Page 104 - Photonics Essentials an introduction with experiments

Photoconductivity

98 Photonic Devices

cludes all the materials from which photodiodes can be made plus in-
sulators and organic materials. As a result, there is a much wider va-
riety of applications for photoconductors than for photodiodes. These
applications include light and motion sensors, photographic film, pho-
tocopiers, and television camera sensors.
Photoconductivity occurs in most materials because the number of
mobile charge carriers is increased upon illumination. The increase in
conductivity is linearly proportional to the photon flux. In some mate-
rials, the absorption of photons increases the number of carriers and
their mobility at the same time. Noncrystalline semiconductors such
as amorphous silicon are examples of this kind of behavior. Equation
5.7 shows that the photocurrent will be nonlinearly proportional to
the photon flux because of its dependence on both the carrier concen-
tration and the mobility. This nonlinear behavior is well suited for
threshold detection. Since amorphous silicon is inexpensive to deposit
and to process compared to crystalline silicon, it is widely used to
make the photoconductive detector elements in motion sensors.
The photoconductive response depends on the ratio of the carrier
lifetime to the transit time between the electrodes. The sensitivity of
the photoconductor is proportional to the carrier lifetime. The quan-
tum efficiency is defined by the number of electrons collected per inci-
dent photon, and it is straightforward to design a photoconductor
with a quantum efficiency much greater than unity. The gain is given
by the ratio of the lifetime to the transit time. Over a considerable
range of applied bias, the transit time will decrease in proportion to
the applied voltage. Thus, the quantum efficiency of a photoconductor
can be tuned by the bias voltage. In comparison, a quantum efficiency
of unity is the best that can be achieved using a photodiode. In addi-
tion, the response of the photodiode does not depend on the bias volt-
age. Photoconductive gain is achieved at the expense of bandwidth.
Photoconductors and photodiodes of the same material can be com-
pared under unity gain conditions, and their performance is quite
similar.
The response of a photoconductor can be engineered. Using a single
material, for example silicon, it is possible to engineer the spectral re-
sponse from the visible to the far infrared. The spectral response is
tuned by the introduction of specific impurities having a well-defined
level with an energy in the band gap that corresponds to the spectral
region of interest.
The sensitivity and the bandwidth can be engineered through both
the geometry of the electrodes and the introduction of specific levels.
The lifetime can be engineered to match the bandwidth of the events
one is detecting, and the resulting gain acts like a built-in amplifier.
This feature has made photoconductive detectors the element of

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.