Excess carrier density

In pulse-power applications, the thyristor is required to supply energy to the load in a very short period of time, which requires that it provide extremely high-current slew rates. During turn-on, excess carrier density increases near the gate area and then spreads throughout the device. As the excess carriers spread, the anode-to-... 

Fig. 64. Calculation of surface recombination velocity vs. electrode potential. Fig. 65. Reciprocal surface recombination velocity vs, excess carrier density Snw.

The charge-induced defect creation mechanism is too slow to be significant at low temperature and the electronic recombination effects reestablish themselves. Low temperature measurements (0-100 K) have been performed using an IR probe beam to modulate the excess carrier density that is in the band tail states (Hundhausen, Ley and Carius... 

Several issues remain to be resolved in studies of electron transport at nanocrystalline semiconductor-electrolyte interfaces. Among these are those related to the variation of excess carrier density with distance (and consequent variations in D and T across the film), the importance of field-assisted transport phenomena, the role of temporary localization of electrodes in traps, the influence of solution species on transient profiles and the like. However, rapid progress is being made in these directions as exemplified by several published studies [338, 349, 352, 353-355]. 

Fig. 5.2-66 Dependence of the functions G+, G , g on Vs for various values of bl- The functions defined in (2.14), (2.15) can be used to obtain the excess carrier densities for majority and minority carriers in accumulation and...

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

Cartiers can also be generated in a semiconductor by the absorption of light or injected into the semiconductor from ap—n or Schottky junction. In either case, as soon as the source is removed the density of those excess carriers begins to decrease exponentially with time. The time it takes for the density to be reduced to 1/ of the original value is defined as the carrier lifetime, T. For siUcon, T is typically in the microsecond range. 

The excess free carrier density decays (via recombination) with those recombination centers that have the largest cross section. 

In metals, the concentration of mobile electrons is enormously high so that the excess charge is confined to a region very close to the surface, within atomic distances [14, 15]. In semiconductors with substantially less charge carrier density, on the other hand, a region of spatial charge distribution can be found [16, 17]. 

Several other sources of external excitation result in metastable defect or dopant creation in a-Si H. Most have the characteristic property that a shift in the Fermi (or quasi-Fermi) energy causes a slow increase in the density of states and that annealing to 150-200 °C reverses the effect. The phenomena are therefore similar in origin to the optically-induced states and fall within the same general description of departures from the thermal equilibrium state induced by excess carriers. 

A similar treatment can be used to calculate the electric field and the electric potential in the metal. However, in a metal, both the electric field and the electric potential drop to zero at a very short distance from the semiconductor/metal interface. This occurs because metals do not support electric fields, and all of the excess charge density resides on the surface of the metallic phase. The surface dipole layer is therefore effectively screened from test charges at any finite distance into the metallic phase, and the width of the electric potential gradient is extremely small. Because charge carriers can pass freely through this extremely thin barrier, only the electric field in the semiconductor significantly affects the electrical properties of semiconductor/metal contacts. 

Taking a p-type electrode as an example, the electrode is illuminated through the electrolyte as illustrated in Fig. 4.6. The diffusion of the excess minority carrier density An is given by the continuity equation [10]... 

The material is assumed to be an insulator, i.e. the thermally-generated charge-carrier density is so small that it makes no noticeable contribution to the transport and therefore can be neglected in the model. All the charge carriers which participate in the transport or are captured in traps are excess charge carriers injected from the contacts. 

An excess of one of the components restricts the type of conduction that can be obtained and, therefore, the compensation hypothesis has to be rejected. Moreover, it is known that ions such as Sn do not increase the carrier density in PbX (Table 3). 

By calculating the carrier density from the Hall coefficient [15], it is concluded the excess atoms replacing Si in CrSi2 are singly charged acceptors and each molecular defect in stoichiometric samples gives 0.5 of a hole to the valence band. 

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