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

The effects of the parasitic gate capacitance and gate resistance (Rg) on the radiofrequency (RF) performance are investigated in LG = 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 (fT) and maximum oscillation frequency (fmax) 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 Rg. When the T‐gate head size is reduced from 1.08 to 0.83 μm, Rg increases by 82%, while fT and fmax 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 Rg in our transistor design and fabrication, leading to improved RF performance when reducing the T‐gate head size.