Young-Shil Kim

High breakdown voltage AlGaN/GaN HEMT by employing selective fluoride plasma treatment

We proposed and fabricated AlGaN/GaN HEMT with high stable reverse blocking characteristics employing fluoride plasma treatment using CF 4 gas. The plasma treatment with various rf power was performed selectively on drain-side gate edge region where electric field was concentrated. Unlike normally-off process, fluoride plasma treatment with attenuated RF power expanded gate depletion region in the direction of drain electrode. Expansion of depletion was confirmed by the change of measured off-state gate-drain capacitance. Expanded gate depletion spread E-field more uniformly with reducing peak of field intensity and prevented from drastic surface potential drop at the gate edge under large reverse bias condition. By the mitigation of field concentration and gradual potential change due to plasma treatment, was leakage current reduced and high breakdown voltage achieved. The breakdown voltage of plasma treated device with optimized rf power was 1400 V while that of untreated sample was 900 V. The leakage current of plasma treated device was 9.5 nA.

Effects of SiO2 Passivation on Oxygen Annealed AlGaN/GaN HEMTs

− The FTS(Fischer-Tropsch synthesis) was carried out over precipitated iron-based catalysts with or without SiO 2 in a fixed-bed reactor at 250 C and 1.5 MPa. The catalysts with SiO 2 showed much higher catalytic activity for the FTS than those without SiO 2 , displaying excellent stability during 144 h of reaction. The X-ray diffraction and N 2 physisorption revealed that the catalysts with SiO 2 showed enhanced dispersion of Fe 2 O 3 compared with those without SiO 2 . Also, the results of temperature-programmed reduction by H 2 showed that the addition of SiO 2 markedly promoted the reduction of Fe 2 O 3 into Fe 3 O 4 and FeO at low temperatures below 260 C. In contrast, surface basicity of the catalysts, which was analyzed by temperature-programmed desorption of CO 2 , decreased as a result of SiO 2 addition. We attribute the high and stable performance of the catalysts with SiO 2 to the improved dispersion and reducibility by the SiO 2 addition.

Normally-off AlGaN/GaN MOS-HEMTs by KOH wet etch and rf-sputtered HfO 2 gate insulator

Normally-off AlGaN/GaN MOS HEMTs were successfully fabricated and investigated by simple KOH wet etch and rf-sputtered HfO₂ as a gate insulator. The proposed KOH wet etch resulted in an adequate recess-depth and smooth etched surface. The gate-recessed HEMT exhibits threshold voltage (V_th) shifts from -3 to 1.5 V after 150 s KOH-wet etch. The breakdown voltage of 1580 V and R_{on, sp} of 8.09 mΩ·cm² was measured in the AlGaN/GaN HEMT with the gate-drain distance of 20 μm-long. The high FOM (figure of merit) of 308 MW/cm² was achieved. Our experimental results indicate that the proposed simple KOH wet etching and rf sputtered HfO₂-gate insulator may be promising for the normally-off AlGaN/GaN MOS HEMTs fabrication.

New AlGaN/GaN HEMTs employing both a floating gate and a field plate

We designed and fabricated AlGaN/GaN high-electron-mobility transistors (HEMTs) employing both a floating gate (FG) and a field plate (FP), which increase the breakdown voltage of AlGaN/GaN HEMTs significantly without sacrificing forward electric characteristics. The electric field strength at the gate–drain region of the proposed AlGaN/GaN HEMT was reduced successfully due to an increase in the number of depletion region edges. The breakdown voltage of the proposed AlGaN/GaN HEMT was 1106 V, while those of the conventional devices with only an FP or FG were 688 and 828 V, respectively. The leakage current of the proposed AlGaN/GaN HEMTs was 1.68 μA under a reverse bias of −100 V while those of the conventional devices with only an FP or FG were 3.21 and 1.91 μA, respectively, under the same condition. The forward electric characteristics of the proposed and conventional AlGaN/GaN HEMTs are similar. The maximum drain current of the proposed AlGaN/GaN HEMTs was 344 mA mm−1 while those of the conventional devices with only an FP or FG were 350 and 357 mA mm−1, respectively. The maximum transconductance of the proposed device was 102.9 mS mm−1, while those of the conventional devices were 97.8 and 101.9 mS mm−1. The breakdown voltage and the leakage current of the proposed device were improved considerably without sacrificing the forward electric characteristics. It should be noted that there were no additional processing steps and mask levels compared to the conventional FP process.

A new vertical GaN SBD employing in-situ metallic gallium ohmic contact

We proposed and fabricated new vertical GaN Schottky barrier diodes (SBDs) employing in-situ metallic gallium (Ga) ohmic contacts which increase the forward current of a vertical GaN SBD considerably. Highly conductive metallic Ga was formed in-situ at the bottom of n+ GaN substrate due to a high thermal budget during n-epi layer growth so that the ohmic contact was well-formed due to the metallic Ga. The forward current density of the proposed device was 625 A/cm2 at 2 V while that of the conventional device was 300 A/cm2. We also employed the floating metal ring and field plate to achieve the high breakdown voltage. The breakdown voltage of the proposed and conventional device was 880 V and 850 V respectively.

The Field Modulation Effect of a Fluoride Plasma Treatment on the Blocking Characteristics of AlGaN/GaN High Electron Mobility Transistors

We designed and fabricated aluminium gallium nitride (AlGaN)/GaN high electron mobility transistors (HEMTs) with stable reverse blocking characteristics established by employing a selective fluoride plasma treatment on the drainside gate edge region where the electric field is concentrated. Implanted fluoride ions caused a depolarization in the AlGaN layer and introduced an extra depletion region. The overall contour of the depletion region was expanded along the drift region. The expanded depletion region distributed the field more uniformly and reduced the field intensity peak. Through this field modulation, the leakage current was reduced to 9.3 nA and the breakdown voltage (V(BR)) improved from 900 V to 1,400 V.

High Voltage AlGaN/GaN High-Electron-Mobility Transistors Employing Surface Treatment by Deposition and Removal of Silicon Dioxide Layer

We have fabricated AlGaN/GaN high-electron-mobility transistors (HEMTs) employing the proposed surface treatment by the deposition and removal of a SiO2 layer. The proposed treatment was applied before the Schottky contact formation. The output DC characteristics of the conventional and proposed devices were almost identical. The same threshold voltage indicates that the deposited SiO2 layer on the proposed device was completely eliminated. The proposed device decreased its leakage current by more than three orders compared with the conventional device and achieved a high breakdown voltage of 1300 V. The improvement in the blocking characteristics of the proposed device is due to the suppression of buffer leakage current, which is induced by the elimination of native oxide and the effect of N2O plasma on the surface of the AlGaN/GaN heterostructure.

Diffusion Effect between Schottky Metals and AlGaN/GaN Heterostructure during High Temperature Annealing Process

We have investigated the change of the Schottky contact surface and the interface between Schottky metals and AlGaN/GaN heterostructure after the annealing process for 35 min at 300 °C. The secondary ion mass spectroscopy (SIMS) and the scanning electron microscopy (SEM) show that the Schottky metals and AlGaN/GaN heterostructure interacted actively during the annealing process. The atoms in Schottky contact and AlGaN/GaN heterostructure diffused interactively and the surface roughness of Schottky contact was increased. After the annealing process for fabricated AlGaN/GaN High-Electron-Mobility Transistor (HEMT), the threshold voltage was shifted by +0.2 V and the leakage current was decreased by 40 %.

Effect of oxygen annealing temperature on AlGaN/GaN HEMTs

We have investigated an effect of oxygen annealing temperature on the leakage current and breakdown voltage of AlGaN/GaN HEMTs. The breakdown voltage of 8 30 V and a low drain leakage current of 1.2 nA/mm at VDS= 50 V and VGS= −5 V are exhibited by employing oxygen annealing at 550°C. The blocking characteristics are improved with increasing annealing temperature up to 550°C due to high density of deep traps generated by oxygen annealing. However, the blocking characteristics of the annealed device were degraded when the annealing temperature exceeds 550°C due to thermal damage on the surface of AlGaN/GaN HEMTs.

High Breakdown Voltage AlGaN/GaN HEMTs by Employing Proton Implantation

The breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) was increased considerably without sacrificing any other electrical characteristics by proton implantation. The breakdown voltage increased from 416 V of conventional device to 719 V by proton implantation with 150 KeV, 1x1014cm-2 fluence after a thermal annealing at 400degC for 5 min under N2 ambient. The increase of breakdown voltage is attributed to the expansion of depletion region under the 2-dimensional electron gas (2-DEG) channel. The depletion region expanded downwards to the GaN buffer layer because implanted protons act as positive ions and attract the electrons in the 2-DEG channel. Proton implantation successfully reduced the electric field concentration so that the breakdown voltage increased.