Shiro Ozaki

3.6 W/mm high power density W-band InAlGaN/GaN HEMT MMIC power amplifier

We demonstrated a W-band high-power-density MMIC power amplifier with 80 nm InAlGaN/GaN HEMTs. The MMIC consists of two-stage cascade units, each of which has two transistors with the same gate periphery for a high gain and low-loss matching circuit. The MMIC achieved a maximum output power of 1.15 W and maximum PAE of 12.3 % at 86 GHz under CW operation. Its power density reached 3.6 W/mm, representing the highest performance of the W-band GaN HEMT MMIC power amplifier.

Millimeter-Wave GaN HEMTs With Cavity-Gate Structure Using MSQ-Based Inter-Layer Dielectric

We have fabricated millimeter-wave gallium nitride high electron mobility transistors (GaN HEMTs) using methyl silsesquioxane (MSQ)-based inter-layer dielectric to suppress current collapse by enhancing the moisture resistance, and removed MSQ around the gate electrode for reducing parasitic capacitance. We clarified that the moisture resistance of conventional benzocyclobutene (BCB) is insufficient to suppress current collapse because water molecules easily permeate BCB, especially when using a cavity-gate structure. On the other hand, the moisture resistance of MSQ is very high because of the excellent hydrophobic property, and the current collapse due to moisture was effectively suppressed. So, improving the moisture resistance with hydrophobic low-k films plays a key role in reducing the current collapse of GaN HEMTs, especially when using a cavity-gate structure. Moreover, the parasitic capacitance of GaN HEMTs was successfully reduced by using the MSQ-cavity, and RF performance was improved by around 20%.

Effect of Atomic-Layer-Deposition Method on Threshold Voltage Shift in AlGaN/GaN Metal–Insulator–Semiconductor High Electron Mobility Transistors

We have investigated the mechanism for threshold voltage (Vth) shift of AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) for power applications. In this study, atomic layer deposited (ALD)-Al2O3 was used in AlGaN/GaN MIS-HEMTs as gate insulator films, and we focused on plasma-induced damages at the GaN/Al2O3 interface, when O2 plasma was used as the oxidant source for the ALD method. We clarified that the deep trap sites which were located around 2.58–3.26 eV from the conduction band edge were generated in the oxidized-GaN layer at the GaN/Al2O3 interface due to plasma-induced damages, and this caused the Vth shift when using O2 plasma. Therefore, we controlled the initial oxidant source, and demonstrated the reductions in the Vth shift and the gate leakage current by applying hybrid–Al2O3 structure (lower H2O vapor–Al2O3/upper O2 plasma–Al2O3) for AlGaN/GaN MIS-HEMTs.

X-Ku Wide-Bandwidth GaN HEMT MMIC Amplifier with Small Deviation of Output Power and PAE

A new design methodology was proposed to obtain wide-bandwidth and flat-group-delay reactive-matching GaN HEMT MMIC amplifiers. Frequency dependence of the optimal source and load impedance of a GaN HEMT are derived as polynomial equations and matching circuits are designed by small signal simulation without the use of large-signal transistor model and large-signal simulation. Fabricated GaN HEMT MMIC amplifiers, which show a small deviation of Pout and PAE in the range of 8-18 GHz, prove that our methodology is suitable for the design of a wide-bandwidth MMIC amplifier.

Current linearity and operation stability in Al2O3-gate AlGaN/GaN MOS high electron mobility transistors

To investigate current linearity and operation stability of metal–oxide–semiconductor (MOS) AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated and characterized the Al2O3-gate MOS-HEMTs without and with a bias annealing in air at 300 °C. Compared with the as-fabricated (unannealed) MOS HEMTs, the bias-annealed devices showed improved linearity of I D–V G curves even in the forward bias regime, resulting in increased maximum drain current. Lower subthreshold slope was also observed after bias annealing. From the precise capacitance–voltage analysis on a MOS diode fabricated on the AlGaN/GaN heterostructure, it was found that the bias annealing effectively reduced the state density at the Al2O3/AlGaN interface. This led to efficient modulation of the AlGaN surface potential close to the conduction band edge, resulting in good gate control of two-dimensional electron gas density even at forward bias. In addition, the bias-annealed MOS HEMT showed small threshold voltage shift after applying forward bias stress and stable operation even at high temperatures.

Effects of air annealing on DC characteristics of InAlN/GaN MOS high-electron-mobility transistors using atomic-layer-deposited Al2O3

We evaluated the DC characteristics of InAlN/GaN metal–oxide–semiconductor high-electron-mobility transistors (MOS-HEMTs) using atomic-layer-deposited (ALD-)Al2O3 by focusing on air annealing to control defect levels in Al2O3 and electronic states at the Al2O3/InAlN interface. We clarified that the transconductance linearity and subthreshold slope were improved by air annealing, indicating a reduction in the number of electronic states at the Al2O3/InAlN interface. Furthermore, the cathodoluminescence study demonstrated that the oxygen-related defects in ALD-Al2O3 were decreased by air annealing. Consequently, we could successfully reduce the threshold voltage shift of InAlN/GaN MOS-HEMTs by using air-annealed ALD-Al2O3.

Systematic investigation of silylation materials for recovery use of low-k material plasma damage

We systematically investigated the recovery ability of some silylation materials for a plasma-damaged porous silica low-k material. In order to evaluate recovery ability, the permittivity and leakage current were measured. The recovery ability is listed in ascending order as ethoxy group, disilazane group, and dimethylamino group, and materials with SiH–(CH3)2 showed a better recovery ability than those with Si–(CH3)3. Recovery mechanisms were also analyzed using Fourier transform infrared spectroscopy (FT-IR) and thermal desorption spectrometer (TDS). Clear differences in the silylation abilities were observed in the silylation rate as well as the amount of silylation. We focused on trimethylsilyldimethylamine (TMSDMA) and tetramethyldisilazane (TMDS) as the recovery material of vapor phase and both materials showed good recovery characteristics. TMSDMA showed rapid recovery because of its higher vapor pressure. TMDS effectively reduced the amount of absorbed water.