Optimizing Photodetectors for Radiation Environments

In an ionizing environment, the photodetector in an optoelectronic data link is a major source of unwanted signal. An analysis is presented of the analog signal response of Si and other photodiode materials to light and to ionizing radiation for single-pass, two-pass, and multipass optical detectors. Since the analog signal analysis represents a worse case calculation, photodiodes in digital data links will exhibit even higher radiation tolerances. Maximum allowable radiation dose rates are determined as a function of the effective collection depth which is defined as the width of the collection region for minority carriers. These calculations demonstrate that the proper selection of the detector material will allow signal detection in the presence of dose rates as high as 2 × 106 rads/sec per mW/cm2 of light power in a realistic photodiode structure. The parameters used in the calculations for Si are verified by experiment.