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The indices of refraction, extinction constants and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz are obtained using THz time-domain spectroscopy. The results correspond well with the Kohlrausch stretched exponential model. Using the Kohlrausch model fit not only provides the mobility of the free carriers in the GaN film, but also estimates the relaxation time distribution function and average relaxation time.

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We have used proton and As + implantation to increase the resistivity of conventional Si (10 -cm) and Si-on-quartz substrates, respectively. High resistivity of 1.6 M-cm is measured that is close to intrinsic Si and semi-insulating GaAs. Very low loss and cross coupling of 6.3 dB/cm and 79 dB/cm (10 m gap) at 20 GHz are measured on these samples, respectively. The very high resistivity and improved rf performance are due to the extremely fast 1 ps carrier lifetime stable even after a 400 C annealing for 1 h. Little negative effect on gate oxide integrity is also observed as evidenced by the comparable stress-induced leakage current and charge-to-breakdown for 30 A oxides.