Impact ionization model for full band Monte Carlo simulation in GaAs

The impact ionization rate in GaAs is derived from a first principle's calculation which includes realistic band structure and a wave-vector- and frequency-dependent dielectric function. The impact ionization rate is highly anisotropic at low electron energy, while it becomes isotropic at higher energy range in which impact ionization events frequently occur. The calculated impact ionization rate is well fitted to a modified Keldysh formula with two sets of power exponents of 7.8 and 5.6, indicating very soft threshold characteristics. Using a full band Monte Carlo simulation which includes the empirical phonon scattering rate based on first principles theory, we derived the impact ionization coefficient. The calculated impact ionization coefficients agree well with available experimental data. Our isotropic model shows better agreement with reported experimental data than a previous anisotropic model, especially in low electric field. The mean energy of secondary generated electrons is found to be expressed as two sets of linear functions of the primary electron energy. (C) 1996 American Institute of Physics.