Page 85 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 79

and
1
v h = 600 · = 6 × 10 cm-sec –1 for holes
5
10 × 10 –4
(The linear relationship between drift velocity and electric field no
–1
longer holds for GaAs when the electric field is larger than 10 V-cm .)
3
The transit time is:
10 × 10 –4
t e = = 1.25 × 10 –10 sec for electrons
8 × 10 6
and
10 × 10 –4
t h = = 1.67 × 10 –9 sec for holes.
6 × 10 5
This example shows that the transit time of electrons and holes can
be quite different. This difference plays a very important role in deter-
mining the device properties of photoconductive detectors.

5.3 Gain and Bandwidth

A frequently used photoconductor design consists of a semiconductor
material with ohmic contacts across which a voltage is maintained, as

10 microns

Figure 5.1. Schematic diagram of a GaAs photoconductive detector created by deposit-
ing two ohmic contacts in an interdigitated array on a GaAs wafer.

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