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.

I. Introduction

The major factors that limit the performance and reliability of AlGaN/GaN high-electron mobility transistors (HEMTs) for high-power radio frequency and high-temperature applications are their high gate leakage and drain current collapse. Significant progress has been made on GaN/AlGaN metal–oxide–semiconductor HEMTs (MOS-HEMTs) using [1], [2], [3], [4]–[6], and [7] as the gate dielectrics to suppress the aforementioned problems but at the expense of a significant decrease in device transconductance [4] and a large threshold voltage shift [5], [6]. The use of dielectrics with high permittivity (high ) could help solve these problems, because a larger dielectric constant could translate to a more efficient gate modulation [8]; thus, a smaller decrease in transconductance and a moderate increase in the threshold voltage could be expected in MOS-HEMTs with high- gate dielectrics. , having a high dielectric constant (20 25) and being highly insulating with a large bandgap (5.6 5.8 eV), has been studied extensively as the gate dielectric in Si MOS field-effect transistors [9]. Recently, dielectric films on a GaN surface using reactive sputtering [8] and atomic layer deposition (ALD) [10] have been reported. However, a major obstacle is the lack of high-quality and thermodynamically stable insulators on III–V semiconductors with a low interface state density and a good interfacial layer comparable to that of the interface.