Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

A Survey of Wide Bandgap Power Semiconductor Devices

A recessed-gate structure has been studied with a view to realizing normally off operation of high-voltage AlGaN/GaN high-electron mobility transistors (HEMTs) for power electronics applications. The recessed-gate structure is very attractive for realizing normally off high-voltage AlGaN/GaN HEMTs because the gate threshold voltage can be controlled by the etching depth of the recess without significant increase in on-resistance characteristics. With this structure the threshold voltage can be increased with the reduction of two-dimensional electron gas (2DEG) density only under the gate electrode without reduction of 2DEG density in the other channel regions such as the channel between drain and gate. The threshold-voltage increase was experimentally demonstrated. The threshold voltage of fabricated recessed-gate device increased to -0.14 V while the threshold voltage without the recessed-gate structure was about -4 V. The specific on-resistance of the device was maintained as low as 4 m/spl Omega//spl middot/cm/sup 2/ and the breakdown voltage was 435 V. The on-resistance and the breakdown voltage tradeoff characteristics were the same as those of normally on devices. From the viewpoint of device design, the on-resistance for the normally off device was modeled using the relationship between the AlGaN layer thickness under the gate electrode and the 2DEG density. It is found that the MIS gate structure and the recess etching without the offset region between recess edge and gate electrode will further improve the on-resistance. The simulation results show the possibility of the on-resistance below 1 m/spl Omega//spl middot/cm/sup 2/ for normally off AlGaN/GaN HEMTs operating at several hundred volts with threshold voltage up to +1 V.

Recessed-gate structure approach toward normally off high-Voltage AlGaN/GaN HEMT for power electronics applications

Recent success with the fabrication of high-performance GaN-on-Si high-voltage HFETs has made this technology a contender for power electronic applications. This paper discusses the properties of GaN that make it an attractive alternative to established silicon and emerging SiC power devices. Progress in development of vertical power devices from bulk GaN is reviewed followed by analysis of the prospects for GaN-on-Si HFET structures. Challenges and innovative solutions to creating enhancement-mode power switches are reviewed.

https://iopscience.iop.org/article/10.1088/0268-1242/28/7/074011/pdf

Gallium nitride devices for power electronic 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).

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 exam- ined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance ðRonÞ and a high breakdown voltage ðVbÞ.

GaN Power Transistors on Si Substrates for Switching Applications Hybrid MOS-FET transistor devices with low on-resistance, high hold-voltages and high breakdown voltage promise to provide high-power, low-loss operation for switching applications.

We fabricated an AlGaN/GaN heterojunction field effect transistor (HFET) with a very low on-state resistance. An undoped Al0.2Ga0.8N(30 nm)/GaN(2 μm) heterostructure was grown on a sapphire substrate using gas-source molecular-beam epitaxy. The undoped GaN layer had a high resistivity (above 10 M Ω ) and the breakdown field of the undoped layer was about 2 MV/cm. Si-doped GaN with a carrier concentration of 5×1019 cm−3 was selectively grown in the source and drain regions for obtaining a very low contact resistance. As a result, a very low ohmic below 1×10−7 Ω  cm2 was obtained. After that, an Al0.2Ga0.8N/GaN HFET was fabricated. The gate width was 20 cm and the gate length was 2 μm. The ohmic electrode materials were Al/Ti/Au and the Schottky electrodes were Pt/Au. The distance between the source and the drain was 13 μm. The HFET was operated at a current of over 20 A. A higher switching speed of HFET was obtained.

A high-power AlGaN/GaN heterojunction field-effect transistor

A GaN semiconductor device, which has a low on-resistance, allows an extremely small leak current while reverse biased voltage is applied, and has an excellent withstand voltage characteristic. The GaN semiconductor device is provided with a III-V nitride semiconductor layer containing at least one hetero junction structure of III-V nitride semiconductors having different band gap energies; a first node electrode arranged on a surface of the III-V nitride semiconductor layer by Schottky junction; a second anode electrode, which is arranged on the surface of the III-V nitride semiconductor layer by Schottky junction, is electrically connected with the first anode electrode, and forms a higher Schottky barrier than that formed by the first anode electrode; and an insulating protecting film, which is brought into contact with the second anode and is arranged on the surface of the III-V nitride semiconductor layer.

GaN SEMICONDUCTOR DEVICE

In this paper, GaN-based MIS-HFET devices on 4-inch Si substrates were fabricated, and the device characteristics were examined. As a result, the breakdown voltage was improved to be over 2.45 kV using high-resistive carbon doped buffer layers with a larger thickness of over 7.3 µm. The maximum drain current was estimated to be over 115 A using MIS-structures. A trade-off between the specific on-resistance (RonA) and the breakdown voltage (Vb) was improved using carbon doped buffer layers, resulting in obtaining RonA = 5.9 mΩcm2 and Vb = 1730 V simultaneously with the gate-width of a 340 mm. Furthermore, the field-plate structure was introduced into MIS-HFET structures. We have examined the suppression of a current collapse phenomenon owing to the combination of the gate field-plate structure and a conductive Si substrate with MIS-HFET devices.

High-power AlGaN/GaN MIS-HFETs with field-plates on Si substrates

We developed new ohmic electrodes combined with an Al-silicide and a molybdenum for AlGaN/GaN HFETs to realize a high power switching application. As a result, the maximum drain current of the HFET was over 55 A and the breakdown voltage was about 800 V. The specific on-state resistance of the HFET was smaller than that of a Si Cool MOSFET. Furthermore, we examined the dynamic characteristics. The turn-off and turn-on delay time were 14.8 nsec. and 8.4 nsec. at the condition of 100 V, respectively. These values were considerbly smaller compared with those of Si Cool MOSFETs.

Over 55 A, 800 V high power AlGaN/GaN HFETs for power switching application

We have studied the activation of Si ion implanted un- and Mg-doped gallium nitride (GaN) for the fabrication of reduced surface field (RESURF) metal–oxide–semiconductor field-effect transistors (MOSFETs). By annealing at 1260 °C for 30 s by using rapid thermal annealing (RTA), the activation ratios of un- and Mg-doped GaN with Si doses of 3×1015 cm-2 were ~100 and 73%, respectively. These values are sufficient for application some semiconductor devices. Hardly any diffusion of the Si atoms implanted in GaN was observed by secondary ion mass spectrometry (SIMS). The activation ratio between un- and Mg-doped GaN was markedly different at low doses. The cause of the difference appears to be Mg compensation in GaN. In addition, we fabricated GaN MOSFETs with ion implanted RESURF zones. We also monitored the field-effect transitor (FET) operation and high breakdown voltage of the GaN MOSFETs. The threshold voltage was +2 V. An enhancement mode operation and a breakdown voltage higher 1500 V at a RESURF length of 20 µm were achieved.

https://iopscience.iop.org/article/10.1143/JJAP.47.5409/pdf

Jiang Li

Papers

A high-power AlGaN/GaN heterojunction field-effect transistor

GaN SEMICONDUCTOR DEVICE

High-power AlGaN/GaN MIS-HFETs with field-plates on Si substrates

Over 55 A, 800 V high power AlGaN/GaN HFETs for power switching application

Si Ion Implantation into Mg-Doped GaN for Fabrication of Reduced Surface Field Metal–Oxide–Semiconductor Field-Effect Transistors