Impurity Ionization inn-Type Germanium

Abstract

Impurity ionization in semiconductors at low temperatures is the excitation of trapped electrons on shallow levels to the conduction band by the impact of electrons heated by a sufficient electric field. The hot-electron mobility and the impact-ionization cross section are studied separately, by the microwave mobility of the avalanche plasma (during short current pulses) and by the transient response of high-impedance samples, respectively. The basis of any interpretation is the correct calculation of some hot-electron distribution functions. During the discharge regime (steady electric field, current increasing by orders of magnitude), the mobility remains nearly constant. For purer samples, the values of the mobility agree within 30% with the hypothesis of pure emission of acoustical phonons. For less pure samples, the mobility decreases with the donor concentration and neutral impurity scattering dominates ionized impurity scattering. The saturation regime is also investigated for a pure sample where some heating of phonons occurs. A precise calculation of the coupled system of Boltzmann equations indicates a mean free path of phonons greater than the transverse width of the sample. Several impact-ionization cross-section laws are tested to compare the results of the transient-pluse analysis with the theoretical ionization rate, which is an average of that cross section weighted by the distribution function. Mobility and ionization rate measurements indirectly give the Auger recombination parameter, which is compared with previous data for much lower energies and with the cascade capture theory.