Photoconductive switching using wideband semiconductor materials
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] A solid state switch concept based on the photoconductive properties of the aluminum gallium nitride (AlGaN) and gallium nitride (GaN) semiconductors has been developed for radar limiters. A device geometry was designed which would allow for matched radio-frequency off-state transmission but provide substantial attenuation of an incident signal in the on-state (illuminated) condition. Several possible device geometries were designed and optimized in CST Microwave Studio®. The off-state transmission and on-state limiting ability of each device was compared to determine the geometry best suited for this project. The chosen design allowed for greater than 99% off-state transmission and an on-state limiting of more than 99% of the incident signal amplitude. Experimental tests to determine the semiconductor's effectiveness to act as a photoconductive switch were performed using highly conductive silver paint as the contacts. Once initial data was taken and the responses better understood, the designed geometry was fabricated using the AlGaN and GaN semiconductors. These devices were then subjected to testing and the results compared with simulated calculations in CST and MATLAB®. The ability of AlGaN and GaN to act as a photoconductive switch when illuminated with 355-nm light has been experimentally verified. Results show a reduction in channel resistance from 12-kΩ to less than 200-Ω, limited by a Schottky contact resistance. Devices manufactured to provide ohmic contacts have shown the ability to reach values of 0.1-Ω when illuminated.
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