A- semiconductor conductivity

The elements Si and Ge of group 14 act as semiconductors. A semiconductor is an element that can, to some extent, conduct electricity and heat, meaning it has the properties of both metal and nonmetals. The abihty of semiconductors to transmit variable electrical currents can be enhanced by controlling the type and amount of impurities. This is what makes them act as on-ofF circuits to control electrical impulses. This property is valuable in the electronics industry for the production of transistors, computer chips, integrated circuits, and so on. In other words, how well a semiconductor conducts electricity is not entirely dependent on the pure element itself, but also depends on the degree of its impurities and how they are controlled. 

The probability of an electron transfer, pet, from an excited donor species to a semiconductor conduction band may be estimated from the following equation ... 

Electron-phonon interaction in a semiconductor is the main factor for relaxation of a transferred electron. There are two different relaxation processes that decrease the efficiency of light conversion in a solar system (1) relaxation of an electron from a semiconductor conduction band to a valence band and (2) a backward electron transfer reaction. The forward and backward electron transfer processes have been already included in the tunneling interaction, HSm-qd, described by Eq. (108). However, the effect of SM e-ph interaction is important for the correct description of electron transfer in the SM-QD solar cell system. In the previous section, we have gradually considered different types of interactions in the quantum dot and obtained the exact expression for the photocurrent (128) where the exact nonequilibrium QD Green s functions determined from Eq. (127) have been used. However, in... 

Transference numbers have been introduced because they can be determined experimentally. On the other hand, the individual mobilities cannot be determined independently as in the case of electrons and holes in a semiconductor conductivity measurements yield only the sum of the cation and anion mobilities (see Eq. 3.1). Accordingly, the mobilities can be evaluated from measurements of the conductivity and the corresponding transference numbers. There are various methods for measuring transference numbers which are not described here (for details, see for example ref. [2]). Since both A and t-, depend on the concentration of the ions, data are usually evaluated from measurements in very dilute solutions because of interactions between ions. According to many investigations, the transference numbers of most ions are not far from = 0.5 i.e. the mobilities of cations and anions of dissolved salts are about the same. Large t-, val-... 

In 1883, copper(I) oxide was the first substance found to have semiconducting properties. A semiconductor conducts an electric current, although not nearly as efficiently as a conductor like copper, gold, or silver. Semiconductor components are now widely used in computer chips, although they are now made from silicon rather than copper(I) oxide. 

In a semiconductor, conduction occurs through the movement of electrons excited to the conduction band and through the movement of holes in the valence band. Holes are positively charged particles that represent the absence of electrons. The dynamical equations of an electron in a crystal in an applied electric field can be rewritten to resemble the equations of motion of a free electron in an electric field. To compensate for the effects of the crystal, the mass of the electron is replaced with an effective mass, m. The effective mass can be negative, representing a hole. A hole with a negative effective mass behaves in the same way as a positive charge and moves in the opposite direction as an electron in an electric field. Both electrons and holes contribute to the current flow. 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Figure C2.16.9. Schematic cross-section and biasing of a metai-oxide-semiconductor transistor. A unifonn conducting channei is induced between source (S) and drain (D) for > V. Voitage is appiied between the gate (G) and the source. Part (A) shows the channei for - V the transistor acts as a triode. The source-...

Thermocouples, bolometers and pyroelectric and semiconductor detectors are also used. The first three are basically resistance thermometers. A semiconductor detector counts photons falling on it by measuring the change in conductivity due to electrons being excited from fhe valence band info fhe conduction band. 

Figure 9.8(a) shows how the conduction band C and the empty valence band V are not separated in a conductor whereas Figure 9.8(c) shows that they are well separated in an insulator. The situation in a semiconductor, shown in Figure 9.8(b), is that the band gap, between the conduction and valence bands, is sufficiently small that promotion of electrons into the conduction band is possible by heating the material. For a semiconductor the Fermi energy E, such that at T= 0 K all levels with E < are filled, lies between the bands as shown. 

Figure 9.8 Conduction band, C, and valence band, V, in (a) a conductor, (b) a semiconductor and (c) an insulator...

A semiconductor laser takes advantage of the properties of a junction between a p-type and an n-type semiconductor made from the same host material. Such an n-p combination is called a semiconductor diode. Doping concentrations are quite high and, as a result, the conduction and valence band energies of the host are shifted in the two semiconductors, as shown in Figure 9.10(a). Bands are filled up to the Fermi level with energy E. ... 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

Fig. 1. Photoexcitation modes iu a semiconductor having band gap energy, E, and impurity states, E. The photon energy must be sufficient to release an electron (° ) iato the conduction band (CB) or a hole (o) iato the valence band (VB) (a) an intrinsic detector (b) and (c) extrinsic donor and acceptor...

The electron current density J has units of A/cm and in a semiconductor results from drift and diffusion. In the absence of concentration gradients, equation 7 reduces to Ohm s law, = nqp E = [Pg.346]

Sihcon (33) is a semiconductor and thus the electrical conductivity. O, is deterrnined by contributions from both electrons and holes, ie,... 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

Semiconducting Properties. Sihcon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stabiUty of its electronic stmcture, sihcon carbide has been studied for uses at high (>500° C) temperature. The Hall mobihty in sihcon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-ty e crystals, activation energy for ioniza tion of nitrogen impurity varies with polytype (50,51). 

Materials are usually classified according to the specific conductivity mode, eg, as insulators, which have low conductivity and low mobihty of carriers. Metahic conductors, which include some oxides, have a high conductivity value which is not a strong (exponential) function of temperature. Semiconductors are intermediate and have an exponential temperature dependence. Figure 1 gives examples of electrical conductivities at room temperature for these various materials. 

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