Simulation of avalanche time in thin GaN/4H-SiC heterojunction avalanche photodiodes

A random ionization-time model is introduced to compute the avalanche time of double carrier multiplication in heterojunction avalanche photodiodes (APDs). The Monte Carlo method is employed to determine the distribution of carriers for both electron- and hole- initiated multiplications in the GaN/4H-SiC heterojunction APDs of multiplication widths, w = 0.1 and 0.2 mu m, incorporating of dead space and hetero-interface effects at high electric field region with respect to time. The carriers that are injected into the GaN layer will undergo multiplication based on material-dependent electron and hole impact ionization coefficients alpha GaN and beta GaN, then cross the heterojunction based on the probability and followed by the multiplication based on material dependent alpha 4H-SiC and beta 4H-SiC in the 4H-SiC layer. The avalanche time is calculated from the instant the parent carrier enters the multiplication region until all carriers leave the multiplication region. Our model is able to show the distribution of carriers with respect to space and time, inclusive of the presence of secondary carriers due to different groups of feedback carriers and dead time. Due to potential difference at hetero-interface, the avalanche time of the GaN/4H-SiC heterojunction APDs is less than that of the GaN and 4H-SiC homojunction APDs of the same multiplication width; hence, they are good candidates for sensing and switching devices.