Recessed 70-nm Gate-Length AlGaN/GaN HEMTs Fabricated Using an $\hbox{Al}_{2}\hbox{O}_{3}/\hbox{SiN}_{x}$ Dielectric Layer

In this letter, a novel process for recessed-gate AlGaN/GaN high-electron-mobility transistors using an Al₂O₃/SiNx dielectric has been developed. The Al₂O₃/SiNx dielectric bilayer was used as a recess etch-mask for short-gate-footprint definition. Recessed-gate devices with a gate length of 70 nm have been fabricated on a molecular-beam-epitaxy-grown layer structure using this process. After the removal of the dielectric layers, excellent dc and small-signal results, a high drain-current density of 1.5 A/mm, a unity gain cutoff frequency of 160 GHz, and a maximum frequency of oscillation of 200 GHz were obtained.

I. Introduction

For microwave high-power and noise applications, AlGaN/GaN high-electron-mobility transistors (HEMTs) have demonstrated excellent device characteristics [1]–[3]. Very few results have been reported for AlGaN/GaN HEMTs with unity current gain frequency exceeding 150 GHz [4]–[6]. Higashiwaki et al. [4] reported an of 181 GHz and a maximum oscillating frequency of 186 GHz for devices with a gate length of 30 nm. Recently, they have further reported results for devices with 's as high as 190 GHz and as high as 251 GHz for devices with of 60 nm [5]. Palacios et al. [6] have reported an of 153 GHz and an of 230 GHz for devices with 100-nm-long recessed gate and an InGaN back-barrier structure in the channel. To maintain excellent gate control of the channel in short-gate-length devices, it is crucial to reduce the distance between the gate and the 2-D electron-gas (2DEG) channel. The aspect ratio is crucial to channel modulation. It is apparent that when is reduced, must also be reduced. This vertical scaling in the HEMT can be achieved through the direct growth of a thin AlGaN barrier layer or through a gate-recess technique. However, using a very thin AlGaN in the HEMT layer can result in devices with high access resistances due to lower carrier concentrations. Higashiwaki et al. [4], [5], [7] adopted thin catalytic chemical-vapor-deposited (Cat-CVD) SiN as the passivation and the gate dielectric layer in order to increase electron concentrations when using ultrathin ( thick) AlGaN layers with relatively high Al concentration (0.4). In the gate-recess technique, it is desirable to limit the recess area underneath the gate in order to reduce access resistance. These precautions in processing are critical if the fullest potentials of AlGaN/GaN HEMTs are to be realized.