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Extensive experimental characterization and TCAD simulation analysis have been used to study the dark current in Avalanche Photo-Diodes (APDs). The comparison between the temperature dependence of measurements and simulations points out that SRH generation/recombination is responsible for the observed dark current. After the extraction of the carrier lifetimes in the GaAs layers, they have been used to predict the APD collection efficiency of the photo-generated currents under realistic operation conditions and as a function of the photogeneration position inside the absorption layer.

We report simulation results for gain and noise in avalanche photodiodes fabricated using heterojunctions of III-V compound semiconductors. We employ a recently developed nonlocal model for impact ionization. The model has been calibrated and validated on devices fabricated and measured in our labs and data from the literature. The model is then used to explore the design trade-offs related to the number of conduction band steps in staircase APDs for X-ray detection based on the GaAs/AlGaAs material system.

We present an improved Random Path Length algorithm to accurately and efficiently estimate the design space of heterostructure avalanche photodiodes (APDs) in terms of gain, noise and bandwidth without any need of full Monte Carlo transport simulations. The underlying nonlocal model for impact ionization goes beyond the Dead Space concept and it is suited to handle staircase structures composed by a superlattice of III-V compounds as well as thick and thin p-i-n APDs. The model parameters have been calibrated on GaAs and $Al_{x}Ga_{1-x}As$ p-i-n APDs in a previous work. In this work GaAs p-i-n APDs are compared to staircase structures in terms of noise and bandwidth.