Majority carrier density

For a weU-designed, weU-made HgCdTe photoconductoi detector (76,77), g-r noise is dominant and may be expressed in terms of a minority carrier densityp and majority carrier density n. Semiconductor noise analysis for the HgCdTe photoconductor yields,... 

According to this equation, the lifetime of excited carriers decreases with increasing majority carrier density and consequently with doping. A similar result is obtained if the recombination process occurs via impurity centers (Shockley-Read equation [20]), which will not be shown here. The recombination rate also influences the stationary density of electrons and holes produced by light excitation. One obtains from Eqs. (6) and (7) ... 

Equation (20) is an approximation being valid only in the depletion layer, where the majority carrier density at the surface (ns for n-type ps for p-type) is smaller than the corresponding bulk concentration. The thickness of the space charge layer can be defined by the relation dsc = eeo/C which is valid for a normal capacitor. Inserting Eq. (20), for the thickness of the space charge layer, one obtains ... 

This relation is valid only for a space-charge region where the majority-carrier density is depleted with respect to the bulk density. The thickness of the space-charge layer, defined as w = decreases as doping increases. For a typical carrier... 

The center range, where the capacity curve is rather flat, i.s characterized by a majority carrier density being depleted with respect to the bulk concentration, i.e. Ms < (j (depletion region) whereas tig pg. In this range the capacity is mainly determined by the linear term in Eq. (5.24) which originates from the ionized donors. 

Theoretically, the luminescence intensity is proportional to the rate of electron-hole recombination, i.e. I oc n z)p z). In the simplest case it is assumed that the majority carrier density is not substantially changed upon illumination. TTien the luminescence intensity is given by... 

Eq. (7.62) is only valid for small changes in majority carrier densities, i.e. Ap po. Since in most cases fairly intense laser pulses were used, a more complex equation must be used which finally leads to quadratic concentration terms in Eq. (7.115). The exact procedure for the evaluation cannot be given here and the reader is referred to... 

Notice that although both electrons and holes are present, current in the neutral regions is dominated by the minority carrier because light and bias induced carrier gradients are larger. Majority carrier transport is described by similar diffusion equations but because the recombination probability is so low, the majority carrier density is effectively constant across the neutral region. Gradients in the carrier density may arise from resistive losses in low mobility materials. 

Contacts must supply a reservoir of carriers ready to enter the semiconductor at the rate required by the ohmic currents. At low fields, the balance is maintained by dielectric relaxation which quickly compensates any deviation from equilibrium by a charge supply from the interior and from the other contact. At high fields departures from this balance occur. When the contact region has an abundance of carriers the ohmic extrapolation is exceeded by space charge supplied currents. These are called by the misnomer space charge limited currents . When, on the other hand, the demand for carriers exceeds the supply, the current becomes emission limited. A very comprehensive account of injection in insulators has been given recently by Lampert and Mark (1970), Lampert and Schilling (1970), and by Baron and Meyer (1970). The injection processes in insulators and amorphous semiconductors differ markedy from minority carrier injection in crystalline semiconductors where the majority carrier density adjusts... 

The capacity-potential relation, given by Eq. (10), is the so-called Mott-Schottky equation which is strictly valid only in the exhaustion region, i.e., for space charges in which the majority carrier density at the surface is smaller than the corresponding bulk concentration < o for n-type and Ps < po for p-type electrodes). 

According to these experimental results, the primary effect here is an accumulation of holes on the surface because the anodic dissolution seems to be a very slow reaction. Working at a potential between f/ft,(dark) and f/f, (light), the bands become flattened due to their shift. Since then the majority carrier density is increased near and on the surface, the recombination rate increases (the recombination rate is proportional to n and p, see Section 1.6). Accordingly, the high recombination is a consequence of the band-edge shift on the surface. In the case... 

The space-charge width, w, which is related to the majority carrier density, N , and the potential drop, space charge region by the equation ... 

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