Page 89 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 83

Figure 5.7. Finally, the hole reaches the negative contact. The sample is now back to its
condition before the absorption of the photon. At this point, the photoconductivity
stops. Assuming that the sample was uniformly illuminated, 12 electrons will flow in
the external circuit for every incident photon absorbed.

The gain of the photoconductor is the ratio of the transit time of the
slower charge carrier to the faster charge carrier:

t h e
G = = (5.4)
t e h
The gain and the bandwidth are interrelated, and this relationship
is expressed by the gain–bandwidth product:
1 1 e V
t h
G·B = · = = (5.5)
2
t e t h t e L
It will be helpful to think about the following two cases:
1. The incoming signal has a duration in time much less that the
transit time of the slower charge carrier. In this case, the action of
photoconductive gain will be to increase the signal amplitude by
elongating the signal in time to the transit time for the slower car-
rier. In this case, signal bandwidth is exchanged for signal ampli-
tude.
2. The incoming signal has a time duration that is longer than the
transit time of the slower charge carrier. In this case, the photo-
conductive gain will increase the signal amplitude with only a mi-
nor degradation in the bandwidth of the signal.

Note that in both cases the full photoconductive gain, given by the ra-
tio of the carrier mobilities, is obtained.

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