Page 83 - Photonics Essentials an introduction with experiments

Source: Photonics Essentials

Chapter
5

Photoconductivity

5.1 Introduction
Photons that are incident on a semiconductor material can be ab-
sorbed if their energy lies above the band gap energy. Energy is con-
served 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 of 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 dif-
fers 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 condi-
tions.

5.2 Conductivity and Mobility

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, :
= Nq ( -cm) –1 (5.1)
The mobility, , is a measure of how “easily” an electronic charge can
propagate through the semiconductor structure. The mobility of an

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