Simulation of negative-effective-mass terahertz oscillators

We present a model calculation of hole current oscillations in doped p+pp+ ballistic diodes using the nonparabolic balance-equation theory and a time-dependent drift-diffusion model. Such an oscillation originates from a negative effective mass (NEM) region in the hole dispersion relation. In the present balance-equation calculation, we consider the scatterings by hole-impurity, hole-acoustic phonon, hole-polar-phonon, and hole-nonpolar-phonon–hole interactions, and yield a “N-shape” velocity-field relation, which are quite different from the two-valley results for electrons in bulk GaAs. We provide a detailed analysis of the resulting oscillations as a function of the applied voltage, base length, base doping, and the dispersion relation. Typical frequencies for a 0.2 μm structure NEM oscillator are in the terahertz range. Qualitative agreement is obtained between the present calculations and the existing Boltzmann results.