Very high-speed ultraviolet photodetectors fabricated on GaN

We report on the temporal and the frequency response of both metal-semiconductor-metal (MSM) and p-i-n ultraviolet photodetectors fabricated on single-crystal GaN. The best MSM devices show a fast 10-90% rise-time of similar to 28 psec under comparatively low ultraviolet excitation of similar to 0.1 W/cm(2) average irradiance. The fast-Fourier transform (FFT) of the pulse response data indicates a bandwidth, f(3dB), of similar to 3.8 GHz at a reverse bias of 25 V. This agrees well with the direct frequency response measurement value of similar to 3.5 GHz. For the p-i-n devices, we measured a rise-time of similar to 43 psec at 15 V reverse bias for a 60 mu m diameter mesa with 1 mu m thick intrinsic region. The FFT of the p-i-n pulse response obtains f(3dB) approximate to 1.4 GHz. Analysis in terms of reverse bias and geometric scaling indicates that the MSM photodetectors are transit-time limited. The p-i-n devices also show evidence of transit-time limited effects based on trends with respect to reverse bias and intrinsic region thickness. However, our larger area p-i-n devices show clear evidence of RC-limited behavior. Modeling of the temporal behavior indicates that a slow component in the time and frequency response data is a consequence of the hole drift velocity. We have also found preliminary evidence of microplasmic effects in the p-i-n devices.