| Semiconductor devices on GaN have been studied in all aspects for many years. Among all the applications, high-speed response of the optical receiver is of special interest. Theoretical simulation showed that metal-semiconductor-metal (MSM) photodiode on GaN is a promising candidate for broad bandwidth application in UV region with impulse response time of picosecond. In this thesis, both real application of the UV photodiode and fundamental physics of the free carriers in GaN will be studied.; In real applications, the device has to be integrated into a package so that the optically generated electrical waveform can be coupled to standard instruments. It is, therefore, important to make the design in such way that the mismatch and discontinuities between the device and the fast circuit can be minimized. To ensure the functionality of the fast package, I carried out numerical simulations of the entire assembly using a distributed circuit approach. The samples were fabricated at Cornell Nonafabrication facility (CNF) with electron-beam lithography. To explore the optimal condition of the device, the feature size was varied from 0.3 mum to 5 mum and active areas were 25 x 25 mum2 and 50 x 50 mum2. These devices were then measured with a digitized sampling oscilloscope and showed several features predicted by theoretical simulations. Firstly, the response time of the entire package could be as fast as 55 ps. Secondly, the inherent response of the MSM photodiode was obscured by the parasitic effect associated with the circuits. The third feature is that under high-level illuminations the electrical impulse broadened significantly. To disclose the dynamic behavior of the free carriers in GaN, a faster measurement method with sub-picosecond time resolution is needed. I conducted electro-optic (EO) sampling experiments, in which the package was removed and the inherent electrical impulses were recorded. The best device showed an ultrafast time response of 3.5 ps, which represents the fastest UV photodiode on GaN reported to date. The impulse waveform was well fit by a bi-exponential function, which indicates the contributions from fast electrons and slow holes respectively. By changing the bias voltages applied on the device, the electron velocity in GaN was derived as a function of average electric field. The result is in a good agreement with a report from an independent research group.; To further increase the bandwidth of the MSM photodiode, I also measured devices with different feature sizes and active areas. These measurements provided useful guidance for future device design. |
