Page 88 - Photonics Essentials an introduction with experiments

Photoconductivity

82 Photonic Devices

The action of the electric field continues. The electron, with a veloci-
ty more than 10 times greater than the hole, traverses the semicon-
ductor and is collected on the positive contact, causing yet another elec-
tron to be introduced at the negative contact. The hole, meanwhile, is
still trying to reach the negative contact. This is shown in Fig. 5.6.
Finally, the hole reaches the negative contact and is collected in the
external circuit. The photoconductivity stops because there are no extra
charge carriers. The length of the photoconductivity event is deter-
mined by the transit time of the slower carrier, the hole. From this time,
we can determine the bandwidth of the detector. During this time, 12
electrons have traversed the sample and the external circuit in order to
maintain charge neutrality. The current due to these 12 electrons was
initiated by the absorption of one photon. The ratio of the number of
electrons collected per incident photon is the photoconductive gain.
The sequence of events illustrated in Figs. 5.2 through 5.7 illus-
trates the origin of photoconductive gain and bandwidth. The band-
width is determined by the transit time of the slower charge carrier.
In the discussion that follows, we will assume that this is the hole.

L L
t h = =
v h
where L is the electrode separation and = V/L is the electric field.
The bandwidth of the photoconductive detector is

1 h V
B = = (5.3)
L 2
t h

Figure 5.6. The electron traverses the space between the contacts, and is about to be
collected by the positive contact. Meanwhile, the hole is still moving more slowly to-
ward the negative contact.

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