A group of UCSD researchers publishes an open-access Applied Physics Letters paper "An amorphous silicon photodiode with 2 THz gain‐bandwidth product based on cycling excitation process" by Lujiang Yan, Yugang Yu, Alex Ce Zhang, David Hall, Iftikhar Ahmad Niaz, Mohammad Abu Raihan Miah, Yu-Hsin Liu, and Yu-Hwa Lo. The paper proposes APD-magnitude gain mechanism in by means of 30nm-thing amorphous Si film deposited on top of the bulk silicon: