CHAPTER VIII
SUMMARY AND CONCLUSIONS
Theoretical expressions. have been derived to compute the'hole. mobility, resistivity and Hall factor as functions of dopant density and temperatu re for silicon doped with boron, gallium and indium. The valence band of silicon was represented by a three-band model which takes into account the nonparabolic nature of the bands. This attribute of the valence band is included in the effective mass calculations. Contributions from scattering by acoustical and optical phonons, ionized impurities and neutral impurities were considered in the calculation of average rel axation time. In addition, our model also takes into account the effect of hole-hole scattering on both lattice and ionized impurity scattering relaxation times, and the effect of interband transitions on the acoustic phonon relaxation time. Thus the model developed in this study represents a more complete theoretical description of the conductivity mobility, Hall mobility, Hall factor and resistivity than previous theoretical models have acknowledged.
As stated in Chapter II, the model used for the valence band of silicon provides values of density-of-s tates effective mass which lead to values of intrinsic carrier density which are well within the limits of experimental error. The greatest deficiency this model has over an exact calculation is that the anisotropy of the b ands has been averaged out. Thus while the temperature dependence of effective mass derived from the exact and approximate models of the valence band is similar, 103