Ki-Sik Im

Normally Off GaN MOSFET Based on AlGaN/GaN Heterostructure With Extremely High 2DEG Density Grown on Silicon Substrate

A normally off GaN MOSFET was proposed by utilizing an extremely high 2-D electron-gas density (> 1014 / cm2) at an AlGaN/GaN heterostructure as source and drain, which can be obtained by controlling the tensile stress accompanied with the growth of GaN on silicon substrate. The fabricated MOSFET with an Al2O3 gate insulator exhibited excellent device performance, such as a threshold voltage of 2 V, drain current of 353 mA/mm, extrinsic transconductance of 98 mS/mm, and field-effect mobility of 225 cm2/V·s.

Effects of TMAH Treatment on Device Performance of Normally Off

Normally off Al2O3/GaN MOSFETs are fabricated by utilizing a simple tetramethylammonium hydroxide (TMAH) treatment as a postgate-recess process. The TMAH-treated device with a gate length of 2.5 μm exhibited excellent device performances, such as a threshold voltage of 3.5 V, a maximum drain current of 336 mA/mm, and a breakdown voltage of 725 V, along with extremely small gate leakage current of about 10-9 A/mm at Vgs = 15 V, which is approximately six orders lower in magnitude compared to that of the device without TMAH treatment.

\hbox{Al}_{2}\hbox{O}_{3}/\hbox{GaN}

Two types of fin-shaped field-effect transistors (FinFETs), one with AlGaN/GaN heterojunction and the other with heavily doped heterojunction-free GaN layer operating in junctionless mode, have been fabricated and characterized. The threshold voltages of both devices shift toward positive direction from large negative value as the fin width decreases. Both devices exhibit high ON-state performance. The heterojunction-free GaN FinFETs show superior OFF-state performance because the current flows through the volume of the GaN channel layer, which can be fully depleted. The proposed GaN nanochannel FinFETs are very promising candidates not only for high performance, but also for high power applications.

MOSFET

A single-nanoribbon Al₂O₃/GaN metal-insulator-semiconductor field-effect transistor (MISFET) has been fabricated. The fabricated device exhibits normally off operation with a threshold voltage of 2.1 V. The device also exhibits superior performances such as a maximum drain current density of 1.51 A/mm, a maximum transconductance of 580 mS/mm, and a field-effect mobility of 293 cm²·V^-1·s^-1. This is because the electron concentration in the GaN channels can be increased due to the enhanced gate controllability, which, thus, effectively screens the field lines from the interface traps or the defects near the channels to improve the electron mobility in the channel. The nanoribbon Al₂O₃/GaN MISFET is a very promising candidate for high-performance normally off GaN FETs.

High-Performance GaN-Based Nanochannel FinFETs With/Without AlGaN/GaN Heterostructure

Heavily doped GaN nanochannel fin-shaped field-effect transistors (FinFETs) without heterojunction have been fabricated and characterized for the first time. Simplified pragmatical technology for GaN epitaxial growth and FinFET process was used to achieve nanodevices with a channel width from 40 to 100 nm and a gate length of 1 μm. They exhibit excellent on-state performance, such as maximum drain current of 670 mA/mm and maximum transconductance of 168 mS/mm. Record off-state performance was measured: extremely low leakage current of ~ 10-11 mA and source-drain breakdown voltage of ~280 V. The subthreshold slope of 68 mV/decade is close to the theoretical limit (60 mV/decade, so far achieved only in SOI MOSFETs) and leads to very high Ion/Ioff ratio of 108 - 109. The proposed heterojunction-free nanochannel GaN FinFET is a very promising candidate not only for high-performance and high-speed integrated circuits but also for high-power applications.

Normally Off Single-Nanoribbon

AlGaN/GaN-based fin-shaped field-effect transistors with very steep side-wall have been fabricated by utilizing electron-beam lithography and subsequent anisotropic side-wall wet etch in tetramethyl ammonium hydroxide solution. The investigate device demonstrated extremely broad transconductance (gm) ranging from ~0 to ~8 V at VD = 10 V, which is essential for high linearity device performance. Pulse measurement showed that the device exhibits negligible gate lag, but still suffers from drain lag. The device with LGD of 17 μm exhibited excellent OFF-state characteristic with subthreshold swing of ~58 mV/decade, low OFF-state leakage current of ~10-12 A, and breakdown voltage of ~400 V at VG = -9 V.

\hbox{Al}_{2} \hbox{O}_{3}\hbox{/GaN}

An AlGaN/GaN-based Ω-shaped nanowire fin-shaped FET (FinFET) with a fin width of 50 nm was fabricated using tetramethylammonium hydroxide (TMAH)-based lateral wet etching. An atomic layer deposited (ALD) HfO2 side-wall layer served as the etching mask. ALD Al2O3 and TiN layers were used as the gate dielectric and gate metal, respectively. The Ω-shaped gate structure fully depletes the active fin body and almost completely separates the depleted fin from the underlying thick GaN buffer layer, resulting in superior device performance. The top-down processing proposed in this work provides a viable pathway towards gate-all-around devices for III–nitride semiconductors.

MISFET

We have demonstrated the lateral multifinger-type Schottky barrier diode (SBD) with bonding pad over active structure fabricated on the AlGaN/GaN heterostructure prepared on sapphire substrate. The fabricated GaN-SBD with size of 9 mm2 exhibited excellent device characteristics such as forward current of 4.5 A at 1.5 V, leakage current of 6 μA at 600 V, and high breakdown voltage of 747 V. The temperature variations of GaN-SBD for the reverse recovery characteristics are negligible and the value of reverse-recovery charge (Q)rr of GaN-SBD is one twentieth of Si-diode at 175°C.

Heterojunction-Free GaN Nanochannel FinFETs With High Performance

We have investigated the photoluminescence (PL) characteristics of AlGaN/GaN heterostructure with an extremely high two-dimensional electron gas (2DEG) density. A very high 2DEG density was obtained by controlling the tensile stress during the growth of the heterostructure on silicon substrate. Strong PL emission peaks corresponding to both the band edge and the 2DEG were observed in the highly tensile-stressed heterostructure. Furthermore, the strong longitudinal optical (LO) phonon replicas (1-LO2DEG and 2-LO2DEG) of the 2DEG peak were also observed in the heterostructure.

AlGaN/GaN FinFET With Extremely Broad Transconductance by Side-Wall Wet Etch

The recessed-gate AlGaN/GaN metal?oxide?semiconductor heterostructure field-effect transistors (MOSHFETs) with a p-GaN back-barrier studied in this work exhibited much lower buffer leakage current than those without the back-barrier. The threshold voltage of the device with the p-GaN back-barrier was controlled by varying the depth of gate recess etching, and a value as high as 2.9 V was obtained with deep gate-recess etching into the channel layer. The device structure has the advantage of both low leakage current and high threshold voltage, which is important for power-switching applications. In contrast, the performance parameters of the device, such as subthreshold slope and field-effect mobility, can be degraded owing to increased plasma damage with increasing recess depth.