Multigain-Stage InGaAs Avalanche Photodiode With Enhanced Gain and Reduced Excess Noise
The design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950–1650 nm wavelength sensing applications are reported. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise–and stable linear-mode operation at much higher avalanche gain–than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact-ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6,000 without avalanche breakdown. Excess noise characterized by an effective impact-ionization-rate ratio below 0.04 were measured at gains over 1,000.