Mobility of carriers

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

Control of alignment of n-conjugated polymers on the substrate is important for excellent performance of the polymer in electronic devices (e.g., higher mobility of carrier in field-effect transistors [134,136]). Details of the molecular structure and molecular assembly of PAEs will be discussed in other chapters. 

For the simplest case of a single set of localized states sitnated at a particular energy Ei, the trap-limited drift mobility of carriers moving in extended states at is readily compnted from equation (3.3). If the effective density of extended states at Ep is Np and the trap concentration is N, then we may write... 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

Tachikawa (1999) also analyzed mobilities of carriers along the silicon chain, and his results should be mentioned separately. As it turned out, the mobility obtained for a positive charge (hole) was several times larger than that for an excess electron. This result suggests that the localization mechanism of a hole and that of an electron are different from each other. Probably, an excess electron is trapped in the defect of the main chain, whereas a hole is not trapped. The defects are mainly structural ones, such as branching points and oxidized sites (Seki et al. 1999). This can lead to a different electron conductivity. Continuation of the polysilane ion radical studies will hopefully result in some important technical applications. 

It is normally unnecessary for the electrochemist to be concerned with the mobility of carriers in most of the semiconductors whose properties have been studied, since the very low conductivity of "small polaron samples would normally preclude their measurement. However, a proviso must be entered here in the case of binary and, more especially, ternary samples. It may well be the case that the majority carriers in a particular material are indeed itinerant (i.e. have mobilities in excess of ca. 1 cm2 V 1s 1), but there is no guarantee that this will be true of the minority carriers generated by optical absorption. Thus, the oxide MnTi03 shows a marked optical charge transfer absorption from Mn(II) to Ti(IV), the latter being the CB. The resultant holes reside on localised sites in the Mn levels, presumably as local Mn(III) centres, and are comparatively immobile. The result is that there is... 

The greater the mobility of carriers is (thus the longer the diffusion length L), the greater is 4> for a given crystal size d and absorption coefficient a. 

It is possible to measure the mobility of carriers directly in the linear region of operation [115]. This is an alternative to determining the mobility through curve-fitting to a transistor model. The mobility measured this way has an unambiguous meaning, but the approach has some limitations. The total current in the transistor deep in the linear region can be treated like a resistor ... 

The microscopic or conductivity mobility This is the mobility of carriers during the time they are eneigeticaJly located in a particular conducting state. It is the quantity appearing in the expression for conductivity a = (n, p)ep It is often misnamed drift mobility in the literature. 

Another aspect of the impurity doping which will be common in mineral particles is illustrated by the effect of Nb(V) on anatase. This ion substitutes isomorphical-ly for Ti(IV) and as an n dopant creates a Shottky barrier at the interface. This assists injection of an electron from a reducing solute into the conduction band (1J[). The flat band is also shifted cathodically. It has recently been claimed that the relative inefficiency of haematite as a photoanode is a function of low mobility of carriers and that this problem may be... 

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