imi2 <2>
       r Hi  [mHJ[>                                               (5.7)



is the Hall factor. We'see that allowing for a difference between the values of conductivity and Hall effective masses due to the anisotropic, nonparabolic nature of the band, enables us to separate the Hall factor into two components: the mass anisotropy factor given by

                M*2
       rAi :    i(5.8)



and the scattering factor given by


             <T. 2>
       rsi      >2                                                (5.9)



These components of the Hall factor will be considered in detail in the next two sections.

5.3 The Mass Anisotropy Factor

     Lax and Mavroides [20] have derived expressions for rA based on the Dresselhaus et al. [28] model of the valence band of germanium and silicon. Their formulation for rA acknowledges the anisotropy, but neglects the nonparabolicity of the bands. In general it is found that rA is less than unity unless the scattering anisotropy becomes extreme [30]. In order to determine the variation of the mass anisotropy factor with changes in temperature and dopant density for the combined valence band of silicon, equation (5.8) was evaluated using the values of combined valence band effective mass obtained from equations (2.28) and (2.29).