Sadahiro Kato

GaN Power Transistors on Si Substrates for Switching Applications

In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were examined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).

High power AlGaN/GaN HFET with a high breakdown voltage of over 1.8 kV on 4 inch Si substrates and the suppression of current collapse

In this paper, we successfully demonstrate an AlGaN HFET with a high breakdown voltage of over 1.8 kV on 4 inch Si substrates. In order to obtain the high breakdown voltage and to improve the crystalline quality of GaN layers, a thick GaN epitaxial layer including a buffer layer with a total thickness of over 6 mum was grown. The breakdown voltage and the maximum drain current were achieved to be over 1.8 kV and 120 A, respectively. Furthermore, the suppression of the current collapse phenomenon is examined. The on-resistance is not so significantly increased up to the high drain off-bias-stress of 1.0 kV.

Normally Off n-Channel GaN MOSFETs on Si Substrates Using an SAG Technique and Ion Implantation

We have demonstrated n-channel gallium nitride (GaN) MOSFETs using a selective area growth (SAG) technique and ion implantation on a silicon substrate. Both MOSFETs realized good normally off operations. The MOSFET using the SAG technique showed a large drain current of 112 mA/mm, a lower leakage current, and a high field mobility of 113 cm2/V . s, which is, to our knowledge, the best for a GaN MOSFET on a silicon substrate.

Over 1.7 kV normally-off GaN hybrid MOS-HFETs with a lower on-resistance on a Si substrate

In this study, normally-off GaN hybrid MOS-HFET devices on 4-inch Si substrates were fabricated, and the device characteristics were examined. As a result, the breakdown voltage (Vb) was improved using a combination of a high-resistive carbon-doped back barrier layer and a thin channel layer of 50 nm. The specific on-resistance (RonA) was estimated to be less than 7.1 mΩcm2 for Lgd = 12 μm, and Vb was estimated to be over 1.71 kV for Lgd = 18 μm. To our knowledge, these values are the best results ever reported for normally-off GaN-based MOSFETs.

Enhancement-mode GaN hybrid MOS-HFETs on Si substrates with Over 70 A operation

We report on the demonstration of enhancement-mode n-channel GaN-based hybrid MOS-HFETs realized on AlGaN/GaN heterostructure on silicon substrates with a large drain current operation. The GaN-based hybrid MOS HFETs realized the threshold voltage of 2.8 V, the maximum drain current of over 70 A with the channel width of 340 mm. This is the best value for an enhancement-mode GaN-based FET. The specific on-state resistance was 16.5 mΩcm2. The breakdown voltage was over 500 V. These results suggest that this structure is a good candidate for power switching applications.

Normally off operation GaN-based MOSFETs for power electronics applications

Gallium nitride (GaN) is a promising electronic semiconductor material for high-power, high-temperature devices due to its remarkable material properties like wide bandgap, large critical electric field and high saturation velocity compared with Si. The metal-oxide–semiconductor (MOS) field-effect transistor (MOSFET) structure can be operated at a positive threshold voltage, namely the normally off mode, which is preferable for power transistors in terms of fail-safe operation. However in order to minimize the power losses in MOSFET operation, good interface quality at SiO2/GaN and low resistance in the n+-contact layer are strongly required. The MOS capacitors were used to characterize the interface states at SiO2/GaN, and the interface state density at Ec − 0.4 eV was less than 1 × 1011 cm−2 eV−1 after annealing at 900 °C for 30 min by the furnace. In addition, the activation annealing of Si-implanted GaN was performed at 1260 °C for 30 s in rapid thermal annealing (RTA) and its sheet resistance was 23 kΩ sq−1. Finally, we have fabricated GaN MOSFETs and have achieved more than 1 A operation in the normally off mode at more than 250 °C. The breakdown voltage was more than 1500 V. We also confirmed more than 100 h of consecutive operation at 250 °C at the moment.

High Integrity SiO2 Gate Insulator Formed by Microwave-Excited Plasma Enhanced Chemical Vapor Deposition for AlGaN/GaN Hybrid Metal–Oxide–Semiconductor Heterojunction Field-Effect Transistor on Si Substrate

High quality SiO2 gate insulator has been demonstrated for GaN metal–oxide–semiconductor (MOS) transistor which has high performance with normally-off operation. The SiO2 films formed on GaN by microwave-excited plasma enhanced chemical vapor deposition (MW-PECVD) and annealed after deposition exhibits a low-interface state density between SiO2 and GaN, a high-breakdown field, and a high charge-to-breakdown. The SiO2 films have been also applied to the gate insulator of AlGaN/GaN hybrid MOS heterojunction field-effect transistor (HFET) on Si substrate. The MOS-HFET show excellent properties with the threshold voltage of 4.2 V and the maximum field-effect mobility of 161 cm2 V-1 s-1.

Soft Switching Controlled AlGaN Based Power Transistors for Induction Heating Applications

This paper reports on our first and series trial to apply the AlGaN heterojunction field effect transistors (HFETs) for in substitution for Si transistors in the induction heating (IH) applications. The on-resistance of the 800 V/50 A AlGaN HFET was 0.11 ohm, and the turn-on and turn-off times were less than 20 ns, respectively, which are lower than that of conventional Si based metal-oxide-semiconductor field effect transitors (MOSFETs). The input capacitance of the AlGaN HFET of up to 350 V was one digit smaller than that of the conventional Si MOSFETs. We fabricated the IH system with a soft switching circuit driven by the AlGaN HFETs. We realized the operation of this IH cookers at an operation voltage of 280 V and current of 8 A at a frequency of 29.6 kHz.

Fabrication of AlGaN/GaN HFET with a High Breakdown Voltage on 4-inch Si (111) Substrate by MOVPE

We investigated an AlGaN/GaN heterostructure field effect transistor (HFET) on Si substrates using a multi-wafer metalorganic vapor phase epitaxy (MOVPE) system. It was confirmed that a GaN film with smooth surface and without any crack was obtained. To increase a resistance of a GaN buffer layer, the carbon (C) -doping was carried out by controlling the V/III ratio and the growth pressure. The breakdown voltage of the buffer layer was dramatically improved by introducing the C. As a result, the breakdown voltage was about 900 V when the C concentration was about ∼8×10 18 cm −3 . After while, an AlGaN/GaN heterojunction FET (HFET) on a C-doped GaN buffer layer was fabricated. We achieved the breakdown voltage of over 1000 V and the maximum drain current of over 150 mA/mm, respectively. It was found that the C doped buffer layer is very effective for improving the breakdown voltage of AlGaN/GaN HFETs.

Investigation of Surface Pits Originating in Dislocations in AlGaN/GaN Epitaxial Layer Grown on Si Substrate with Buffer Layer

An AlGaN/GaN heterostructural layer with a crack-free smooth surface was grown on multiple buffer layers formed on a Si(111) substrate. On the AlGaN surface, pit arrays forming a network structure were observed by atomic force microscopy (AFM). In order to clarify the origin of these pit arrays, the AlGaN/GaN layer was investigated using transmission electron microscopy (TEM). As a result, similar network structures of threading edge dislocations in the AlGaN/GaN layer were observed by plan-view TEM. It was thus confirmed that the surface pit arrays observed by AFM represent the surface termination of edge dislocations formed at the small-angle boundaries of slightly misoriented crystal domains.