Page 103 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 97

has changed this situation. Recombination takes place when a hole
meets an electron on a recombination site. Sensitization has reduced
the hole occupancy of the type 1 sites by six orders of magnitude in
this example. The main result is that all recombination through the
type 1 centers has been cut off. Recombination now takes place almost
entirely on the type 2 sites, where there are both electron and hole
sites available.
Our objective is to calculate the electron lifetime for the sensitized
material. Since generation equals recombination in the steady state,
we can write

n
f = = n(p r1 vs n1 + p r2 vs n2 )
n

1 1
n = =
10 N r1 ·10 ·10 –15 + N r1 ·10 ·10 –20
7
7
–6
p r1 vs n1 + p r2 vs n2
1 1 10 13
= sec = 10 –2 sec (5.23)
10 –14 + 10 –13
N r1 N r1
In analyzing Eq. 5.3, it can be seen that the longer lifetime for elec-
trons reflects the recombination properties of the type 2 centers. The
electron lifetime has been increased by five orders of magnitude from
10 –7 seconds to 10 –2 seconds, and the sensitivity of the photoconductor
is increased by this amount.
The calculation of the hole lifetime for the sensitized material is left
as an exercise at the end of the chapter.
This example shows that it is possible to increase the lifetime of one
of the carriers by addition of an appropriate impurity (or vacancy)
into a photoconductor. The requirements for sensitization are:

The density of the type 2 centers is greater than that of the type 1
centers.
The recombination properties of the type 2 centers are different
from those of the type 1 centers.
The level of illumination is high enough to saturate the type 1 cen-
ters with one type of carrier.

5.7 Summary
Photoconductivity occurs when the absorption of light creates electron
hole pairs that are mobile in an electric field. Photoconductive detec-
tors differ from photodiodes in several important ways. Photoconduc-
tive detectors can be made from a wide variety of materials including
those in which it is not possible to form a p-n junction. This range in-

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