Comparison of waveguide avalanche photodiodes with InP and InAlAs multiplication layer for 25 Gb/s operation

2.5 and 10  Gb/s InP/InGaAs avalanche photodiodes (APDs) have been widely used in optical communication systems. However, the study on InP/InGaAs APDs above 10  Gb/s is insufficient. Recently, high-speed and high-sensitivity APDs for 100  Gb/s or even 400  Gb/s optical communication systems have drawn a lot of attention. On basis of the physical model for frequency response of APD including the dead space effect, a waveguide separate absorption, grading, charge, and multiplication (WG-SAGCM) InP/InGaAs APD has been designed for 25  Gb/s operation. Also, the frequency response of WG InAlAs/InGaAs APD was also simulated, which is perfectly in accordance with the experimental data. The comparison between InP/InGaAs APD and InAlAs/InGaAs APD with the same thickness of multiplication layer shows that the speeds of carriers in the nonionization layers are also important for the gain-bandwidth characteristics of SAGCM WG-APD. The higher drift velocity of carriers returned from multiplication layer and the lower drift velocity of carriers injected into multiplication layer result in a higher gain-bandwidth product and a higher dc gain. This work is helpful for the design of high-speed APDs.