Picosecond Response of Ultraviolet Photodiodes on GaN

LLE Review, Volume 97 25 Experimental results on the fabrication, packaging, and testing of very fast metal-semiconductor-metal photodiodes (MSM-PD’s) made on gallium nitride (GaN) have been previously reported.1 The devices—with feature sizes ranging from 0.3 μm to 5 μm—were packaged in a circuit that was designed to easily couple the electrical transients out of the device, thus making them suitable for practical applications. A temporal response of 55±5-ps full width at half maximum (FWHM) was measured in all devices, independent of feature size. External bias was changed from 1 V to 10 V, and the device area was decreased by a factor of 4 to reduce the total capacitance, neither of which had a significant effect on the measured speed. Only high illumination levels produced a change in the device response. This change was attributed to space-charge screening effect. These results led to the conclusion that the device response was dominated by the packaging and measurement system. Theoretical calculations2–5 have predicted, however, that the steady-state peak electron velocity in GaN is around 3 × 107 cm/s, which is higher than that in GaAs. This implies that the inherent speed in GaN detectors should be substantially faster than in GaAs devices. Joshi et al.,6 in particular, using Monte Carlo simulations, studied the dynamic response of GaN MSM-PD and predicted a FWHM of 3.5 ps for a device with 0.25-μm feature size under low-bias and low-level illumination.