TABLE OF CONTENTS Photons that are incident on a semiconductor material can be absorbed if their energy lies above the band gap energy. Energy is conserved by the breaking of a bond: that is, the promotion of an electron from the valence band to the conduction band. The presence of these additional charge carriers, one electron and one hole, increases the conductivity o f the semiconductor. This is photoconductivity. If the semiconductor has been processed with ohmic contacts and is placed in a circuit, it will behave like a light-controlled resistor. Ingenious implementations of photoconductivity have been devised by both mankind and by nature. Detection of light by photoconductivity differs from the detection of light by a photodiode in one significant way. The photoconductive detector can be designed to have built-in gain. That is, the absorption of a photon can lead to the generation of many electrons in the resulting photocurrent, whereas a photodiode has a gain that is less than or equal to unity under normal operating conditions.
The electrical conductivity of a semiconductor material is the product of the density of free charge carriers N, the charge on the electron, and the mobility of the charge carrier, :
| (5.1) |  |
The mobility, , is a measure of how easily an electronic charge can propagate through the semiconductor structure. The mobility of an electron is different from the mobility of holes, and for the vast majority of semiconductors, it turns out that e
