Seeing into the ultraviolet (UV) would be useful for a variety of purposes, from tracking missile launches to studying distant stars. Silicon-based detectors, however, are not the best means of capturing UV light. To improve the technology, researchers at North Carolina State University (NCSU; Raleigh, NC), working with the US Army Night Vision Laboratory (Ft. Belvoir, VA), have developed a visible-blind UV digital camera based on gallium nitride.
The camera contains a 32 x 32 array of gallium nitride/aluminum gallium nitride (GaN/AlGaN) heterostructure PIN photodiodes. A base layer of n-doped AlGaN with approximately 20% aluminum is topped by an undoped GaN layer and a p-doped GaN layer. The entire structure is built on a polished sapphire substrate through which light can shine. The photodiode is sensitive to light from approximately 320 to 365 nm. Wavelengths shorter than 320 nm are absorbed by the AlGaN base layer, and those longer than 365 nm pass through the GaN. Increasing the aluminum content in either the base or the top layers can change the photodiode's bandwidth.