A physics-based frequency dispersion model of GaN MESFETs

A physics-based model for GaN MESFETs is developed to determine the frequency dispersion of output resistance and transconductance due to traps. The equivalent circuit parameters are obtained by considering the physical mechanisms for current collapse and the associated trap dynamics. Detrapping time extracted from drain-lag measurements are 1.55 and 58.42 s indicating trap levels at 0.69 and 0.79 eV, respectively. The dispersion frequency is in the range of megahertz at elevated temperature, where a typical GaN power device may operate, although at room temperature it may be few hertz. For a 1.5 × 150 μm GaN MESFET with drain and gate biases of 10 V and -1 V, respectively, 5% decrease in transconductance and 62% decrease in output resistance at radio frequencies (RFs) from their DC values are observed. The dispersion characteristics are found to be bias dependent. A significant decrease in transconductance is observed when the device operates in the region where detrapping is significant. As gate bias approaches toward cutoff, the difference between output resistance at dc and that at RF increases. For drain and gate biases of 10 and -5 V, output resistance decreases from 60.2 kΩ at dc to 7.5 kΩ at RF for a 1.5 μm × 150 GaN MESFET.