Semiconductor, conductivity definition

Every semiconductor, by definition, has a filled valence band, separated by an energy difference (band gap) from the vacant conduction band. In an undoped state, conductivity is observed only by thermal population of the conduction band by electrons sufficiently energetic to jump across the band gap to the conduction band. This is an energetically unfavorable process and occurs only infrequently. An undoped semiconductor therefore has very low conductivity and is generally considered to be insulating. 

Figure 7.13. The definitions of ionization potential, Ie, work function, , Fermi level, EF, conduction level, Ec, valence level Ev, and x-potential Xe without (a) and with (b) band bending at the semiconductor-vacuum interface.

Starting with this definition the semiconductor diemical sensors can be arbitrary classified with respect to following features the type of electrophysical characteristics diosen for monitoring, such as electric conductivity, thermal-electromotive force, work function of electron, etc. type and nature of semiconductor adsorbent used as an operational element of the sensor and, finally, the detection method used for monitoring the adsorption response of electrophysical characteristics of die sensor. 

According to the electronic theory, a particle chemisorbed on the surface of a semiconductor has a definite affinity for a free electron or, depending on its nature, for a free hole in the lattice. In the first case the chemisorbed particle is presented in the energy spectrum of the lattice as an acceptor and in the second as a donor surface local level situated in the forbidden zone between the valency band and the conduction band. In the general case, one and the same particle may possess an affinity both for an electron and a hole. In this case two alternative local levels, an acceptor and a donor, will correspond to it. 

The fundamental division of materials when electrical properties are considered is into metals, insulators, and semiconductors. An insulator is a material that normally shows no electrical conductivity. Metals and semiconductors were originally classified more or less in terms of the magnitude of the measured electrical conductivity. However, a better definition is to include in metals those materials for which the... 

In his article mainly mode-locked tunable dye lasers are discussed. Giant pulse ruby lasers (3 nsec pulse halfwidth) have been successfully used to probe electron densities as a function of time in a rapidly expanding plasma 22). The electron lifetime in the conduction band can be determined with nanosecond semiconductor lasers. By absorption of the laser pulse the electrons in the semiconductor probe are excited into the conduction band, resulting in a definite conductivity. The mean lifetime is obtained by measuring the decrease of conductivity with time 26). 

In the tables for both Cu-S and Cu-Se (Tables 6.4 and 6.5), the column denoting conductivity type has been deleted (these semiconductors are always p-type), and, in its place, the phase (and/or composition) has been given. In some cases, particularly for the sulphides, where no XRD pattern was seen (except for CuS), no phase (or composition) was proven and therefore no entry is given in the table. This was not a problem for Cu-Se, since XRD spectra were always clear and definitive. 

These are quantized vibrations of conduction electrons in a metal or semiconductor 4°). The quantized energy levels of the collective longitudinal vibrations of the electron gas are quasi-particles in the sense of the definition given above and are called plasmons. The frequency of this longitudinal vibration, the plasma frequency u>P, is given by 40>... 

Fig. 3.15 Band model for an intrinsic semiconductor. The valence band is totally filled and the conduction band empty. Conduction occurs via promotion of electrons from Ey to Ecy the conductivity increasing with increase in temperature, (a) Definition of energy levels (b) Variation of density of available states with...

By definition, the electrical conductivity or due to a concentration n of carriers, all with the (mean) mobility p., is given by a = n e x, e being the electronic charge, and in a simple semiconductor, it is in fact the activation of n (= electrons = nhoies) that produces the activation of a... 

By definition, a semiconductor does not have a continuum of states (as do metallic conductors) but rather a band structure. The filled levels, called the valence band, are an energetically closely spaced array of orbitals composed of the valence electrons of the material. A gap exists between the top edge of this band and the lower edge of a similar closely spaced array of orbitals that are unoccupied in the ground state, that is, the conduction band. The gap separating these bands is called the band gap. 

Figure 5.2 illustrates a typical spherically-shaped small TiOi particle, taken here as a model of small semiconductor systems, with a definition of the conduction and... 

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