Bands band edge

The efficiency of the electron transfer reactions illustrated in Fig. 1 governs a semiconductor s ability to serve as a photocatalyst for a redox reaction. This efficiency is, in turn, a function of the positions of the semiconductor s conduction and valence bands (band-edge positions) relative to the redox potentials of the adsorbed substrates. For a desired electron transfer reaction to occur, the potential of the electron acceptor should be below (more positive than) the conduction band of the semiconductor, whereas the potential of the electron donor is preferred to be above (more negative than) the valence band of the semiconductor. For an efficient organic synthesis via oxidative photocatalysis, the substrates must have potentials more negative than the valence band of the semiconductor. For an efficient organic synthesis via reductive photocatalysis, it is the reverse. 

Another important concept in band theory is that of effective mass. In a semiconductor, most of the charges reside at the edge of the conduction or valence band. Band edges can be approximated to parabolic bands, by analogy with the free electron dispersion law, Eq. (3)... 

The major question is where to put the Fermi level with respect to the flat-band band edges (step 3). Marking this position will determine on which side of the heterojunction most of the band bending occurs. When one semiconductor is very heavily doped the choice is easy. Mark the Fermi level with respect to the flat band edge of the heavily-doped material. There will be very httle band bending in that material (see Figure 3.22). 

E is tire density of states between E and E + AE. A simpler way of calculating n is to represent all tire electron states in tire CB by an effective density of states at tire energy E (band edge). The electron density is tlien simply n = NJ (Ef. 

Micic O I ef a/1996 Highly efficient band-edge emission from InP quantum dots Appi. Phys. Lett. 68 3150... 

Fig. 1. Band-edge energy diagram where the energy of electrons is higher in the conduction band than in the valence band (a) an undoped semiconductor having a thermally excited carrier (b) n-ty e doped semiconductor having shallow donors and (c) a -type doped semiconductor having shallow acceptors.

Fig. 2. Representation of the band edges in a semiconductor p—n junction where shallow donor, acceptor energies, and the Fermi level are labeled Ejy E, and E respectively, (a) Without external bias is the built-in potential of the p—n junction (b) under an appHed forward voltage F. ...

Fig. 4. Schematic cross section and the band diagram of a double heterostmcture showing the band-edge discontinuities, AE and AE used to confine carriers to the smaller band gap active layer, (a) Without and (b) with forward bias. See text.

Fig. 6. Band edge positions of several semiconductors ia contact with an aqueous electrolyte at pH 1 ia relation to the redox (electrode) potential regions (vs the standard hydrogen electrode) for the oxidation of organic functional groups (26,27).

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

In Modulation Spectroscopy, which is mosdy used to characterize semiconductor materials, the peak positions, intensities and widths of features in the absorption spectrum are monitored. The positions, particularly the band edge (which defines the band gap)> are the most useful, allowing determination of alloy concentration. 

Bande,/. edge, border band gang. Bandeisen, n. hoop iron, band iron. Bandenlinie, /. band line. 

Band-feder, /. flat spring, -fdrderer, m. belt conveyor, -kante, /. edge of a band, band head, -mass, n. tape measure, -stabl, m. band steel, strip steel, bandstreifig, a. banded, streaked, striped. Band-trockuer, m. belt drier, -wunn, m. tapeworm. 

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