This paper describes the device fabrication process and characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) aimed for millimeter-wave applications. We developed three novel techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N/GaN HFETs with a gate length of 30 nm had a maximum drain current density of 1.6 A/mm and a maximum transconductance of 402 mS/mm. The use of these techniques led to a current-gain cutoff frequency of 181 GHz and a maximum oscillation frequency of 186 GHz.

We fabricated 50-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates by using a conventional process under low temperatures, below 300C, to prevent fluorine contamination and suppress possible diffusion of the Si-δ-doped sheet in the electron-supply layer, and measured the DC and RF performance of the transistors. The DC measurement showed that the maximum transconductance gm of a 50-nm-gate HEMT is about 0.91 S/mm. The cutoff frequency fT of our 50-nm-gate HEMT is 362 GHz, which is much higher than the values reported for previous 50-nm-gate lattice-matched HEMTs. The excellent RF performance of our HEMTs results from a shortening of the lateral extended range of charge control by the drain field, and this may have been achieved because the low-temperature fabrication process suppressed degradation of epitaxial structure.