Ionisation dead space and the super-APD

Abstract

In this paper we have seen carriers ionise when the energy gained from the field exceeds the energy loss to phonons by the ionisation threshold energy. Carriers are injected either optically or by impact ionisation, are generated 'cool' and they must travel some distance down the electric field before they 'heat' and their energy distribution achieves equilibrium with the local field. During this 'dead space' region their ionisation coefficient is very small. The impact ionisation process then loses some of it's randomness, also reducing the scatter in M and hence reducing excess noise. In thin devices the measured noise falls, and the effect is well described by treatments which allow for the the effects of dead space. This noise reduction in thin avalanche regions is also seen in a wide variety of III-V materials and in SiC and these results have demonstrated that APDs can be simultaneously both quiet and fast.