Avalanche photodiodes with an impact-ionization-engineered multiplication region

The avalanche photodiode (APD) is frequently the photodetector of choice for high-bit-rate, long-haul fiber optic communications, owing to its internal gain, which provides a sensitivity margin compared to PIN photodiodes. Since the multiplication region of an APD plays a critical role in determining the gain, the multiplication noise, and the gain-bandwidth product, numerous research programs have focused on optimizing the multiplication region in order to improve the APD performance. We describe a new multiplication region structure that achieves very low multiplication noise by impact ionization engineering (I/sup 2/E), which utilizes heterojunctions to achieve greater localization of impact ionization than spatially uniform structures. By placing thin layers with relatively low threshold energy (multiplication layer) on each side of a region with higher ionization coefficients (the separation layer), impact ionization is enhanced at the edges in the twin multiplication layers and is suppressed in the center, where the carriers are energized in transit.