Characterization of gain, quantum efficiency, and noise-equivalent power in III-V avalanche photodiodes

We report a comprehensive noise and sensitivity analysis of GaAsSb/AlGaAsSb separate-absorption–charge– multiplication (SACM) avalanche photodiodes (APDs) grown on InP substrate and operating at a wavelength of 1550nm. Devices with diameters of 150μm, 250μm, and 350μm were characterized under reverse biases of −45V and −65V using sinusoidally intensity-modulated optical excitation. Voltage noise spectral density measurements were used to extract input-referred current noise, responsivity, quantum efficiency, noise-equivalent power (NEP), and signal-to-noise ratio (SNR) as functions of optical power. All metrics exhibit pronounced power-dependent transitions whose onset depends strongly on reverse bias and, to a lesser extent, device geometry. At −65V, enhanced avalanche gain improves low-power sensitivity but leads to earlier saturation behavior, manifested as responsivity flattening, increased input-referred noise, and degradation of NEP and SNR at moderate optical powers. In contrast, −45V operation preserves a wider linear operating range but exhibits a delayed and more abrupt sensitivity degradation at the highest optical powers. Among all devices, the 150μm APD operated at −65V achieves the lowest NEP and highest SNR at the maximum measured optical power, representing the best overall sensitivity in this study. These results highlight the competing roles of avalanche gain and excess noise in GaAsSb/AlGaAsSb SACM APDs and identify bias- and power-dependent operating regimes relevant for optimized SWIR detection.