KEYWORDS: Field effect transistors, Transistors, Molybdenum, Gallium nitride, Personal digital assistants, Atomic layer deposition, Interfaces, Diodes, Dielectrics, Chemical species
We have investigated AlGaN/GaN high-electron mobility transistors (HEMTs) with a high κ gate dielectric using hafnium silicate (HfSiOx). The (HfO2)/(SiO2) laminate structure was deposited on the AlGaN surface by a plasma enhanced atomic layer deposition, followed by a post-deposition annealing at 800 °C. The HfSiOx-gate HEMT showed good transfer characteristics with a high transconductance expected from its κ value and a subthreshold swing of 71 mV/ decade. In addition, we observed excellent capacitance-voltage (C-V) characteristics with negligible frequency dispersion in the metal-oxide-semiconductor (MOS) HEMT diode. The detailed C-V analysis showed low state densities in the order of 1011 cm-2 eV-1 at the HfSiOx/AlGaN interface.
The selective and low-damaged etching of p-type GaN or AlGaN layer is inevitable process for AlGaN/GaN high-power transistors. We have investigated an electrochemical etching of p-GaN layer grown on AlGaN/GaN heterostructures, consisting of an anodic oxidation of p-GaN surface and a subsequent dissolution of the resulting oxide. The p-GaN layer was electrochemically etched by following the pattern of the SiO2 film that acted as an etching mask. Etching depth was linearly controlled by cycle number of triangular waveform at a rate of 25 nm/cycle. The AFM, TEM and μ-AES results showed that the top p-GaN layer was completely removed after 5 cycles applied, and the etching reaction was automatically sopped on the AlGaN surface. I-V and C-V measurements revealed that no significant damages were induced in the AlGaN/GaN heterostructures.
Surface characterization and control technologies were applied to GaN and AlGaN surfaces. It was found
that a unique "air-gap CV" technique is effective in evaluating surface state density on free AlGaN surfaces. A
photoelectrochemical process, utilizing a mixed solution of propylene glycol and tartaric acid, was employed to
form a thin oxide layer on GaN and AlGaN. We observed an enhancement of drain current in the AlGaN/GaN
HEMT having a narrow channel width of 200 nm after the oxidation of the channel walls by the electrochemical
process. To improve the uniformity of the effective electric field in the channel, a multi-mesa-channel (MMC)
AlGaN/GaN HEMT has been proposed and developed. With forming a periodic trench just under the gate region
by an ECR-plasma assisted dry etching, the MMC HEMT has parallel mesa-shaped channels with 2-dimensional
electron gas (2DEG) surrounded by the top- and side-gate electrodes.
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