Ultralow-noise avalanche photodiodes

Abstract

InP/In0.53Ga0.47As avalanche photodiodes (APDs) have been widely deployed in high-bit-rate, long-haul fiber optic communication systems due to the higher sensitivity, relative to a PIN photodiode, afforded by internal gain of the APD. Owing to their materials and structural limitations it is uncertain whether the performance of InP-based APDs will be adequate for 10 GB/s systems and subsequent higher- speed systems. One of the impediments for the InP-based APDs is the fact that InP has roughly equal electron and hole ionization rates. This result in a symmetric multiplication process with relatively high multiplication noise and the gain-bandwidth product of an APD are primarily determined by the structure of the multiplication region. Recently, it has been reported that submicron scaling of the multiplication region thickness leads to lower multiplication noise and higher gain-bandwidth products. This is due to the nonlocal nature of impact ionization, which can be neglected if the thickness of the multiplication region is much greater than the 'dead length', the distance over which carriers gain sufficient energy to impact ionize. The advantage of thin multiplication regions, i.e., those for which, the dead space accounts for a significant portion of the total thickness, is that the number of ionization chains that result in multiplication greatly in excess of the average gain is reduced, which in turn yields lower noise for a given gain. In this paper we describe materials and structural modifications to the thin multiplication regions that result in even lower excess noise. For gains <EQ 20 APDs with thin AlxGa1-xAs multiplication layers have achieved excess noise factors less than twice the shot noise. We have also shown that ultra low noise can be achieved with an Impact-Ionization-Engineered approach that utilizes heterojunctions to incorporate adjacent regions with low and high ionization rates.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.