Charge transport in non-crystalline semiconductors

Abstract

A description is given of the theory of the transport properties of electrons in non-crystalline solids and liquids. Two different mechanisms by which an electron can move are distinguished. One is that familiar in crystalline semiconductors and metals, in which the mobility is not thermally activated and a mean free path can be defined. The other is thermally activated hopping which occurs particularly in impurity conduction, and in which the electron jumps from one localised state to another with the emission or absorption of phonons. When the electron states are localised, the conductivity tends to zero with the temperature, even though the density of states is everywhere finite. Some experimental work is described which distinguishes the two types of charge transport and shows what happens at the transition between them. This includes impurity conduction in compensated semiconductors and conduction in cerium sulphide containing a high concentration of vacancies. The model established by this work is applied to chalcogenide glasses and to amorphous germanium. It is shown also that it provides a satisfactory explanation for the behaviour of threshold switches making use of a conducting glass as the heart of the device.