Silicon avalanche photodiodes (Si APDs) have low dark current and low excess noise factor, and are currently used in many of NASA’s missions. Noise equivalent power (NEP) of 40 to 50 fW/(Hz)1/2 over 140-MHz bandwidth has been demonstrated for Si APDs. Si APDs have very low responsivity for wavelengths longer than 1.1 mm, and cannot be used in future NASA lidar missions that require low noise and large-area photodetectors operated in the shortwave infrared (SWIR) region. The need for high-performance fiber optic communications receivers has provided the impetus for substantial progress during the last two decades in the understanding and performance of InP-based APDs that exhibit high responsivity in the wavelength range from 0.9 to 1.7 mm.

The technology described here was developed for large-area, high-performance photodetectors sensitive in SWlR wavelengths. The technology can be used in active remote sensing optical instruments such as NASA’s ASCENDS, ACE, and Doppler Wind LIDAR, and in NASA’S free space optical communications. In addition to NASA applications, the developed technology has numerous commercial applications such as range-finding and ladar applications, optical time domain reflectomery (OTDR), and free-space optical communications.

The devices employ the separate absorption, charge, and multiplication (SACM) structure that has been widely used for telecommunications receivers with the addition of two critical design features: an ultra-low-noise impact ionization engineered multiplication region, and an epitaxially grown quenching barrier layer to provide negative feedback.

The basic idea of impact ionization engineering is to place thin layers with relatively low ionization threshold adjacent to layers with higher ionization threshold. The lower noise of the impact ionization engineering structure is a result of the spatial modulation of the probability distribution for impact ionization. The heterojunction results in a more spatially localized process, which, in turn, reduces the noise. Another unique feature is an epitaxially grown quenching barrier layer. It is very difficult for InP-based linear-mode APDs to achieve stable high gain (>50) due to the inherent positive feedback process involved in the impact ionization process and the strong dependence of ionization coefficients on electric field inside the multiplication region. The quenching barrier layer provides negative feedback to the avalanche process by dynamically adjusting the electric field inside the multiplication region. The dynamic negative feedback provided by the quenching barrier will further regulate the impact ionization process and make it more deterministic, which is expected to further reduce the excess noise factor.

This work was done by Mark Itzler and Xudong Jiang of Princeton Lightwave Inc. for Goddard Space Flight Center. GSC-16736-1