Electron transport is caused by an electric field, a thermal gradient, or a concentration gradient. In the latter two cases, a uniform condition is established as a result of the transport unless external sources are used to maintain the nonuniform conditions. The transport of electrons under these conditions may cause transfer of energy from one part of the sample to the other, but the electrons do not gain energy from the external sources in the process of transport. On the other hand, when the transport is caused by an electric field, electrons are continuously supplied with energy from the source of the electric field at a rate J · ε (J is the current density and ε is the electric field), and it would appear that the total energy of the electron system should go on increasing indefinitely. This, however, does not happen as this gain of energy is balanced by transfer of energy to the lattice atoms through the process of collisions. It has been seen that an electron is scattered by the lattice either by emitting or by absorbing a phonon. The lattice absorbs energy from the electron when a phonon is emitted and it delivers energy to the electron when a phonon is absorbed. In the absence of the electric field the absorption and emission processes are so balanced that there is no net transfer of energy from the electron system to the lattice system or the vice versa. It means in effect that the temperature, the thermodynamic coefficent determining transfer of energy from one system to another, of the electron and that of the lattice system are identical.
KeywordsMicrowave Anisotropy Sulphide Mercury Cadmium
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