Optimization of a HOT LWIR HgCdTe Photodiode for Fast Response and High Detectivity in Zero-Bias Operation Mode

Fast response is an important property of infrared detectors for many applications. Currently, high-temperature long-wavelength infrared HgCdTe heterostructure photodiodes exhibit subnanosecond time constants while operating under reverse bias. However, nonequilibrium devices exhibit excessive low-frequency 1/ f noise that extends up to MHz range, representing a severe obstacle to their widespread application. Present efforts are focused on zero-bias operation of photodiodes. Unfortunately, the time constant of unbiased photodiodes is still at the level of several nanoseconds. We present herein a theoretical investigation of device design for improved response time and detectivity of long-wavelength infrared HgCdTe photodiodes operating at 230 K in zero-bias mode. The calculation results show that highly doped p -type HgCdTe is the absorber material of choice for fast photodiodes due to its high electron diffusion coefficient. The detectivity of such a device can also be optimized by using absorber doping of N A = 1 × 10 17 cm −3 . Reduction of the thickness is yet another approach to improve the device response. Time constant below 1 ns is achieved for an unbiased photodiode with absorber thickness below 4 μ m. A tradeoff between the contradictory requirements of achieving high detectivity and fast response time is expected in an optically immersed photodiode with very small active area.