handbook of modern sensors

422 14 Light Detectors
of generated carriers per second per unit of volume. For a CdS cell, µ n τ n ≫ µ p τ p ;
hence, conductance by free holes can be ignored. Then, the sensor becomes an n-type
semiconductor. Thus,
σ = ef µ n τ n . (14.18)
We can define the sensitivity b of the photoresistor through a number of electrons
generated by one photon (until the carrier life span ends):
b = τ n
t t
, (14.19)
where t t = l 2 /Vµ n is the transit time for the electron between the sensor’s electrodes,
l is the distance between the electrodes, and V is the applied voltage. Then, we arrive
at
b = µ n τ nV
l 2 . (14.20)
For example, if µ n = 300 cm 2 /V s, τ n = 10 −3 s, l = 0.2 mm, and V = 1.2 V, then
the sensitivity is 900, which means that a single photon releases 900 electrons for
conduction, making a photoresistor work as a photomultiplier. Indeed, a photoresistor
is a very sensitive device.
It can be shown that for better sensitivity and lower cell resistance, the distance
l between the electrodes should be reduced, and the width d of the sensor should
be increased. This suggests that the sensor should be very short and very wide. For
practical purposes, this is accomplished by fabricating a sensor in a serpentine shape
(Fig. 14.14B) where the electrodes are connected to the leads.
Depending on the manufacturing process, the photoresistive cells can be divided
into the sintered type, single-crystal type, and evaporated type. Of these, the sintered
type offers high sensitivity and easier fabrication of large sensitive areas, which
eventually translate into lower-cost devices. The fabrication of CdS cells consists of
the following steps.
1. Highly pure CdS powder is mixed with appropriate impurities and a fusing agent.
2. The mixture is dissolved in water.
3. The solution in a form of paste is applied on the surface of a ceramic substrate
and allowed to dry.
4. The ceramic subassemblies are sintered in a high-temperature oven to form a
multicrystal structure. At this stage, a photoconductive layer is formed.
5. Electrode layers and leads (terminals) are attached.
6. The sensor is packaged into a plastic or metal housing with or without a window.
To tailor a spectral response of a photoresistor, the powder of step 1 can contain some
variations; for instance, the addition of selenide or even the replacement of CdS for
CdSe shifts the spectral response toward longer wavelengths (orange and red).
To illustrate, how photoresistors can be used, Fig. 14.15 shows two circuits. Circuit
A shows an automatic light switch which turns lights on when illumination drops (the
turn-off part of the circuit is not shown). Circuit B shows a beacon with a free-running
multivibrator, which is enabled at darkness, when the resistance of a photoresistor
becomes high.