Recent Advances in Avalanche Photodiodes

摘要

Avalanche photodiodes (APDs) have been utilized for a wide range of commercial, military, and research applications. In recent years, perhaps the primary driving force for research and development of APDs has been optical communications. It is well known that the internal gain of APDs provides higher sensitivity in optical receivers than PIN photodiodes, however, at the cost of more complex epitaxial wafer structures and bias circuits. APDs have been successfully deployed for 2.5 Gb/s receivers and there are numerous industrial efforts actively developing APDs for 10 Gb/s systems. Communications APDs utilize the SACM structure, which has separate layers for multiplication, electric field profiling, and absorption. The multiplication region is typically InP and the absorber is In{sub}0.53Ga{sub}0.47As. While these APDs have achieved excellent receiver sensitivities at 2.5 Gb/s there are three factors that limit their performance at higher bit rates. Since they operate under normal incidence, the absorption region must be approximately 2μm thick in order to achieve good (> 60%) quantum efficiency. The associated transit times limit the bandwidth to 10 GHz at low gain. At higher gains, the relatively low gain-bandwidth product (<100 GHz) restricts the frequency response. The gain-bandwidth product and the high excess noise are consequences of the reasonably unfavorable ionization coefficients of InP. Much of the recent work on APDs has focused on developing new structures and incorporating alternative materials that will yield lower excess noise and higher speed while maintaining optimal gain levels.