Numerical Modeling of Graded Bandgap Long Wavelength Infrared HgCdTe Avalanche Photodiodes

We present an in-depth numerical study of the design of a long wavelength infrared (LWIR) avalanche photodiode (APD) that incorporates a compositional gradient to improve performance in terms of gain, noise, and dark current. The modeling uses a combination of drift–diffusion to assess dark current mechanisms and Monte Carlo (MC) simulations to assess gain and noise, which are impact ionization-related properties. The HgCdTe material parameters used in this study were carefully tuned to published experimental data for devices with a uniform composition, and then applied to the design of a proposed device with a compositional gradient. The proposed device is shown to significantly reduce the impact of band-to-band tunneling, which extends the operational bias range, resulting in an APD with higher gain and lower dark current. Furthermore, it is shown that in certain cases, absorption of illumination within the multiplication region can degrade the superb gain and noise performance of HgCdTe APDs, an effect that is eliminated with the proposed molar grading. This manuscript also discusses the implementation of a robust technique to simulate smoothly graded composition within the MC framework.