Comparison of Thin AlxGa1-xAsSb Avalanche Regions for High-Speed Optical Communication

Employing avalanche photodiodes (APDs) significantly enhances the signal-to-noise ratio (SNR) of high-speed optical communication links, compared to conventional photodiodes. The lower limit of the conventional InP avalanche region is due to the onset of band-to-band tunneling current. This article reports on the experimental excess noise and dark current characteristics of Al ${}_{x}$ Ga ${}_{{1}-{x}}$ AsSb APDs with avalanche regions (100nm intended avalanche region width) and ${x} \lt {0.85}$ for high-speed applications. The bandgaps range from 1.40 to 1.49 eV, corresponding to ${x} = {0.68}$ and 0.78. Under pure electron injection, the APDs exhibit excess noise factors of 2–3 at a gain of 10, comparable to that of ${x} = {0.85}$ and better than InP and InAlAs APDs. The APDs’ experimental dark current density is lower than the tunneling current densities of InP and InAlAs with similar avalanche widths. Simulations of the optical receiver’s sensitivity showed that the receiver’s sensitivity is unaffected by the Al content when using a ~114-nm avalanche width. Therefore, Al ${}_{x}$ Ga ${}_{{1}-{x}}$ AsSb with ${x} ={0.68}$ to 0.78 can be the smaller-bandgap alternative to the widely reported ${x} = {0.85}$ , for reducing the bandgap difference between the InGaAs absorber and the AlGaAsSb avalanche region in high-speed, low-noise APDs.