Page 221 - Organic Electronics in Sensors and Biotechnology

198 Cha pte r S i x

I
2I

I
I = 0
V

(a) (b)

FIGURE 6.4 (a) Typical photoresistor. (b) The conductivity of a photoresistor
increases in approximate proportion to the intensity of light, providing a
simple low-cost light sensor.

be derived from the internal amplification); F is minimized if either α
n
or α is equal to zero, in which case the noise factor reduces to 2. Intri-
p
cate device architectures have been developed to maximize the differ-
ence in the ionization coefficients and so approach the ideal situation
F = 2 (which, for a typical gain of 100, would yield a 50-fold improve-
ment in the signal-to-noise ratio). The relative complexity of APDs and
their stringent manufacturing requirements lead to high fabrication
costs, with high-end avalanche photodiodes often selling for several
thousand U.S. dollars.
Photoresistors (Fig. 6.4) —also known as photoconductors or
2
photoconductive cells—are closely related to photodiodes in the
sense that they too use a semiconductor as the photoactive medium.
However, unlike photodiodes, they contain no junction and so pro-
vide no driving force for charge separation in the absence of an
applied electric field. The density of photogenerated charge and
hence the photoconductivity increases in direct proportion to the
intensity of incident light. Photoresistors are typically fabricated by
a simple two-step process, in which a thick layer of photosensitive
material such as CdS is first deposited onto a substrate, and inter-
digitated metal electrodes are then deposited by thermal evapora-
tion on top of the active layer. Photoresistors have the benefit of
being tolerant to high operating voltages of several hundred volts,
enabling direct AC usage and, due to their simplicity of fabrication,
are extremely low-cost devices. However, they are inherently less
sensitive than photodiodes since they require an applied voltage to
operate (which generates a dark current and associated noise, see
Sec. Shot Noise), and they also tend to respond slowly on a > 10 ms
time scale. Photoresistors made from mercury telluride and cad-
mium telluride offer a means of accessing the 2 to 15 μm range
of the optical spectrum (which is inaccessible to photodiodes),