Common Semiconductors

By replacing the quantity (1/ne) by the coefficient the relation between the electric current density and the perpendicular applied magnetic induction is given by the following equation  

Gunn effect Laser effect Photovoltaic effect Piezoelectric effect Piezoresistance Transistor effect Tunnel effect Varactor effect 

High-frequency generation and amplification GaAs, InP, CdTe Laser diode GaAs, InAs, InSb 

High-frequency switch, oscillator, amplifier Si, Ge, GaAs Parametric amplification, tunning diode Si, Ge, GaAs 

Industrial preparation. Several methods can be used for preparing the pure element. Polycrystalline silicon for use in electronics can be produced using three common methods  

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An extensive compilation of the properties of compound semiconductors may be found in the Landolt-Bn mstein reference books (13,14). Various subvolumes in the series cover the properties of elemental. III—V, II—V, and other less common semiconductors. Information may also be found concerning semiconductor technology. Another useful source of information is the EMIS data review series (15). These books describe the properties and technology of GaAs, HgCdTe, InP, AlGaAs, InGaAs, and the III—V nitride compounds. 

A schematic of epitaxial growth is shown in Fig. 2.11. As an example, it is possible to grow gallium arsenide epitaxially on silicon since the lattice parameters of the two materials are similar. On the other hand, deposition of indium phosphide on silicon is not possible since the lattice mismatch is 8%, which is too high. A solution is to use an intermediate buffer layer of gallium arsenide between the silicon and the indium phosphide. The lattice parameters of common semiconductor materials are shown in Fig. 2.12. 

Semiconductors. In Sections 2.4.1, 4.5 and 5.10.4 basic physical and electrochemical properties of semiconductors are discussed so that the present paragraph only deals with practically important electrode materials. The most common semiconductors are Si, Ge, CdS, and GaAs. They can be doped to p- or n-state, and used as electrodes for various electrochemical and photoelectrochemical studies. Germanium has also found application as an infrared transparent electrode for the in situ infrared spectroelectrochemistry, where it is used either pure or coated with thin transparent films of Au or C (Section 5.5.6). The common disadvantage of Ge and other semiconductors mentioned is their relatively high chemical reactivity, which causes the practical electrodes to be almost always covered with an oxide (hydrated oxide) film. 

Semiconductors are materials that contain a relatively small number of current carriers compared to conductors such as metals. Intrinsic semiconductors are materials in which electrons can be excited across a forbidden zone (bandgap) so that there are carriers in both the valence (holes, p-type) and conduction (electrons, ra-type) bands. The crucial difference between a semiconductor and an insulator is the magnitude of the energy separation between the bands, called the bandgap (Eg). In the majority of useful semiconducting materials this is of the order of 1 eV some common semiconductors are listed in Table 1. 

Table 1, Band Positions " for Some Common Semiconductor Photocatalysts...

Semiconductor electrodes, which have much lower charge carrier densities (1013—1019 carriers/cm3), typically absorb in the infrared but exhibit much lower absorption by charge carriers than metals of comparable film thickness, and frequently show a transparency window in much of the visible spectrum due to a substantial band-gap energy, before absorbing again in the ultraviolet. For example, Sn02 and ZnO, like many common semiconductor electrode materi-... 

Table I. Common Semiconductor Materials and Methods for Growing Them...

Figure 2. Thermal conductivity of several common semiconductor materials plotted against the best estimates for the critical resolved shear stress (CRSS) for the crystal. As explained in the text, materials with low thermal conductivity and low CRSS are hardest to grow.

Table 15.1 Band positions of some common semiconductors used for photocatalytic processes [2, 3, 6, 7, 95].

On the other hand, many organic compounds have a redox potential at a higher energy than the valence band edge of common semiconductor oxides and, therefore, they can act as electron donors and thus yield a radical cation (Fig. 1), which can further react, for example, with H20, 02 , or 02. 

The driving force of the electron transfer process in the interface is the difference of energy between the levels of the semiconductor and the redox potential of the species close to the particle surface. The thermodynamically possible processes occurring in the interface are represented in Fig. 9 the photogenerated holes give rise to the D -> D + oxidative reaction while the electrons of the conduction band lead to the A -> A reductive process. The most common semiconductors present oxidative valence bands (redox potentials from +1 to + 3.5 V) and moderately reductive conduction bands (+ 0.5 to - 1.5 V) [115]. Thus, in the presence of redox species close or adsorbed to the semiconductor particle and under illumination, simultaneous oxidation and reduction reactions can take place in the semiconductor-solution interface. 

Molecular beam methods are now widely used for the preparation of common semiconductors such as GaP and their intergrowth with other compounds. Thin films of iron phosphides of 0.5 to 25 J,m thickness can be electrodeposited from sulfate solutions. Depending on the deposition conditions (time, pH, temperature), the phosphides FeP, Fe2P, and/or FesP occur. The preparation of higher polyphosphides, for example, KP15, as thin films demonstrates the range of still yet unexplored preparative methods. This is also valid for the electrolysis of phosphates. The latter techniques is mostly suited for metal-rich phosphides. ... 

Taken together, these three structures (cubic diamond, zinc blende, and wurtzite) encompass the majority of the common semiconductor materials in use today. These structures, which share the common feature of having an average of four valence electrons per atom, are called adamantine solids. 

The band gaps of common semiconductors can vary from near-zero (fig = 0.1 eV for Hgo.8Cdo.2Te) to greater than 3 eV (fig = 3.2eVfor SrTiOs). 

Figure 19.2 Band gaps, together with valence and conduction band edges of common semiconductors, placed alongside the standard redox potentials (versus normal hydrogen electrode (NHE)) of the OijOY and OH/ OH redox couple.

Table 1. Band positions in some common semiconductors [30],...

Table 1 provides a list of these values for the most commonly accessible semiconductor powders suspended in aqueous acid, along with a conversion of the band gap to an absorption onset wavelength A Eg). The band-edge positions [30] can also be adjusted by control of the particle size of the irradiated semiconductor. Quantization effects can shift these values by more than 100 nm this allows control of the onset wavelength and of the band positions of several common semiconductors. 

By shift from a reactive solvent (e.g., water) to an inert solvent (e.g., acetonitrile), this problem can be easily solved. Acetonitrile is oxidized just outside the potential window of the band gap of common semiconductors, so the adsorbed substrate can be selectively oxidized. Without an oxidative source for producing hydroxyl radicals, however, there are fewer opportunities for the incorporation of OH groups. 

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