Abstract
This thesis reports results of experimental and theoretical investigations of impact ionisation in semiconductors. The impact ionisation rates for electrons and holes (alpha and beta respectively) are important parameters which determine the performance of several high-field semiconductor devices, including avalanche photodiodes (APDs). These ionisation rates depend on the threshold energies for ionisation, which are determined by the details of the band-structure for a semiconductor material. However, little is known about the band-structure features responsible for the favourable noise-performance of some materials (eg. Si) or relatively poor noise-performance of others (eg. Ge and most III-V materials). A better understanding of the relation between the band-structure and the resulting ionisation thresholds and rates would help guide the search for materials and structures with desirable properties. Impact ionisation in Si and Ge has been studied both experimentally and theoretically. Threshold energies in Si and Ge were calculated from pseudopotential band-structures. The key band-structure features leading to a large asymmetry in the ratio of thresholds for electrons and holes in Si, and the small ratio in Ge have been identified. The breakdown voltages in Si and Ge APDs have been measured under hydrostatic pressure. The results for Si and Ge are very different, and reflect the difference in the dominant ionisation processes calculated in these materials. Threshold energies have also been calculated for a selection of relaxed and strained GexSi1-x alloys. The relaxed alloys are expected to exhibit "Si-like" characteristics for a Ge mole fraction of 0 < X < 0.35. The effects of uniaxial strain on GexSi1-x alloys lead to a significant increase in the lowest-threshold ratio, making GexSi1-x/Si APDS an attractive proposition. However, a first-order Monte Carlo simulation of the effect of strain on ionisation rates indicates that the enhancement may be less than expected, as a result of the extreme softness of the ionisation process in Si. The role of satellite valleys in ionisation rate enhancement in multiple quantum well APDs has been studied using a Monte Carlo simulation. It is shown that the enhancement of the electron ionisation rate in multiple quantum well (MQW) APDs is determined by band-edge discontinuities in the satellite valleys, rather than in the r valley as previously assumed for semiconductors where the threshold field for the Gunn effect is lower than that for impact ionisation. Simulations of a model GaAs/Al0.45Ga0.55As MQW show no enhancement attributable to the band-edge discontinuities.