Poole-Frenkel process

Ben-Chorin etal. (1994) Nanoporous Al/PS/c-Si/Al Poole-Frenkel process... 

The results of several studies were interpreted by the Poole-Frenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summarized (57). 

A limitation of the model of Abkowitz and coworkers is the assumption that the thermal generation process is field independent. A depletion model that includes a field-dependent thermal generation process has been proposed by Kasap et al. (1987). The model is based on the thermal generation of carriers from localized states via a Poole-Frenkel lowering of a Coulomb potential. The model assumes that the distribution of localized states is exponential in energy. According to Kasap et al., the field dependence of the thermal generation process is... 

In order to predict absolute dielectric strengths we need to have more detailed information than is yet available about electronic states and mobilities in polymers. For the present we can only conclude that there is satisfactory agreement between the form of the theoretical results, based on a rather general electronic model, and the best experimental results. To the extent that the model is a very reasonable one, we can say that we can understand intrinsic breakdown behaviour. Measurement of pre-breakdown currents, especially with pointed electrodes which impose regions of very high field strength at their tips when embedded in the material, suggests that electronic carrier production either by injection from the electrodes (Schottky emission) or from impurities (Poole-Frenkel effect) may play a part in the breakdown process. More work is required, however, before this can be fully understood. 

In special cases, inelastic processes contribute too, for example, by the Poole-Frenkel effect [61]. In photoelectrochemistry, finally, the photocurrent is initiated by vertical electron/hole pair generation with following charge separation. 

In addition to the Poole-Frenkel effect and the field-induced tunneling from traps to conduction band states, the Zener effect (field-induced transitions from valence band to conduction band) and various forms of avalanche breakdown effects, can give a bulk conductivity rising sharply with field. These effects are difficult to assess in the present systems, because little is known about the electronic states in amorphous oxides, the electronic transport process, or the lattice vibration spectrum. 

Fig. 5 Schematic representation of photo excitation processes in an n-type oxide film (a) localized state to localized state, (b) localized state to conduction band, (c) valence band to localized state, (d) excitation to localized state followed by escape to conduction band by Poole-Frenkel effect, (e) excitation to localized state followed by electron tunneling into the electrolyte.

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