Construction of improved phototransistors to provide increased base-collector capacitance is described. Silicon or germanium semiconductor wafers are used as examples. Operation of transistors and electrical properties are discussed and illustrations of the devices are included

Mccann, D. H.

English

NASA Technical Reports Server

+ N Region Light Flux	 Emitter 
Collector 
July 1972	 B72-10347 
.
NASA TECH BRIEF 
Marshall Space Flight Center 
NASA Tech Briefs announce now 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. 
Overlap Diffusion for Increasing Phototransistor Dynamic Range 
The problem: 
The present phototransistors operating in the charge-
storage mode have very limited dynamic ranges. There 
are severe limitations on both the maximum voltage at 
which these devices may be operated and the maximum 
base-collector capacitance which can be obtained from 
them. 
The solution: 
The base-collector capacitance in phototransistors is 
increased by a factor of five by use of one of the two 
methods of n+ diffusion. 
How it's done: 
As shown in the Figure 1, one configuration of the 
improved phototransistor is constructed of a silicon or 
germanium semiconductor wafer. The lower portion 
forms the collector region to provide an n-type con-
ductivity for n-p-n type phototransistors (p-type con-
ductivity for p-n-p type). Above the collector region is 
a base region which is formed by diffusion of impurities 
into the substrate exhibiting p-type conductivity for an 
n-p-n device (n-type for a p-n-p device). A third distinct 
emitter region is formed by diffusion of n+ impurities 
into the base region (p+ impurities for p-n-p device) to 
create a second junction. This stage completes the for-
mation of conventional photo transistor. To depart from 
the conventional device, a second base-collector junction 
is formed by an n+
 (p+ for p-n-p) conductivity layer, as 
in the case of an emitter region. This layer is out of the 
emitter-collector path and does not contribute to the 
base-collector junction breakdown. At the same time, 
this layer increases the base-collector capacitance. 
A second type of construction that increases the base-
collector capacitance is shown in Figure 2. This photo-
SECTION A-A 
Figure 1.
(continued overleaf) 
This document was prepared under the sponsorship of the National
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Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000346 2020-03-17T03:30:22+00:00Z
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SECTION A-A

Figure 2. 
transistor includes an additional base-collector junction 
which is provided by diffusion of n region into the base 
periphery (p for p-n-p). This additional region connects 
to the collector region by a common peripheral boundary. 
Again, being out of the emitter-collector path, this region 
provides additional base-collector capacitance without a 
decrease in the maximum base-collector operating voltage. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Code A&TS-TU 
Huntsville, Alabama 35812 
Reference: B72-10347
Patent status: 
This invention is owned by NASA, and a patent appli-
cation has been filed. Inquiries concerning non-exclusive 
or exclusive license for its commercial development 
should be addressed to: 
Patent Counsel 
Marshall Space Flight Center 
Code A&TS-PAT 
Huntsville, Alabama 35812 
Source: D. H. McCann of

Westinghouse Electric Corp.

under contract to

Marshall Space Flight Center

(M FS-2 0407) 
B72-10347	 Category 01 


Overlap diffusion for increasing phototransistor dynamic range

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