Mobility edge level

Fig. 2-33. Electron energy and state density in amorphous semiconductors A and C = diffuse band tail states B = gap states, cmc = mobility edge level for electrons MV = mobility edge level for holes ...

The distribution of formation energies therefore follows the density of valence band tail states. The dangling bond energy level is estimated to be about 0.8 eV above the valence band mobility edge and the band tail extends at least 0.5 eV into the gap, so that the energies are reasonably consistent with the known defect densities. The only justification for the model is that the creation of the defect occurs without a large... 

If the Fermi level lies above the mobility edge the electron wavefunctions are sufficiently delocalised that transport is possible and the material is metallic. If the Fermi energy lies below the mobility edge the electrons are localised and the material is insulating. In this case conduction can occur by either ... 

Such an observation of strong VG and T dependent EA can be explained using the multi trap and release model which assumes a semiconductor with Fermi level closer to the band edge and upon applying VG the Fermi level moves through the distribution of band tail states. As a result EA is reduced as the density of injected charge carriers are increased above the mobility edge [4, 6-11],... 

Fio. 52. Summary of the density-of-states results for five different a-Si H films with differing amounts of phosphorus doping (see description in text). The films have slightly different deduced band gaps, and for purposes of comparison the curves are all normalized to the conduction-band (mobility) edge E. The position of the bulk Fermi level in each film is indicated. [From Lang et at. (1982a).]... 

Since the electron density is strongly peaked alE, it is possible to treat the drift mobility problem as a two-level problem a conducting level near the mobility edge with an effective density of states and a single trap level at E. This trap level has the unusual feature that its energy position E and number density are both time dependent. 

Generally, the aim of doping a semiconductor is to control the electronic properties exclusively by shifting the Fermi energy. In the study of a-Si H, the question arose early as to whether the incorporation of dopants causes side effects as well. The formation of a phosphorus impurity band 0.13 eV below the conduction-band mobility edge E. has been proposed by LeComber et al (1977) from their results of Hall effect experiments. An arsenic impurity level 0.35 eV below E,. and a boron impurity level 0.42 eV above the valence-band mobility edge Ey have been inferred by Jan et al (1979,... 

In a-Si H one has not yet been able to observe the nonmetal-metal transition that is expected at large F, when is moved below or above the Fermi level at the surface. There should not be any difficulty in principle except for the question of whether the mobility edges E and E inde follow the internal potential F(x). The mobility edges separate in three dimensions the localized and extended states. In a narrow 20-50-A-wide slice of the potential well F(x) near the surface, it is likely that at least the low-eneigy extended states become localized because of the decrease in dimensionality. As a consequence, E (or E ) may not follow F(x) up to the surface but instead meet the surface with a horizontal slope. This will decrease the conductance compared to the value calculated fixrm Eq. (14). 

In the critical and insulating regimes, the resistivity ratio and positive MR increase as the extent of disorder increases. The field-induced transitions, from the metallic to the critical and from the critical to the insulating regimes, show that the mobility edge and Fermi level are situated rather close. Hence, due to interchain transport and disorder, conducting polymers are at the metal-insulator boimdary. 

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