High-speed resonant-cavity SAM avalanche photodiodes

Separate absorption and multiplication (SAM) APDs have been widely deployed in long-wavelength, high-bit-rate optical transmission systems. At high bit-rates (>10 GBit/s), however, the bandwidth and the gain-bandwidth-product of the SAM APDs has become a limitation. Recently, we have developed a resonant-cavity SAM APD structure that circumvents both of these performance limitations. Since the quantum efficiency is decoupled from the transit time in the resonant-cavity structure, bandwidths >20 GHz have been achieved in the low-gain regime. At high gains, the response of an APD is determined by the gain-bandwidth-product. The use of very thin multiplication layers in the resonant-cavity structures has led to low noise (/spl beta///spl alpha/=k<0.25) and gain-bandwidths as high as 180 GHz. The external quantum efficiency of these resonant-cavity SAM APDs is about 80 %, resulting in a record bandwidth-efficiency product as well. The breakdown voltage (V/sub b/) is <15 V and the dark current is <10 nA at 90% V/sub b/.