Design of High-Aspect-Ratio T-Gates on N-Polar GaN/AlGaN MIS-HEMTs for High $f_{\max }$

This letter discusses the design of high-aspect-ratio T-gates on molecular beam epitaxy (MBE)-grown nitrogen-polar (N-polar) GaN/AlGaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) for high power-gain cutoff frequency (f_max). A 351-GHz f_max is demonstrated, which is the highest published to date for an N-polar GaN HEMT. Novel 80-nm-long 1.1-m-tall T-gates with a 370-nm-tall stem were used to simultaneously minimize gate resistance (R_g) and parasitic gate-drain capacitance (C_gd). The device on -resistance (R_on) of 0.42 mm was obtained by employing n⁺ GaN MBE-regrown ohmic contacts and by scaling the lateral separation between regrown source-drain regions to 250 nm. Within the design space explored, this letter experimentally demonstrates that f_max is increased by reducing the gate width and the T-gate top length.