We are developing a new kind of gamma-ray imaging device that will have submillimeter spatial resolution and excellent energy resolution. The device is composed of a slab of semiconductor detector partitioned into an array of detector cells by photolithography with a monolithic circuit chip called a multiplexer (MUX) for readout. The application is for an ultra-high-resolution, modular SPECT system for functional brain imaging. In this paper, we describe preliminary results obtained with a Hughes 48 × 48 array of Ge PIN photodiodes with MUX readout originally developed as an infrared focal-plane-array detector. The device operates at cryogenic temperatures. We show that the device functions as an array of individual gamma-ray detectors and describe initial results for single pixel gamma-ray response, including linearity, quantum efficiency and energy resolution as a function of gamma-ray energy. A measured energy resolution of 1.5% FWHM is found for 99mTc 140 keV gamma rays. When used as an imaging detector, the spatial resolution of this device should be similar to the pixel spacing (125 μm).