Enhanced Gain-Bandwidth Product and Performance in Thin Heterostructure Avalanche Photodiodes

It is well known that the excess noise factor of an avalanche photodiode (APD), which is a measure of its gain fluctuation, can be reduced by decreasing the thickness of the avalanche multiplication region. This noise reduction is attributable to the increased importance of the dead-space effect in thin layers, which prevents a carrier from impact ionizing before it travels a sufficient distance enabling it to acquire a minimum ionization threshold energy. Recently, it has been observed that there are yet two dead-space related mechanisms that can further reduce the excess noise factor. They are: 1) the initial-energy effect and 2) the bandgap-boundary effect, which is observed in heterostructure APDs [1]. The initial energy is referred to the energy that an injected carrier acquires just before entering the multiplication layer, which can be due to the presence of a strong electric-field gradient in a doped region just before the multiplication layer. This initial energy reduces the first dead space of the parent carrier, resulting in the localization of the first impact ionization. On the other hand, the bandgap-boundary effect occurs when the initial-energy effect is deliberately incurred by means of using two (or more) bandgap engineered intrinsic layers in the multiplication region. For example, in a heterostructure APD with a two-layer side-by-side Al0.6Ga0.4As/GaAs multiplication regions, as shown in Fig. 1, the high bandgap intrinsic Al0.6Ga0.4As layer, termed the energy-buildup layer, is used to energize the carriers that are injected into it while the low-bandgap GaAs layer is used as the primary layer for hosting the ionization events. Moreover, the width of the energy-buildup layer can be optimized to maximize the benefit of the initial-energy effect, thus minimizing the excess noise. The optimization strategy for minimizing the excess noise factor using an Al0.6Ga0.4As/GaAs heterostructure APD has been developed in [2] demonstrating a significant theoretical reduction in the excess noise factor. The performance advantages rendered by the bandgap-boundary effect have been demonstrated in the class of heterostructure devices termed “impact-ionization-engineered” APDs [3]. In this paper, the initial-energy and bandgap-boundary mechanisms are shown to improve the gain-bandwidth product (GBP) beyond the limits previously known. We will use the rationale of the modified dead-space multiplication theory (MDSMT) [1] for gain noise, which captures both the initial-energy and the bandgap-boundary effects in heterostructure APDs, to extend our recursive technique [4] for the prediction of the mean impulse response to arbitrary heterostructure APDs.