mixing of optical and, acoustical phonon scattering. This agrees with experimental results [5,8]. The relaxation time for scattering by nonpolar optical phonons is given by [46]
-1 m* 3/ Dg~l r (no+l)r _i
T -1 = i T112 - D 1~/2
oi - x D.
+ 'D } i = 1,2,3 (3.12)
where 0D is the Debye temperature, no = (exp(OD/T)-l)- is the phonon distribution function, and W is a constant which determines the relative coupling strength of the holes to the optical phonon mode compared to the acoustical phonon mode
D 2fi 2 C 2
W o (3.13)
2ko a 0D2
2
.where D0 is the optical deformation potential constant. The first term
0
in the brackets of equation (3.12) corresponds to optical phonon emmission and is relevant only when this is energetically possible (>e D/T). The second term in the brackets corresponds to optical phonon absorption.
3.5 Ionized Impurity Scattering
The Columbic interaction between ionized impurities and charge carriers drifting through the cyrstal under the action of an applied electric field causes scattering of the charge carriers. Scattering by ionized impurities was first considered by Conwell and Weisskopf [47]. The basic assumption is that the Coulomb field is cut off at half the