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

This paper reports on state-of-the-art millimeter-wave power performance of N-polar GaN-based metal–insulator–semiconductor high-electron-mobility transistors at 30 and 94 GHz. The performance is enabled by our N-polar deep recess structure, whereby a GaN cap layer is added in the access regions of the transistor to simultaneously enhance the access region conductivity while mitigating dc-to-RF dispersion. The impact of lateral scaling of the drain access region length is examined using the tradeoff between breakdown voltage and small-signal gain. Load-pull measurements are presented at 94 GHz, corresponding to the target device operating frequency in W-band, where the device demonstrated a peak power-added efficiency (PAE) of 28.8% at 16 V and record-high maximum output power density of 8 W/mm at 20 V. Additional load-pull measurements at 30 and 10 GHz demonstrate the viability of this device across a wide frequency range where the peak power remained constant at 8 W/mm and with peak PAEs of 56% and 58%, respectively.

Demonstration of Constant 8 W/mm Power Density at 10, 30, and 94 GHz in State-of-the-Art Millimeter-Wave N-Polar GaN MISHEMTs

This article reviews the state-of-the-art millimeter-wave (mm-wave) power amplifiers (PAs), focusing on broadband design techniques. An overview of the main solid-state technologies is provided, including Si, gallium arsenide (GaAs), GaN, and other III–V materials, and both field-effect and bipolar transistors. The most popular broadband design techniques are introduced, before critically comparing through the most relevant design examples found in the scientific literature. Given the wide breadth of applications that are foreseen to exploit the mm-wave spectrum, this contribution will represent a valuable guide for designers who need a single reference before adventuring in the challenging task of the mm-wave PA design.

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A Review of Technologies and Design Techniques of Millimeter-Wave Power Amplifiers

The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications, such as wireless communication and radar systems. However, the performance and reliability of GaN-based high-electron-mobility transistors (HEMTs) are limited by the high channel temperature induced by Joule heating in the device channel. Integration of GaN with high thermal conductivity substrates can improve the heat extraction from GaN-based HEMTs and lower the operating temperature of the device. However, heterogeneous integration of GaN with diamond substrates presents technical challenges to maximize the heat dissipation potential brought by the ultrahigh thermal conductivity of diamond substrates. In this work, two modified room-temperature surface-activated bonding (SAB) techniques are used to bond GaN and single-crystal diamond. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties from room temperature to 480 K. A relatively large thermal boundary conductance (TBC) of the GaN/diamond interfaces with a ∼4 nm interlayer (∼90 MW/(m2 K)) was observed and material characterization was performed to link the interfacial structure with the TBC. Device modeling shows that the measured TBC of the bonded GaN/diamond interfaces can enable high-power GaN devices by taking full advantage of the ultrahigh thermal conductivity of single-crystal diamond. For the modeled devices, the power density of GaN-on-diamond can reach values ∼2.5 times higher than that of GaN-on-SiC and ∼5.4 times higher than that of GaN-on-Si with a maximum device temperature of 250 °C. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling, especially for GaN-on-diamond devices.

Interfacial Thermal Conductance across Room-Temperature-Bonded GaN/Diamond Interfaces for GaN-on-Diamond Devices.

High frequency and high efficiency operation is one of the premier interests in the signal and energy conversion applications. The wide bandgap GaN based devices possess superior properties and have demonstrated exceeding performance than Si or GaAs devices. In order to further exploit the potential of GaN electronics, monolithic power integration is proposed. Firstly, this paper discusses the structure and properties of GaN power devices to explain the choice of lateral integration in the view of GaN power ICs. Then the state-of-the-art performance of GaN power integration in two major application areas is reviewed, which are the microwave power amplification and DC-DC power conversion. The GaN power integration technologies in MMIC platforms are summarized in terms of the gate length, operation frequency and power added efficiency of ICs. On the other hand, the smart GaN power IC platforms have boosted the development of DC-DC power converters. Demonstrations of high frequency (>1 MHz) and high efficiency (>95 %) converters with various kinds of integration technology and topology are reviewed. Lastly novel integration schemes and methods are introduced to stimulate new thoughts on GaN power integration road.

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GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review

This letter reports on an InAlGaN/GaN high-electron-mobility transistor (HEMT) employing a SiC/diamond-bonded heat spreader with a record high output power density of 22.3 W/mm. A quaternary In-added InAlGaN barrier enabled both the large current of over 1 A/mm and the high breakdown voltage of 257 V. The drain bias was increased as high as 100 V for the S-band load-pull measurement, leading to high power operation. Furthermore, the thermal resistance was reduced by 60%, from 18.8 to 7.2°C/W, by employing the SiC/diamond heat spreader. This large heat dissipation effect was clearly observed in the output power density for the load-pull measurement. Our results demonstrate that the GaN HEMT with an In-added barrier layer is promising not only for millimeter-wave applications but also for high output power microwave amplifiers.

An Over 20-W/mm S-Band InAlGaN/GaN HEMT With SiC/Diamond-Bonded Heat Spreader

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.

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

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.

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

This paper demonstrates highly efficient GaN high-electron-mobility transistors (HEMTs) on GaN substrates with reduced interface contamination. By applying a hydrofluoric acid-based pre-growth treatment to a GaN substrate, the Si impurity concentration at the interface between the GaN substrate and the epitaxial layer can successfully be reduced. RF performance was enhanced by pre-growth treatment owing to the suppression of Si-induced parasitic loss. As a result, GaN HEMTs on GaN substrates exhibited an excellent power-added efficiency of 82.8% at a 2.45 GHz. To the best of our knowledge, this exceeds that of the previously reported discrete GaN HEMTs at around this frequency range.

Over 80% power-added-efficiency GaN high-electron-mobility transistors on free-standing GaN substrates

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.

https://iopscience.iop.org/article/10.7567/JJAP.52.11NG04/pdf

Shiro Ozaki

Papers

An Over 20-W/mm S-Band InAlGaN/GaN HEMT With SiC/Diamond-Bonded Heat Spreader

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

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

Over 80% power-added-efficiency GaN high-electron-mobility transistors on free-standing GaN substrates

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