voltages greater than 400 volts as well as forward 
voltages less than 3 volts at a forward current of 1.0 
ampere for temperatures from 27 to 400°C. 
How it's done: 
Material used in the fabrication of these devices is 
prepared by the WE growth of thin (approximately 
30 pm), high resistivity layers on low resistivity liquid 
encapsulated Czochralski (LEC) substrates. The growth 
is carried out in an open tube system using phosphine 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000672 2020-03-17T03:42:31+00:00Z
(PH 3 ) with HC1 gas transport of Ga. Both the PH 3 , as 
a 5% mixture in H 2 , and the HC1 are ultra pure grade. 
The other reagents are 99.9999% Ga and H 2 purified 
by Pd diffusion. HC1 at 3 ml/min standard temperature 
and pressure (STP) diluted with 50 ml/min of H 2 is 
passed over Ga at temperature close to 780°C to form 
GaC1 vapor. The GaC1 vapor is then mixed with the 
main H 2 stream at 450 ml/min including 10 nil/min of 
5% PH 3 in H2 mixture, at 930°C. From the mixing 
zone, the combined reactant gas stream is passed over 
the horizontally supported substrates located at 800°C 
to 850°C in a temperature gradient of 10°C/cm. Epi-
taxial layers are grown at a rate of approximately 10 
microns/hr. The epitaxial layer thickness is in the range 
of from 25 to 35 pm and uniform to within ± 1 .im for 
a given wafer. 
The epitaxial layers are doped with nitrogen, an iso-
electric impurity, to a concentration of approximately 
10 19 molecule s/cm 3 , but are otherwise not intentionally 
doped. 
Carrier densities in the nitrogen doped epitaxial 
layers as low as 3 x 10 14 /cm 3 were observed by Au-GaP 
Schottky barrier C-V analysis. Epitaxial layers grown in 
the same reactors, but without nitrogen doping, have 
background carrier concentrations of approximately 
10 1
 6/cm3. 
Junctions are formed at a depth of approximately 6 
jim in the thin epitaxial layer by a closed ampoule Zn 
diffusion using either a ZnAs2 or metallic Zn source. 
Diffused wafers are lapped and polished on the substrate 
side to a final thickness of approximately 150 pm. A 
thin film of Au-Ge eutectic alloy is vacuum evaporated 
on the substrate side to improve the n-type side ohmic 
contact. A thin film of Al is then evaporated onto the 
p-type side to reduce spreading resistance in the p-type 
layer. The wafers are then diamond scribed and cleaved 
into chips. 
In order to minimize thermally induced strain in the 
devices during high-temperature operation, the n-type
sides of the chips are alloyed to Au-plated 99.5% W - 
0.5% Ni stress relief pads (see figure) with Au-Ge pre-
forms. The W-Ni stress relief pads have a thermal expan-
sion coefficient which closely matches that of GaP. The 
chip-stress pad sub-assemblies are then alloyed to Au-
plated headers and contacted with multiple Al ultra-
sonic bonds. 
Notes: 
1. These thin film devices can be used to temperatures 
in excess of 400°C which is far above the maximum 
operating temperature for Si devices. 
2. This work represents a major improvement in GaP 
material and device technology, and makes possible 
the fabrication of GaP active components, such as 
transistors and monolithic light emitting arrays with 
a memory. 
3. The following documentation may be obtained from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: NASA CR-2098, Development of 
High Temperatures Gallium Phosphide Rectifiers 
4. Technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B72-10673 
Patent status: 
NASA has decided not to apply for a patent. 
Source: M. G. Craford and D. L. Keune of 
Monsanto Company
under contract to
Lewis Research Center 
(LEW-1 1804) 
B72-10673
	 Category 01
	 L


High temperature gallium phosphide rectifiers

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