Effects of parasitic gate capacitance and gate resistance on radiofrequency performance in L_G = 0.15 μm GaN high-electron-mobility transistors for X-band applications

The effects of the parasitic gate capacitance and gate resistance (R_g) on the radiofrequency (RF) performance are investigated in L_G = 0.15 μm GaN high-electron-mobility transistors with T-gate head size ranging from 0.83 to 1.08 μm. When the device characteristics are compared, the difference in DC characteristics is negligible. The RF performance in terms of the current-gain cut-off frequency (f_T) and maximum oscillation frequency (f_max) substantially depend on the T-gate head size. For clarifying the T-gate head size dependence, small-signal modeling is conducted to extract the parasitic gate capacitance and R_g. When the T-gate head size is reduced from 1.08 to 0.83 μm, R_g increases by 82%, while f_T and f_max improve by 27% and 26%, respectively, because the parasitic gate–source and gate–drain capacitances reduce by 19% and 43%, respectively. Therefore, minimizing the parasitic gate capacitance is more effective that reducing R_g in our transistor design and fabrication, leading to improved RF performance when reducing the T-gate head size.