Abstract
For high-bit-rate, long-haul systems, the avalanche photodiode (APD) is frequently the pho-todetector of choice owing to its internal gain, which provides a sensitivity margin relative to PIN photodiodes. APDs can achieve 5–10 dB better sensitivity than PINs, provided that the multiplication noise is low and the gain-bandwidth product of the APD is sufficiently high. The multiplication region of an APD plays a critical role in determining the gain, the multiplication noise, and the gain-bandwidth product. Low multiplication noise and high gain-bandwidth products have been achieved by submicron scaling of the thickness of the multiplication region.1–9 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. However, when the dead space accounts for a significant portion of the multiplication region, 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. Recently, it has been shown that the noise of APDs with thin multiplication regions can be reduced even further by incorporating new materials and impact ionization engineering with beneficially designed heterostructures.10,11 High gain-bandwidth products have been achieved by incorporating low-noise multiplication regions into resonant-cavity and waveguide structures.7,12
© 2002 Optical Society of America
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