  We present a new optimised model of Brookes-Herring ionized impurity
scattering for use in Monte Carlo simulations of semiconductors. When
implemented, it greatly decreases the execution time needed for simulations
(typically by a factor of the order of 100), and also properly incorporates the
great proportion of small angle scatterings that are neglected in the standard
algorithm. It achieves this performance by using an anisotropic choice of
scattering angle which accurately mimics the true angular distribution of
ionized impurity scattering.
