Y. Irokawa

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

Characteristics of MgO/GaN gate-controlled metal–oxide– semiconductor diodes

Gate-controlled n+p metal–oxide–semiconductor diodes were fabricated in p-GaN using MgO as a gate dielectric and Si+ implantation to create the n+ regions. This structure overcomes the low minority carrier generation rate in GaN and allowed observation of clear inversion behavior in the dark at room temperature. By contrast, diodes without the n+ regions to act as an external source of minority carriers did not show inversion even at measurement temperatures of 300 °C. The gated diodes showed the expected shape of the current–voltage characteristics, with clear regions corresponding to depletion and inversion under the gate. The MgO was deposited prior to the Si implantation and was stable during the activation annealing for the Si-implanted n+ regions.

Photodegradation of toluene over TiO2−xNx under visible light irradiation

We report the photooxidation of toluene over nitrogen doped TiO(2) (TiO(2-x)N(x)) under visible light irradiation. The photocatalytic oxidation of toluene in air over TiO(2-x)N(x) powders was studied using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), gas chromatography (GC), ion chromatography (IC), and gas chromatography mass spectrometry (GC-MS), focusing on the photocatalytic decomposition processes of toluene. Results obtained indicate that toluene, weakly adsorbed on the catalyst surface, is initially photooxidized to benzaldehyde which adsorbs onto the TiO(2-x)N(x) surface more strongly, leading to the formation of ring-opening products such as carboxylic acids and aldehydes. No gaseous intermediates were detected during the photooxidation. Major intermediates adsorbed at the catalyst surface were oxalic acid, (COOH)(2), acetic acid, CH(3)COOH, formic acid, HCOOH, and pyruvic acid, CH(3)COCOOH, whereas more complicated carboxylic species, including propionic acid, CH(3)CH(2)COOH, isovaleric acid, (CH(3))(2)CHCH(2)COOH, and succinic acid, (CH(2)COOH)(2), were also found in the early stage of the photooxidation. These intermediate products were gradually photodegraded to CO(2) and H(2)O under visible light irradiation.

Electrical activation characteristics of silicon-implanted GaN

Electrical activation studies of Si-implanted GaN layers on sapphire were made as a function of annealing temperature (1100–1400°C). For an ion dose of 1.0×1014cm−2, the optimum annealing temperature was 1400°C, exhibiting a nearly 100% electrical activation efficiency and a low sheet resistance of ∼450Ω∕square at room temperature. From variable temperature Hall-effect measurements, Si-implanted GaN films annealed below 1200°C displayed deep ionization levels of ∼280meV, whereas samples annealed above 1300°C had shallow ones of ∼11meV. For lateral Schottky diodes fabricated on Si-implanted GaN layers annealed below 1200°C, capacitance frequency and thermal admittance measurements showed a typical dispersion effect characteristic of a single deep donor with an activation energy of ∼133meV. These results illustrate that deep donor levels created by the Si implantation in GaN layers apparently annihilate and transit to shallow levels produced by the Si ion substitution for Ga in the GaN lattice (SiGa) by ann...

Comparison of Surface Passivation Films for Reduction of Current Collapse in AlGaN/GaN High Electron Mobility Transistors

Three different passivation layers (SiN x , MgO, and Sc 2 O 3 ) were examined for their effectiveness in mitigating surface-state-induced current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). The plasma-enhanced chemical vapor deposited SiN x produced ∼70-75% recovery of the drain-source current, independent of whether SIH 4 /NH 3 or SiD 4 /ND 3 plasma chemistries were employed. Both the Sc 2 O 3 and MgO produced essentially complete recovery of the current in GaN-cap HEMT structures and ∼80-90% recovery in AlGaN-cap structures. The Sc 2 O 3 had superior long-term stability, with no change in HEMT behavior over 5 months aging.

Design of junction termination structures for GaN Schottky power rectifiers

Abstract Junction termination extension (JTE) structures for GaN power Schottky rectifiers were investigated using a quasi-three-dimensional simulator. The use of single JTE edge termination was found to produce an almost fivefold increase in reverse breakdown voltage ( V B ) over an unterminated rectifier fabricated on the same bulk GaN substrate. The use of p + guard rings or planar junction termination with oxide field plates also offers significant enhancements in V B relative to unterminated rectifiers. V B was found to be a strong function of the JTE doping concentration and the p + guard-ring spacing.

Temperature dependent characteristics of bulk GaN Schottky rectifiers on free-standing GaN substrates

The performance of Schottky rectifiers fabricated with dielectric overlap edge termination on epitaxial layers grown on a free-standing GaN template is reported. The power figure-of-merit (VB)2/RON where VB is the reverse breakdown voltage and RON is the on-state resistance was 11.5 MW cm−2. The forward turn-on voltage was ∼3.5 V at 25 °C, with an on-state resistance of ∼5×10−3 Ω cm2. The reverse recovery time was ⩽50 ns in switching from forward bias to reverse bias. The reverse breakdown showed a temperature coefficient of −0.45 V/C.

160-A bulk GaN Schottky diode array

Pt Schottky rectifier arrays were fabricated on 200-μm-thick, freestanding GaN layers. Even with the reduced dislocation density in these layers (∼105 cm−2) relative to conventional GaN on sapphire (>108 cm−2), rectifiers fabricated on the freestanding GaN show a strong dependence of reverse breakdown on contact diameter. We show that by interconnecting the output of many (∼130) smaller (500 μm×500 μm) rectifiers, we can achieve high total forward output current (161 A at 7.12 V), low forward turn-on voltage of ∼3 V, and maintain the reverse breakdown voltage. The on/off ratio of the rectifier array was ∼8×107 at 5 V/−100 V.

Activation characteristics of ion-implanted Si+ in AlGaN

Multiple-energy Si+ implantation in the range 30–360 keV into Al0.13Ga0.87N for n-type doping was carried out at room temperature, followed by annealing at 1150–1375 °C for 5 min. Activation efficiencies close to 100% were obtained for ion doses of 1.0×1015cm−2 after annealing at 1375 °C, with a resulting sheet resistance of 74Ω∕square. By sharp contrast, the activation efficiency at 1150 °C was only 4% for this dose, with a sheet resistance of 1.63×104Ω∕square. The activation efficiency was also a function of dose, with a maximum activation percentage of only 55% for lower doses of 1.0×1014cm−2 annealed at 1375 °C. This is due to the comparatively larger effect of compensating acceptors at the lower dose and is also lower than the corresponding activation of Si in pure GaN under these conditions (78%). The measurement temperature dependence of sheet carrier density showed an activation energy of 23 meV, consistent with the ionization energy of Si in AlGaN.

Comparison of Interface State Density Characterization Methods for SiO2 / 4 H ­ SiC MOS Diodes

Four different methods for calculating interface state density in vertical SiO 2 /4H-SiC metal-oxide semiconductor (MOS) capacitors were employed on the same samples. The Terman, ac, quasi-static, and Hi-Lo methods were used to extract surface state densities from the SiO 2 /SiC interface. Surface state densities from 10 1 1 to 10 1 2 cm - 2 eV - 1 were obtained, depending on the method employed. The Hi-Lo method is particularly susceptible to underestimating the trap density if UV light is not used to empty all the deep-lying traps during measurement.