Valence band of semiconductor

Fig. 8-24. Redox reaction currents via the conduction and the valence bands of semiconductor electrode as functions of electrode potential of semiconductor anodic polarization corresponds to Figs. 8-20, 8-21 and 8-22. i (i )= anodic (cathodic) current in (ip) = reaction crnrent via the conduction (valence) band BLP = band edge level pinning FLP = Fermi level pinning.

Data on Valence Bands of Semiconductors (Room Temperature)... 

PART A. DATA ON VALENCE BANDS OF SEMICONDUCTORS (ROOM TEMPERATURES)... 

The relationship between electron momentum in conduction and valence bands of semiconductors determines the coupling between the two states and the... 

TABLE 4. Band Properties of Semiconductors 4.1. Data on Valence Bands of Semiconductors (Room Temperature)... 

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

The band edges are flattened when the anode is illuminated, the Fermi level rises, and the electrode potential shifts in the negative direction. As a result, a potential difference which amounts to about 0.6 to 0.8 V develops between the semiconductor and metal electrode. When the external circuit is closed over some load R, the electrons produced by illumination in the conduction band of the semiconductor electrode will flow through the external circuit to the metal electrode, where they are consumed in the cathodic reaction. Holes from the valence band of the semiconductor electrode at the same time are directly absorbed by the anodic reaction. Therefore, a steady electrical current arises in the system, and the energy of this current can be utilized in the external circuit. In such devices, the solar-to-electrical energy conversion efficiency is as high as 5 to 10%. Unfortunately, their operating life is restricted by the low corrosion resistance of semiconductor electrodes. 

Charging of the surface accompanying adsorption process and resulting in the change of the energy profile of the bottom of the conductivity band and, naturally, the ceiling of the valence band in semiconductors... 

The lowest level of the conduction band for metals Vc and the highest level of the valence band for semiconductors is very often used as a reference point for the energies of the Fermi levels in this book, however, the energy of a free electron at rest in a vacuum will be used as the reference point for the scale of the Fermi levels (cf. Fig. 3.2.). 

Energy level diagram for an n-type semiconductor-metal photoelectrolysis cell in which the flat-band potential lf(b lies above the H+/H2 potential, whereas the 02/H20 potential lies above the valence band of the n-type semiconductor. 

For instance, the more efficiently the photoholes are trapped from the valence band of an n-type semiconductor, the higher is the probability that the photoelectrons in the conduction band reach the surface and can reduce a thermodynamically suitable electron acceptor at the solid-liquid interface. This is illustrated with an example taken from a paper by Frei et al, 1990. In this example methylviologen, MV2+, acts as the electron acceptor and TiC>2 as the photocatalyst. Upon absorption of light with energy equal or higher than the band-gap energy of Ti02, a photoelectron is formed in the conduction band and a photohole in the valence band ... 

Fig. 2-16. Electron state density distribution and electron-hole pair formation in the conduction and valence bands of intrinsic semiconductors Cf > Fermi level of intrinsic semiconductors.

The concentration of electrons, n, in the conduction band of n-type semiconductors and the hole concentration, p, in the valence band of p-type semiconductors are given by Eqn. 2-7 and Eqn. 2-10, respectively. The concentration of ionized donors, IVd-, and the concentration of ionized acceptors, iVx-, are derived by using the Fermi function approximated by the Boltzmann function as shown in Eqns. 2-18 and 2-19, respectively ... 

Because the oxidation of sulphide has a pronounced effect on sulphide mineral flotation, oxidation will produce metal ions on the mineral surface and these ions will react with collectors to render the surface hydrophobicity. From the DOS shown in Fig. 9.13, Fig. 9.15 and Fig. 9.16, marmatite and (Zn, Cu) S are the intrinsic semiconductors and sphalerite is a broad band semiconductor. The top of the valence band of above three materials are dominantly occupied by Fe (3d),... 

---