High-speed InP/InGaAsP/InGaAs avalanche photodiodes

High-speed operation of a long-wavelength (1.0µm ≲ λ ≲ 1.65µm) avalanche photodiode (APD) has been achieved with a heterojunction structure consisting of a wide-bandgap (InP) multiplication region and a narrow-bandgap (InGaAs) absorption region separated by an intermediate-bandgap (InGaAsP) "grading" layer. Previous InP/InGaAs APD\´s without the "grading" layer (SAM-APD structure) have exhibited low dark current and high avalanche gain, but they have not performed well at high bit rates because the bandwidth is restricted by hole trapping at the valence band discontinuity of the InP/InGaAs interface. In this paper we demonstrate that the incorporation of a "grading" layer results in a dramatic improvement in the speed of response while maintaining low dark current, good quantum efficiency, and high avalanche gain. By incorporating one of these APD\´s into a high-speed optical receiver, sensitivity measurements have been obtained for 1.3µm and 1.5µm wavelengths at bit rates of 420 Mb/s and 1 Gb/s.