Resonant-cavity avalanche photodiodes and narrow spectral response photodiodes

The performance of conventional photodiodes is limited by an intrinsic tradeoff between quantum efficiency and bandwidth. We have successfully demonstrated that resonant-cavity photodiodes can simultaneously achieve high quantum efficiency and wide bandwidths. The resonant-cavity approach increases the absorption through multiple reflections between two parallel mirrors. In addition, the wavelength selective spectral response offers potential advantages for applications where filtering is needed such as wavelength division multiplexing. In this paper, we report on a resonant-cavity, separate absorption and multiplication avalanche photodiode (APD). An avalanche gain of more than 40 has been obtained. The peak external quantum efficiency was measured to be approximately 80% at 890 nm with a spectral response linewidth of less than 7 nm. Without cavity enhancement, the efficiency of these APDs would be less than 5% because the absorbing layer is only 500 angstroms thick. The structure was designed to provide electron injection into the multiplication region because we have determined experimentally that for Al 0.4 Ga 0.6 As, the electron ionization coefficient is larger than that of the hole. Based on the conventional formulation for excess noise, the ratio of ionization coefficients is as low as (beta) /(alpha) equals 0.2.