Conductivity, free-carrier concentration

Electrical conductivity, a, is given by the product of free-carrier concentration, n, carrier mobility, p, and carrier charge, e ... 

The proportionality constant between the applied electric field and the resulting drift velocity is called the charge carrier mobility, jx. For electrons, = q r /ml ), for holes, ftp = 7(Trn/mj ). It should be noted that, owing to differences in the effective masses of electrons and holes, their mobilities within a semiconductor may be markedly different. The electrical conductivity, a, of a semiconductor is related to the free carrier concentrations by ... 

At slightly lower frequencies, for u>2 = ujp/eoo, n goes to zero and the reflectance rises to values near unity. The determination of ujr=o when the free-carrier concentration and are known allows determination of the conductivity effective mass. For non-parabolic CBs, the values of m so obtained for different filling factors of the CB are different from those measured at the bottom of the CB. 

Classically, the experimental curve interpretation is often limited to the definition of a distribution function in which the Debye-type relaxation superposition is accounted for by two parameters a and jS. From this description, we learn nothing about the physical processes involved this is often the relaxation frequency zone which permits connection of the observed evolutions to a particular polarisation phenomenon. Moreover, for materials with large free carrier concentration, the conduction and polarisation effects are separated in an arbitrary way, assuming that the contribution to the conduction phenomena is limited to a term e" conduction = (Tdc/cyto over the whole frequency range. 

A is a constant related to the free carrier concentration, the ideal glass transition temperature, a pseudo activation energy. Examples of conductivity temperature relations are shown in Figure 1.9. 

A new evidence supporting influence of increased mobility on ionic conductivity comes from work on segmented polyurethanes. It was discovered that addition of plasticizer to polyurethane, or increase in weight percent of soft segments (internal plasticization of polyurethane), or increase in free carrier concentration, each increases the ionic... 

Due to the small gap of SmS there is at room temperature and even down to the lowest temperatures, an appreciable free carrier concentration in the conduction band. Thus for optical experiments in the far infrared and for energies less than the phonon fi equencies the plasma free carrier absorption dominates the spectrum... 

In general, the dielectric constant is a complex quantity, k = k — Ik". The dielectric constant k is controlled by the conductivity at very low frequencies, and by various polarization mechanisms at higher frequencies. Electronic polarization and molecular dipole moments can be estimated from the refractive indices and dielectric constants. The dissipation factor D = k" k of a dielectric is sensitive to charge carriers and reorientable dipoles. Accurate loss measurements are capable of detecting free-carrier concentrations of 10 /cm. Various contributions to the loss can be separated by determining the dependence on temperature and frequency. 

For insulators, Z is very small because p is very high, ie, there is Htde electrical conduction for metals, Z is very small because S is very low. Z peaks for semiconductors at - 10 cm charge carrier concentration, which is about three orders of magnitude less than for free electrons in metals. Thus for electrical power production or heat pump operation the optimum materials are heavily doped semiconductors. 

In case when the major input into the change of concentration of free carriers is provided by double-ionized oxygen vacancies which is valid at high temperatures the concentration of conductivity electrons is [e] w 2[Vq ], which, recalling expressions (1.121), (1.124) and (1.125) brings us to formula... 

The nature of light absorption in a crystal is of no significance for theory. What is important here is that this absorption be photoelectrically active, i.e., results in a change of the concentration of free carriers in a crystal. This process may take the form either of the so-called intrinsic absorption accompanied by the transition of an electron from the valency to the conduction band, or of the so-called impurity absorption caused by an electronic transition between the energy band and the impurity local level. 

The conductivity K induced by radiation absorption at dose rate I (eV°=cm-3 s-1) is given by K = uc, where c the is free ion concentration and u is the sum of mobilities of positive and negative carriers. The establishment of steady state requires equal rates of generation and recombination, or IGR /100 = kc2 where k is the second-order recombination rate constant. Eliminating c between these... 

Figure 4.22 Schematic diagram of a field effect transistor. The silicon-silicon dioxide system exhibits good semiconductor characteristics for use in FETs. The free charge carrier concentration, and hence the conductivity, of silicon can be increased by doping with impurities such as boron. This results in p-type silicon, the p describing the presence of excess positive mobile charges present. Silicon can also be doped with other impurities to form n-type silicon with an excess of negative mobile charges.

---