Researchers at Massachusetts Institute of Technology Lincoln Laboratory (Lexington, MA) have achieved ultraviolet photon counting with gallium nitride (GaN) avalanche photodiodes (APDs). The device structure, grown in a vertical chloride-transport hydride vapor-phase-epitaxy reactor on top of an unintentionally doped 10- to 15-mm-thick GaN layer, included a 0.6-mm-thick silicon-doped layer, an unintentionally doped layer, and a 0.2-mm-thick zinc-doped layer. A voltage source in series with a load resistor (50 to 200 kohm) was used to temporarily bias the device above the breakdown voltage. The device underwent avalanche breakdown when the bias exceeded -86 V, while the load resistor reduced the load on the APD as its own internal resistance dropped during breakdown. Peak current values greater than 0.5 mA were measured for an effective gain on the order of 107. The maximum photon detection efficiency was 13% at 325 nm, room temperature, and less than 425-kHz dark-count rate. Contact Alexander McIntosh at [email protected].