Conductivity extrinsic

However, with local impurities in the material, local energy centers may exist in the forbidden gap. Here, electrons reside on energy levels that may be characterized as local energy wells. With a certain amount of added energy electrons can come up from the well and reach the conduction band (n-type impurities). This can considerably increase conductivity (extrinsic conduction). According to the nature of the impurities, this added conduction may be by electrons or holes as charge carriers. 

The electrical properties of the homogeneous alloys (Hall effect, conductivity, thermoelectric power) were studied at room temperature on electrically homogeneous samples prepared under identical conditions. The measurements showed that the alloys in systems 1 through 5 have n-type conduction while those in system 6 have p-type conduction. Extrinsic conduction and a high carrier density have been established for the alloys in all the systems. The electrical conductivity and mobility (Fig. 2) decrease monotonically with an increase in the content of A B component in the solution. The thermoelectric power and the effective mass of the electrons have low values and vary little with the composition. 

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

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...

These equations represent expressions for the extrinsic ionic conductivity of the material as exhibited by the shortened defect notation of equation 12, showing that Na vacancies are created by doping and not primarily generated from thermal energy. 

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

It is because these extrinsic electrons can so readily be activated thermally to the conduction band, that great care must be taken in producing the elementary semiconductors to a high state of purity, by such processes as zone refining. 

Diffusion in the extrinsic region can readily be modified by doping, although knowledge of the mechanism by which the diffusion takes place is important if this is to be immediately successful. For example, sodium chloride structure materials that conduct by a vacancy mechanism can have the cation conductivity enhanced by doping with divalent cations, as these generate compensating cation vacancies. The inclusion of cadmium chloride into sodium chloride can be written ... 

Intrinsic and Extrinsic Defects in Insulators Ionic Conductivity... 

INTRINSIC AND EXTRINSIC DEFECTS IN INSULATORS IONIC CONDUCTIVITY... 

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