This innovation comprises technology that has the ability to measure at least two ultraviolet (UV) bands using one detector without relying on any external optical filters. This allows users to build a miniature UVA and UVB monitor, as well as to develop compact, multicolor imaging technologies for flame temperature sensing, air-quality control, and terrestrial/counter-camouflage/biosensing applications.

The structure is designed for back illumination and contains six AlGaN layers with different doping, Al percentage, and two contacts — A and B. The cut-off wavelength of AlGaN can be tuned from 200 nm to 365 nm by changing the Al percentage. There are three band-edges in this structure that correspond to AlxGa1–xN, AlyGa1–yN, and AlzGa1–zN — x, y, and z should be designed to be x>y >z for back illumination.

When photons are injected from the backside, they will be absorbed at different layers depending on the wave-length of the photons. Electrically, the device is a back-to-back pin structure along the vertical direction. When B is biased positively, and A is connected to the ground, the bottom pin is forwardly biased and acts as a current variable resistor with resistance becoming negligible when the bias on B is high enough. While the bottom pin is forward biased, the top pin junction is reverse biased and acts as a detector. Because the depletion mainly happens in the n-AlzGa1–zN layer, only the photons absorbed in n-AlzGa1–zN will be converted into photon-current. When the bias is applied in an opposite manner, in which B is biased negatively and A is connected to the ground, the bottom pin is biased in reverse and acts as an active detector. The depletion region is mainly in n-

AlyGa1–yN and the photons with wxxGa1–xN layer. Most of the photo-electrons will be recombined locally without generating photocurrent.

By charging the polarity of the bias, the detector can selectively detect two different wavebands: wywz (no absorption). Practically, wx, wy, and wz are tunable between 250 nm to 300 nm. The percentage of Al in the p+ layer in the center can be any number between y and z. As a result, the two Idetection bands do not have to be continuous.

This work was done by Laddawan Miko, David Franz, and Carl M. Stahle of Goddard Space Flight Center and Feng Yan and Bing Guan of MEI Technologies, Inc. For more information, download the Technical Support Package (free white paper) at under the Physical Sciences category. GSC-15163-1

NASA Tech Briefs Magazine

This article first appeared in the April, 2010 issue of NASA Tech Briefs Magazine.

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