Monte Carlo Simulation of Hot Carrier Transport in Heterogeneous Ge/AlxGa1-xAs (0≤x≤0.8) Multilayer Avalanche Photodiodes

Impact ionization in novel Ge/AlxGa1-xAs (0 <= x <= 0.8) multilayer structures is investigated using numerical simulation. The simulated effective electron ((alpha) over bar) and hole ((beta) over bar) ionization coefficients reveal that a large (beta) over bar/(alpha) over bar ratio of up to 8 can be obtained in a 25-period Ge (50 nm)/Al0.8Ga0.2As (50 nm) multilayer structure, much larger than that in the Ge and AlxGa1-xAs homojunctions, attributed by the reduction in (alpha) over bar. The substantial difference in phonon scattering cross-sections in the AlxGa1-xAs barrier and the Ge well played an important role in determining the (beta) over bar/(alpha) over bar ratio in these structures. Breakdown voltage analysis indicates that hot carriers' drift in the AlxGa1-xAs barriers and Ge wells sampled the transport properties of both materials, despite the electron and hole ionizations are dominated by that of Ge within the electric field range studied.