Program Summary
Avalanche photodiodes (APD) are a prevalent device structure for a number of applications, such as laser rangefinders, optical communications, and three-dimensional imaging laser radar (LADAR) due to their internal photo-electronic signal gain. Current state-of-the-art APDs rely upon intrinsic material characteristics to produce impact ionization and resulting gain. However, this gain is usually accompanied by excess noise as both electrons and holes can initiate multiplication. APDs can operate in linear mode, in which the photocurrent is proportional to the signal received and is below the breakdown voltage of the APD, or in Geiger mode, in which the photocurrent saturates at any level of optical signal received and operates above the breakdown voltage of the APD. Although Geiger mode APDs (GmAPDs) are much more sensitive, they are prone to upset and long dead times. Conversely, linear mode APDs (LmAPDs) can resolve signal amplitude and would be preferred over GmAPDs if their sensitivity can be increased without generating excess noise.
The Gain Enhancement by Novel Impact Ionization (GENII) Microsystems Exploration (?E) topic aims to identify, develop, and demonstrate new approaches to achieve low excess noise, high gain infrared LmAPD structures with cutoff wavelength ≥ 2.0 ?m, operating at ≥240K, with low dark current. Modern bandgap engineering techniques have been developed to bring about new optoelectronic device structures that feature high sensitivity, low noise, high temperature operation, or a combination thereof. Such techniques can be applied to novel APD structures in order to maintain exquisite performance at higher operating temperatures and longer cutoff wavelengths. GENII seeks to explore and expand such bandgap engineering strategies as applied to LmAPDs to determine the trade space for future full scale DARPA Microsystems Technology Office (MTO) solutions.