AlGaN/GaN MOS-HEMT With $\hbox{HfO}_{2}$ Dielectric and $\hbox{Al}_{2}\hbox{O}_{3}$ Interfacial Passivation Layer Grown by Atomic Layer Deposition

We have developed a novel AlGaN/GaN metal–oxide–semiconductor high-electron mobility transistor using a stack gate $\hbox{HfO}_{2}/\hbox{Al}_{2}\hbox{O}_{3}$ structure grown by atomic layer deposition. The stack gate consists of a thin $ \hbox{HfO}_{2}$ (30-$\hbox{\rm{\AA}}$) gate dielectric and a thin $\hbox{Al}_{2}\hbox{O}_{3}$ (20- $\hbox{\rm{\AA}}$) interfacial passivation layer (IPL). For the 50-$\hbox{\rm{\AA}}$ stack gate, no measurable $C$–$V$ hysteresis and a smaller threshold voltage shift were observed, indicating that a high-quality interface can be achieved using a $\hbox{Al}_{2}\hbox{O}_{3}$ IPL on an AlGaN substrate. Good surface passivation effects of the $\hbox{Al}_{2}\hbox{O}_{3}$ IPL have also been confirmed by pulsed gate measurements. Devices with 1- $\mu\hbox{m}$ gate lengths exhibit a cutoff frequency $(f_{T})$ of 12 GHz and a maximum frequency of oscillation $(f_{\rm MAX})$ of 34 GHz, as well as a maximum drain current of 800 mA/mm and a peak transconductance of 150 mS/mm, whereas the gate leakage current is at least six orders of magnitude lower than that of the reference high-electron mobility transistors at a positive gate bias.