Figures of merit in high-frequency and high-power GaN HEMTs

The most important metrics for the high-frequency and high-power performance of microwave transistors are the cut-off frequency f_T, and the Johnson figure of merit FoM_Johnson. We have simulated a state-of-the-art, high-frequency and high-power GaN HEMT using our full band Cellular Monte Carlo (CMC) simulator, in order to study the RF performance and compare different methods to obtain such metrics. The current gain as a function of the frequency, was so obtained both by the Fourier decomposition (FD) method and the analytical formula proposed by Akis. A cut-off frequency f_T of 150 GHz was found in both the transit time analysis given by the analytical approach, and the transient Fourier analysis, which matches well with the 153 GHz value measured experimentally. Furthermore, through some physical considerations, we derived the relation between the FoM_Johnson as a function of the breakdown voltage, V_BD, and the cut-off frequency, f_T . Using this relation and assuming a breakdown voltage of 80V as measured experimentally, a Johnson figure of merit of around 20 × 10 ¹²V/s was found for the HEMT device analyzed in this work.