Photoconductor records image when holes and electrons are trapped inside it due to incident photons. Image can be read out by exposing photoconductor to scanning laser beam. Photons from scanning laser empty traps, generating photocurrent. Image information is obtained by detecting this photocurrent synchronously with laser scan

Maserjian, J.

English

NASA Technical Reports Server

Juhe 1975	 B75-10112 
NASA TECH BRIEF 
NASA Pasadena Office 
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 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Laser Scanned Image Sensors Using Photoconductors With Deep Traps 
.
Improved image sensors used on TV cameras can 
be made with photoconductors containing deep traps 
for carriers, electrons and holes. Basically, the 
photoconductor records an image when the holes and 
the electrons are trapped inside it due to the incident 
photons. This image in the form of accumulated 
trapped carriers can be instantly read out by exposing 
the photoconductor to a scanning laser beam. 
Photons from the scanning laser empty the traps, 
generating a photocurrent. Image information is 
obtained by detecting this photocurrent synchro-
nously with the laser scan. 
A simplified diagram of the process is shown in the 
illustration. Photons of incident light from an image 
strike the photoconductor with energy hv (where h is 
Planck's constant and v is the frequency) greater than 
the band gap AE of the photoconducting semicon-
ductor. The incident photons generate electron-hole 
pairs which are trapped in empty trap states. The 
process continues during the period of exposure,
typically a few seconds, until the accumulated trapped 
carriers have integrated the incident flux of photons. 
The accumulation of trapped carriers (electrons 
and holes) is instantly read out by emptying the traps 
by exposure to a laser beam of photon energy hv1, 
where LEt <hv1 <iE, and AEt is the maximum trap 
energy. During the laser exposure, the photocurrent is 
released by maintaining a constant voltage bias across 
the photoconductor at all times during image sensing, 
using two parallel plate conducting contacts. 
The process is analogous to conventional vidicons in 
that it integrates the image of incident photons in the 
form of an electrical charge and then senses the 
integrated charge. In conventional vidicons, the 
charge is normally stored on the opposite surface of 
the photoconductor (or photodiodes for a silicon 
vidicon). The charge then is read off at each point by a 
scanning electron beam. In the new process, the 
image is integrated in the form of trapped charge 
carriers which are released by the scanning laser 
Incident	 Transparent 
Light	 Conducting 
hv>E	 Plates 
-v +
Scanning 
Laser	 Photoconductor 
Beam	 With Deep Traps 
Et<hvi<E 
Note: Only one plate needs to be transparent If Image 
Photocurrent
	 light and laser beam are Incident from same side. 
Detector
Laser Scanned Image Sensor Using Photoconductor With Deep Traps
(continued overleaf) 
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=19750000112 2020-03-19T20:05:55+00:00Z
beam. The image is sensed through an electrical 
circuit. The laser can be scanned by nonmechanical 
means such. as acoustic optic X-Y deflection systems. 
The proposed method can improve the image 
sensors in several ways: 
a. High-energy electron optics used in standard 
image sensors can be eliminated. This avoids some 
serious reliability and failure problems, such as 
limited cathode life and the degradation of photo-
sensitive material (particularly silicon in silicon 
vidicons) due to X-ray generation. 
b. Since no discrete structures (such as diode arrays) 
are needed and little lateral spread is anticipated, 
very high resolution may be obtained approaching 
fundamental optical limits. 
c. System sensitivity should be equivalent to the most 
sensitive vidicons (photon noiselimited) because 
of the same absorption mechanisms, leading to 
high quantum efficiency and the same kind of 
integrating feature. 
d. A wide choice of photoconductors is possible 
because .of the minimal requirements on the band 
gap and the presence of deep traps. The preferred 
band gap is in the neighborhood of 1 to 2.5 eV. 
The upper limit is set by practical optical cutoff 
wavelengths; the lower limit is set by excessive 
dark currents due to thermal generation (unless 
cooled). Deep traps are easily introduced into 
semiconductors and often occur naturally-
e. Finally, images may be stored for long times 
(hours) without cooling. The long storage time is 
required for transferring the image data to tapes. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP75-101 12 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,865,975). Inquiries concerning non-
exclusive or exclusive license for its commercial 
development should be addressed to: 
Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Joseph Maserjian of 
Caltech/JPL
under contract to

NASA Pasadena Office
(NPO-13131)
[I 
Categories: 03 (Physical Sciences) 
02 (Electronics Systems) 
B75-10112
	 04 (Materials)


Laser scanned image sensors using photoconductors with deep traps

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