Thermal Conductivity of Metals and Semiconductors as a Function

THERMAL CONDUCTIVITY OF METALS AND SEMICONDUCTORS AS A FUNCTION OF TEMPERATURE... 

Thermal Conductivity of Metals and Semiconductors as a Function of Temperature.12-220... 

Returning now to thermal conductivity, Eq. (4.40) tells us that any functional dependence of heat capacity on temperature should be implicit in the thermal conductivity, since thermal conductivity is proportional to heat capacity. For example, at low temperatures, we would expect thermal conductivity to follow Eq. (4.43). This is indeed the case, as illustrated in Figure 4.25. In copper, a pure metal, electrons are the primary heat carriers, and we would expect the electronic contribution to heat capacity to dominate the thermal conductivity. This is the case, with the thermal conductivity varying proportionally with temperature, as given by Eq. (4.42). For a semiconductor such as germanium, there are less free electrons to conduct heat, and lattice conduction dominates—hence the dependence on thermal conductivity as suggested by Eq. (4.41). 

The highest energy occupied allowed band of a metal, or conduction band, is only partially filled with electrons, up to the so-called Fermi level. Hence, electrons located close to this Fermi energy are easily excited to the unoccupied level of the band, where they behave as free electrons. In a semiconductor (like in an insulator), the highest occupied allowed band is totally filled, and called valence band (VB), whereas the conduction band (CB) corresponds to the lowest unoccupied allowed band, which is completely empty. The injection of electrons in the CB occurs either thermally (in an intrinsic semiconductor) or through doping (extrinsic semiconductor). Electrons in the conduction band of metals or semiconductors move in delocalized states, and their wave function can be approximated to that of a free electron, that is, a progressive plane wave... 

In conducting polymers, the extra carriers added upon doping are able to drift under an applied electrical field. In semiconducting polymers, no carriers are available except those thermally excited across the gap. However, negative (positive) carriers can be injected into the material by metallic contacts when the barrier between the metal work function and the LUMO (HOMO) molecular levels is overcome. Then, the injected carriers can move inside the semiconductor if a bias field is applied. Injection of carriers and their transport is a fundamental issue for all electronic devices and transistors in particular. In the following, main transport properties of organic semiconductors (both small molecules and polymers-based) used as active materials in transistors will be reviewed. 

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