Trap-controlled band transport

Instead, transport is most likely a hopping process involving states associated directly with the silicon backbone. Trap-controlled band transport. 

Trap-Controlled Hopping. In trap-controlled hopping, the scenario described for trap-controlled band mobility applies. However, the microscopic mobility is associated now with carriers hopping in a manifold of localized states. Overall temperature and field dependence reflects the complicated convolution of the temperature and field dependence of both the microscopic mobility and the trap kinetic processes. Glearly, the observed behavior can now range from nondispersive to anomalously dispersive behavior as before, depending on the energy distribution of transport-interactive traps. 

Figure 35 Electron transport, (a) Quasifree electrons, band mobility (b) trap-controlled band mobility (c) hopping transport.

The transport of these directly or indirectly formed charge carriers can be described by a band model and/or a hopping mechanism. Important parameters controlling the transport are bandwidths, concentration, energetic and spatial distribution of defects and traps. 

A solid or liquid dielectric inserted between two electrodes can support only a limited voltage. Several physical mechanisms can lead to a current instability and to breakdown, e.g., thermal instabilities in materials with thermally activated conductivities, transitions from trap-controlled transport to band transport, impact ionization, etc. (Zeller, 1987). Which one of the different mechanisms ultimately determines the dielectric strength depends on materials parameters, geometry, voltage pulse forms (including history), temperature, etc. (O Dwyer, 1973). 

It was pointed out later by Gill [16] that, although the model was able to predict the correct field dependence, and even its correct magnitude, it could be objected that the use of a trapped controlled mechanism assumes a transport in delocalized bands (see section 5.1.4). Experimental values of the microscopic mobility, which range from 10 to 10 , do not agree with such a transport. However, this objection could be removed by the model developed by Jonscher and Ansari [17], which assumes a thermally stimulated hopping transport in an energy distribution of localized states. 

Some workers have correlated the pitting resistance and the number of pre-pitting transients (see further) to the electronic properties of the passive film considered as a n-type semiconductor and have found that the number of transients increased with the concentration in deep localized electronic traps in the band gap. Since the electronic defects likely result from the presence of atomic defects (such as cation vacancies), it suggests that the accumulation of these defects is responsible for the film breakdown. It was also found that UV irradiation increases the resistance to pit nucleation, which may result from some modifications induced in the electric field controlling the transport of vacancies. ... 

The results taken as a whole reveal the existence of at least three different trap species in the band gap of Sb(As)j Sei noncrystaUine semiconductors. These species are located at energies 0.22, 0.34, and 0.45 eV, respectively, below the conduction band edge and control the electron transport properties of the material. It seemed that Sb and As introduce a new set of detectable charge-carrier traps. 

The model rests on the following assumptions (1) Carriers arriving at a trap are instantaneously captured with a probability close to one and (2) the release of trapped carriers is controlled by a thermally activated process. The resulting effective mobility is related to the mobility J,o in the transport band by an equation of the form... 

Several completely different experiments support our interpretation of the time-of-flight transport process and the conclusions we have drawn about the distribution of band-tail states. The time-resolved photoinduced absorption experiments of Ray etal. 9% ) support the view that the photogenerated holes are concentrated in the vicinity of an energy E, which moves deeper into the localized state distribution, linearly with temperature and logarithmically with time. Furthermore, the time decay of the photoinduced absorption, which is controlled by the more mobile of the two carriers (electrons), has the t form expected from the multiple trapping model (see, for example, Orenstein eta/., 1982). Thea = r/300°K temperature dependence for a reported by Tauc (1982) is in excellent agreement with the electron time-of-flight results. 

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