Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency device applications, owing to highly desirable III-nitride physical properties. However, these devices are still hounded by issues not previously encountered in their more established Siand GaAs-based devices counterparts. Metal–insulator–semiconductor (MIS) structures are usually employed with varying degrees of success in sidestepping the major problematic issues such as excessive leakage current and current instability. While different insulator materials have been applied to GaN-based transistors, the properties of insulator/III-N interfaces are still not fully understood. This is mainly due to the difficulty of characterizing insulator/AlGaN interfaces in a MIS HEMT because of the two resulting interfaces: insulator/AlGaN and AlGaN/GaN, making the potential modulation rather complicated. Although there have been many reports of low interface-trap densities in HEMT MIS capacitors, several papers have incorrectly evaluated their capacitance–voltage (C–V) characteristics. A HEMT MIS structure typically shows a 2-step C–V behavior. However, several groups reported C–V curves without the characteristic step at the forward bias regime, which is likely to the high-density states at the insulator/ AlGaN interface impeding the potential control of the AlGaN surface by the gate bias. In this review paper, first we describe critical issues and problems including leakage current, current collapse and threshold voltage instability in AlGaN/GaN HEMTs. Then we present interface properties, focusing on interface states, of GaN MIS systems using oxides, nitrides and high-κ dielectrics. Next, the properties of a variety of AlGaN/GaN MIS structures as well as different characterization methods, including our own photo-assisted C–V technique, essential for understanding and developing successful surface passivation and interface control schemes, are given in the subsequent section. Finally we highlight the important progress in GaN MIS interfaces that have recently pushed the frontier of nitride-based device technology.

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Insulated gate and surface passivation structures for GaN-based power transistors

High-performance AlGaN/GaN diodes are realized on 8-in Si wafers with Au-free CMOS-compatible technology. The diodes are cointegrated on the same substrate together with the AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors and with only one extra lithographic step. The diode anode and the transistor gate are processed together and the same metallization is used for both, avoiding extra metal deposition dedicated to the Schottky junction. A gated edge termination allows obtaining low reverse leakage current (within 1 μA/mm at -600 V), which is several orders of magnitude lower than the one of conventional Schottky diodes processed on the same wafer. Recess is implemented at the anode, resulting in low diode turn-on voltage values.

Au-Free AlGaN/GaN Power Diode on 8-in Si Substrate With Gated Edge Termination

This paper describes recent technological advances on III-nitride-based transistors for power switching applications. Focuses are placed on the progress toward enhancing the breakdown voltage, lowering the ON-resistance, suppressing current collapse, and reducing the leakage current in AlGaN/GaN high-electron mobility transistors (HEMTs). Recent publications revealed that the tradeoff relation between ON-resistance and breakdown voltage in AlGaN/GaN HEMTs exceeded the SiC limit and was getting close to the GaN limit; however, the breakdown voltage achieved was still lower than the theoretical impact ionization limit. A novel process featuring strain-controlled annealing with a metal stack, including Al gave rise to significant reduction in the sheet resistance in AlGaN/GaN heterostructures, suggesting the possibility of dramatic reduction in ON-resistance of GaN-based power devices. Some of the interesting approaches to suppress current collapse indicated that surface trapping effects must be controlled by the optimization of surface processing as well as by the reduction of bulk traps in the epitaxial layers. Close correlation between the local gate leakage current and point defects exposed on the free-standing GaN substrate demonstrated that further reduction of defects on bulk GaN substrates is truly required as future challenges.

Low-Loss and High-Voltage III-Nitride Transistors for Power Switching Applications

In this paper, we give an overview of the recent progress in GaN-based high-electron-mobility transistors (HEMTs) developed for mainstream acceptance in the power electronics field. The comprehensive investigation of AlGaN/GaN HEMTs fabricated on a free-standing semi-insulating GaN substrate reveals that an extracted effective lateral breakdown field of approximately 1 MV/cm is likely limited by the premature device breakdown originating from the insufficient structural and electrical quality of GaN buffer layers and/or the GaN substrate itself. The effective lateral breakdown field is increased to 2 MV/cm by using a highly resistive GaN substrate achieved by heavy Fe doping. Various issues relevant to current collapse are also discussed in the latter half of this paper, where a more pronounced reduction in current collapse is achieved by combining two different schemes (i.e., a prepassivation oxygen plasma treatment and a field plate structure) for intensifying the mitigating effect against current collapse. Finally, a novel approach to suppress current collapse is presented by introducing a three-dimensional field plate (3DFP) in AlGaN/GaN HEMTs, and its possibility of realizing true collapse-free operation is described.

https://iopscience.iop.org/article/10.7567/JJAP.55.070101/pdf

AlGaN/GaN high-electron-mobility transistor technology for high-voltage and low-on-resistance operation

(Al)GaN recess-free normally OFF technology is developed for fabrication of high-yield lateral GaN-based power devices. The recess-free process is achieved by an ultrathin-barrier (UTB) AlGaN/GaN heterostructure that features a natural pinched-off 2-D electron gas channel. The top–down manufacturing technique overcomes the challenges in etching of AlGaN barrier with well-controlled depth and uniformity, which is especially attractive for fabrication of normally OFF GaN-based high-electron-mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS-HEMTs) on large-size Si substrate. With SiNx passivation grown by low-pressure chemical-vapor deposition, on-resistance of the UTB-AlGaN/GaN-based power devices can be significantly reduced. High-uniformity low-hysteresis normally OFF HEMTs and Al2O3/AlGaN/GaN MIS-HEMTs are successfully demonstrated on the UTB AlGaN/GaN-on-Si platform. It is also a compelling technology platform for manufacturing high-performance GaN-based lateral power diodes, and normally OFF p-(Al)GaN heterojunction field-effect transistors.

Ultrathin-Barrier AlGaN/GaN Heterostructure: A Recess-Free Technology for Manufacturing High-Performance GaN-on-Si Power Devices

Crack-free AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on a 200 mm Si substrate by metal–organic chemical vapor deposition (MOCVD) is presented. As grown epitaxial layers show good surface uniformity throughout the wafer. The AlGaN/GaN HEMT with the gate length of 1.5 µm exhibits a high drain current density of 856 mA/mm and a transconductance of 153 mS/mm. The 3.8-µm-thick device demonstrates a high breakdown voltage of 1.1 kV and a low specific on-resistance of 2.3 mΩ cm2 for the gate–drain spacing of 20 µm. The figure of merit of our device is calculated as 5.3×108 V2 Ω-1 cm-2.

https://iopscience.iop.org/article/10.7567/APEX.6.026501/pdf

Uniform Growth of AlGaN/GaN High Electron Mobility Transistors on 200 mm Silicon (111) Substrate

High-growth-rate AlGaN buffer layers and atmospheric-pressure growth of low-carbon GaN for AlGaN/GaN HEMT on the 6-in.-diameter Si substrate metal-organic vapor phase epitaxy system

It is difficult to control the surface temperature gradient over a bowing GaN on a large-diameter silicon substrate by metal organic chemical vapor deposition (MOCVD) because the wafer bows convexly to store compressive strain during growth. In an attempt to grow uniform AlGaN/GaN on 6-in. (6'') silicon substrates using a 7?6'' reactor, we described in this paper the control of the surface temperature gradient over the wafer and the mass transport at the edge of the wafer. We attempted to grow Al0.23GaN/AlN/GaN/SLS/Al0.5GaN/AlN on six 8-in. (8'') silicon substrates using a 6?8'' reactor. The standard deviations of total thickness were less than 2.0% on wafer and 0.31% wafer to wafer. The growth rate of strained-layer superlattice (SLS) was as high as 2.8 ?m/h. The typical electron mobility was 1670 cm2?V-1?s-1 at a sheet carrier density of 1.11?1013 cm-2.

https://iopscience.iop.org/article/10.7567/JJAP.52.08JB06/pdf

Control of Thickness and Composition Variation of AlGaN/GaN on 6- and 8-in. Substrates Using Multiwafer High-Growth-Rate Metal Organic Chemical Vapor Deposition Tool

Control of Thickness and Composition Variation of AlGaN/GaN on 6- and 8-in. Substrates Using Multiwafer High-Growth-Rate Metal Organic Chemical Vapor Deposition Tool (Special Issue : Recent Advances in Nitride Semiconductors)

Hayato Shimamura

Papers

Uniform Growth of AlGaN/GaN High Electron Mobility Transistors on 200 mm Silicon (111) Substrate

High-growth-rate AlGaN buffer layers and atmospheric-pressure growth of low-carbon GaN for AlGaN/GaN HEMT on the 6-in.-diameter Si substrate metal-organic vapor phase epitaxy system

Control of Thickness and Composition Variation of AlGaN/GaN on 6- and 8-in. Substrates Using Multiwafer High-Growth-Rate Metal Organic Chemical Vapor Deposition Tool

Control of Thickness and Composition Variation of AlGaN/GaN on 6- and 8-in. Substrates Using Multiwafer High-Growth-Rate Metal Organic Chemical Vapor Deposition Tool (Special Issue : Recent Advances in Nitride Semiconductors)