Boundary effects on multiplication noise in thin heterostructure avalanche photodiodes: Theory and experiment

The history-dependent recurrence theory for multiplication noise in avalanche photodiodes (APDs), developed by Hayat et al, is generalized to include inter-layer boundary effects in heterostructure APDs with multilayer multiplication regions. These boundary effects include the initial energy of injected carriers as well as bandgap-transition effects within a multilayer multiplication region. It is shown that the excess noise factor can. be, significantly reduced if the avalanche process is initiated with an energetic carrier, in which case the initial energy serves to reduce the initial dead space associated with the injected carrier. An excess noise factor reduction up to 40% below the traditional thin-APD limit is predicted for GaAs, depending on the operational gain and the multiplication-region's width. The generalized model also thoroughly characterizes the behavior of dead space as a function of position across layers. This simultaneously captures the effect of the nonuniform electric field as well as the anticipatory nature of inter-layer bandgap-boundary effects. Such anticipatory behavior of the dead space is ignored in simplified models where the dead space is assumed to change abruptly at the layer boundary. The theory is applied to recently fabricated thin Al0.6Ga0.4As/GaAs heterostructure APDs exhibiting low excess noise factors. The excess noise factor predictions are in very good agreement with experiment. In one device, where the initial-energy effect is pronounced, the measured excess noise factor is 36% below the value predicted by previous analytical multiplication models which ignore the initial-energy effect.