Читать книгу Flexible Thermoelectric Polymers and Systems - Группа авторов - Страница 22

The temperature dependence of the conductivity of semiconductors is quite different from metals. The temperature can affect both the charge carrier density and the charge carrier mobility of semiconductors. There are three temperature ranges in terms of the temperature effect on the charge carrier density. At the low temperature range below the saturation temperature (T _s), the thermal energy is less than the ionization energy of the doping atoms. As a result, not all the doping atoms ionize. The electron (or hole) density in the conduction band (or valence band) depends on the thermal excitation of the electrons (or holes) from the dopant to the conduction band (or valence band). The charge carrier density increases with the increasing temperature. In the medium temperature of T _s < T < T _i with T _i being the intrinsic temperature, the thermal energy is high enough for the ionization of all the dopants, but it is lower than the energy bandgap between the valence band and the conduction band. The charge carrier density equals the dopant density, and it is constant in this temperature range. At the high temperature range of T > T _i, the thermal energy can excite the electrons from the conduction band to the valence band. This produces electrons in the conduction band and holes in the valence band. In this high temperature range, both the electron and hole densities increase with the increasing temperature.

There are two temperature regimes for the temperature dependence of the charge carrier mobility of semiconductors. At low temperature regime, the ionized impurities are the charge carrier scattering centers. In this temperature range, the charge carrier mobility increases with the increasing temperature because the scattering area decreases. At high temperature, the scattering by the lattice vibration becomes the dominant factor for the charge carrier mobility. Similar to metals, the charge carrier mobility decreases with the increasing temperature in the high temperature regime.

Therefore, the temperature dependence of the conductivity of semiconductor is more complicated than that of metals. At low and high temperature range, the conductivity of semiconductor increases with the increasing temperature. The conductivity can be less sensitive to the temperature or even decrease in the medium temperature range.

The temperature dependence of the conductivity is usually used to determine whether a conductor is metallic or semiconductive. In general, if the conductivity decreases with the increasing temperature, it is metallic. In contrast, if the conductivity increases with the increasing temperature, it is considered as semiconductive behavior. This is also frequently used to judge the charge transport mechanism of non‐conventional materials, such as carbon nanotubes, graphene, and charge‐transfer organic salts.