ON THE MECHANISM OF LOW-TEMPERATURE IMPURITY BREAKDOWN

Low-temperature impurity breakdown is investigated theoretically on th e basis of a model that takes into account self-consistently the charg e-exchange of the excited state of the impurities and all electron sca ttering processes that affect the impact-ionization rate. The analysis includes impact ionization of the ground and excited states of the im purities, impact ionization of neutral impurities, electron-electron c ollisions, and scattering of electrons by phonons. Numerical calculati ons are performed for n-GaAs. It is shown that there are three mechani sms which lead to a negative differential resistance and under real co nditions can form an S-shaped section of the current-voltage character istic. The first mechanism is due to the decrease in the electron trap ping coefficient of shallow impurities with increasing frequency of in terelectronic collisions, when they start to control the distribution function near the percolation level. This mechanism initiates instabil ity at low currents. The second mechanism is due to the decrease in th e electron energy losses with increasing current as a result of the we akening of the inelastic scattering by impurities as the impurities ar e ionized. The third mechanism is the well-known overheating instabili ty. (C) 1996 American Institute of Physics.