  The Geiger photodiodes, or single photon avalanche photodiodes, for a
radiation tolerant solid-state photomultiplier (SSPM) are being designed using
Aluminum Gallium Arsenide (AlGaAs) and are less than 1 micrometer thick.
Studies on the changes in dark current compared to silicon from thermal neutron
damage were conducted demonstrating that the design concept is feasible, as the
device sensitivity to incident radiation is low relative to silicon devices. To
achieve the required high fields for Geiger mode operation within a thin
device, a triple mesa structure was developed, and simulations show a favorable
field profile that constrains the highest electric fields to the middle of the
device and keeps the field strength at all corners and surfaces 3-6x lower than
the field in the center at breakdown.
