Gallium Nitride Devices Research Articles

Analytical Design Space of Power Amplifiers Including the Class-A/B/J Continuum for Dynamic Load Modulation

In this paper, an analytical design space of power amplifiers (PAs) including the Class-A/B/J continuum for dynamic load modulation (DLM) at the current generator plane (CGP) is proposed based on the theory of load modulated (LM) continuous Class-B/J PAs. By introducing a biasing operation factor ρ, the theory of DLM PAs provides an analytical design space for all the operation modes from Class-B/J mode to Class-A mode. The analytical design space of DLM PAs shows that high efficiency is maintained at a large dynamic range of output power back-off (OPBO) with a purely resistive load modulation at the CGP when Class-B/J mode is performed. For deep Class-AB mode, the analytical design space of the DLM PAs shows that a combination of resistive and reactive load modulation at the CGP is used to maintain the high efficiency at the OPBO. The effectiveness of the proposed analytical design space is validated by load-pull measurements of a bare-chip gallium nitride (GaN) device.

A Battery Energy Storage System for Single-Phase Residential Application with Paralleled GaN Devices

In this paper a battery energy storage system (BESS) for single-phase residential application is proposed. The idea is to apply Gallium Nitride (GaN) devices in order to achieve the advantage of very low switching power loss and relatively high efficiency. The GaN-based BESS includes a bidirectional half-bridge dc-dc converter and a full-bridge single-phase grid-connected inverter with paralleled GaN devices. The power stage design, GaN paralleling, and controller design are analyzed. Moreover, OrCAD Spice and MATLAB SIMULINK simulation results are discussed and validated.

Optically-activated cascode configuration for 650 V GaN FET devices and packaging parasitic inductance effects

In this paper, a novel optically-activated cascode structure is proposed to be used with a normally-on gallium nitride (GaN) field-effect transistor (FET) device to achieve an overall normally-off configuration. Using this novel configuration, cost-effective infrared (IR) lasers can be utilized instead of expensive ultraviolet (UV) lasers to activate this structure which includes a wide-bandgap-material device (GaN FET). Furthermore, the effect of parasitic inductance available in the package and connections of this proposed configuration is evaluated using Silvaco TCAD simulations. In practice, one high-power normally-on FET device is connected in series with a low-power optical switch (OS) to make the proposed overall normally-off cascode structure. The capability of being optically-activated for the proposed structure has many advantages over the conventional electrically-activated cascode structures including but not limited to: more immunity to electromagnetic interference (EMI), using only one main bias as the power source, using cost-effective long-wavelength laser instead of expensive short-wavelength lasers to trigger the GaN devices, reduced current and voltage ringing during switching transitions, etc. Comprehensive device modeling and parasitic inductance analysis for this new proposed optical cascode GaN FET device are provided in this work.

Gallium nitride (GaN) devices as a platform technology for radiation hard inertial confinement fusion diagnostics.

Inertial Confinement Fusion (ICF) is undergoing more detailed research to increase neutron yield and will require high resolution imaging near the target. Neutron damage to diagnostics remains a serious issue in understanding and achieving ICF. We have demonstrated that Gallium Nitride (GaN) optoelectronic devices have exceptional neutron radiation hardness, by systematic testing of neutron radiation effects in GaN devices and materials with elevated neutron fluence levels and a broad neutron energy spectrum. During the 2013-2017 run cycles at Los Alamos Neutron Science Center (LANSCE), we irradiated various GaN materials and devices with fast and thermal plus resonance neutrons at several beamlines. This paper presents a radiation hardness study for Aluminum Gallium Nitride and Gallium Nitride (AlGaN/GaN) deep UV LEDs irradiated at the LANSCE 4FP60R beamline. The fluence level was up to 2.4 × 1013 neutrons/cm2 for neutrons with energies greater than 0.1 MeV. The device performance was monitored in real time. After three years of irradiation studies, we found that the GaN devices maintained operation in the forward active region. The current and voltage relation (I-V curves) varied insignificantly in the linear region. Our results demonstrate the radiation hardness needed for laser fusion diagnostics at least up to 1017 neutron yield per shot, if the diagnostics is placed 1 m away from the target, where the neutron fluence per shot is approximately 8 × 1011 n/cm2. The GaN devices can operate for multiple shots.

Fast and Distributed Thermal Model for Thermal Modeling of GaN Power Devices

A new thermal model based on a distributed and fast modeling approach for the modeling of gallium nitride (GaN) power devices is presented in this paper. The model is based on the application of Green’s function theory to obtain an analytical solution of the heat conduction equation. The model comprises an accurate, spatially distributed, and fast algorithm that calculates a 2-D thermal response at the 2-D electron gas level of GaN power devices in the steady-state and the transient regime. In comparison with finite element method finite-element model simulations, the model achieves a significant reduction of the computational time while retaining very good accuracy. The model shows strong capabilities for thermal analysis with respect to parameters that have a significant impact on the thermal behavior. Moreover, the nonlinear effect associated with the temperature-dependent thermal conductivity is encompassed together with the impact of package and ambience. As a validation study, the thermal behavior of packaged GaN devices is experimentally characterized by means of infrared thermography. An excellent agreement between the model results and experiments is observed.

Computational study of Fermi kinetics transport applied to large-signal RF device simulations

A detailed description and analysis of the Fermi kinetics transport (FKT) equations for simulating charge transport in semiconductor devices is presented. The fully coupled nonlinear discrete FKT equations are elaborated, as well as solution methods and work-flow for the simulation of RF electronic devices under large-signal conditions. The importance of full-wave electromagnetics is discussed in the context of high-speed device simulation, and the meshing requirements to integrate the full-wave solver with the transport equations are given in detail. The method includes full semiconductor band structure effects to capture the scattering details for the Boltzmann transport equation. The method is applied to high-speed gallium nitride devices. Finally, numerical convergence and stability examples provide insight into the mesh convergence behavior of the deterministic solver.

Comparative Evaluation of Front- and Back-End PFC IPT Systems for a Contactless Battery Charger

Inductive power transfer (IPT) is widely discussed as an alternative to contact charger for plug-in hybrid and electric vehicles. This paper analyzes a contactless battery charger back-end power factor correction (PFC) concept, reducing the primary-side circuit complexity and physical size. For that purpose, the main volume and loss factors of a contactless battery charger are identified, discussing the partitioning of tasks to perform a unity power factor operation. An analysis of the most commonly used front-end PFC and the proposed back-end PFC configuration is performed, with the aim of highlighting benefits and limitations of each variant. Moreover, performance of silicon (Si) and gallium nitride (GaN) devices has also been evaluated, defining the main advantages and drawbacks of both semiconductor technologies in different IPT scenarios. The study is verified experimentally on a 3.2 kW contactless battery charger.

A High-Bandwidth Integrated Current Measurement for Detecting Switching Current of Fast GaN Devices

Gallium nitride (GaN) devices are suitable for high-frequency power converters due to their excellent switching performance. To maximize the performance of GaN devices, it is necessary to study the switching characteristics, which requires measuring the switching current. However, GaN devices have a fast switching speed and are sensitive to parasitic parameters, so the current measurement should have a high bandwidth and should not introduce excessive parasitic inductance into the power converters. Traditional current measurements are difficult to meet these requirements, especially for fast GaN devices. This paper presents a high-bandwidth integrated current measurement for detecting the switching current of fast GaN devices. By effectively utilizing the parasitic inductance in the circuit, a single-turn coil is embedded in the printed circuit board. This coil could pick up a sufficiently strong voltage signal, which is then processed to reconstruct the switching current. Moreover, corrections are carried out to further improve the accuracy. The current measurement has a small insertion impedance and a high bandwidth with a small influence on the parasitic inductance of the converter. The accuracy of the current measurement is experimentally verified by a 40 V GaN-based double pulse test circuit with a load current up to 25 A.

Active Thermal Control of GaN-Based DC/DC Converter

The new packaging technologies used for gallium nitride (GaN) devices avoid wire bonds and leads in order to completely utilize their switching performance. This also means that thermomechanical fatigue that used to exist within the device may no longer be a reliability problem. And one of the main bottlenecks for reliability will be the solder joints between the device and the printed circuit board (PCB). To limit the thermal cycling induced failures in these points, an active thermal control scheme using a two step gate driver designed for GaN is presented in this paper. In contrast to the active thermal control techniques employing variable switching frequency control, this method does not alter the converter operation frequency; instead, it merely controls the GaN device slew rates. A simple temperature control algorithm that actively varies the device losses with the objective to minimize thermal cycling is proposed. The solder fatigue due to thermal cycling has been discussed. The effectiveness of this active thermal control scheme has been analyzed also in comparison with losses and validated with analysis, simulation, and experimental results.

Power Electronic Devices and Systems Based on Bulk GaN Substrates

Wide-bandgap power semiconductor devices offer enormous energy efficiency gains in a wide range of potential applications. As silicon-based semiconductors are fast approaching their performance limits for high power requirements, wide-bandgap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) with their superior electrical properties are likely candidates to replace silicon in the near future. Along with higher blocking voltages wide-bandgap semiconductors offer breakthrough relative circuit performance enabling low losses, high switching frequencies, and high temperature operation. ARPA-E’s SWITCHES program, started in 2014, set out to catalyze the development of vertical GaN devices using innovations in materials and device architectures to achieve three key aggressive targets: 1200V breakdown voltage (BV), 100A single-die diode and transistor current, and a packaged device cost of no more than ȼ10/A. The program is drawing to a close by the end of 2017 and while no individual project has yet to achieve all the targets of the program, they have made tremendous advances and technical breakthroughs in vertical device architecture and materials development. GaN crystals have been grown by the ammonothermal technique and 2-inch GaN wafers have been fabricated from them. Near theoretical, high-voltage (1700-4000V) and high current (up to 400A pulsed) vertical GaN diodes have been demonstrated along with innovative vertical GaN transistor structures capable of high voltage (800-1500V) and low RON (0.36-2.6 mΩ-cm2). The challenge of selective area doping, needed in order to move to higher voltage transistor devices has been identified. Furthermore, a roadmap has been developed that will allow high voltage/current vertical GaN devices to reach ȼ5/A to ȼ7/A, realizing functional cost parity with high voltage silicon power transistors.

(Invited) Gallium Nitride Device Technology for Commercial RF Applications

Advances in gallium nitride (GaN) device technology over the last 25 years have enabled commercial radio frequency (RF) applications to benefit from its high-power handling capability and speed. Because of these advances, GaN has emerged as a leading technology for future 4G and 5G communications systems. This presentation will cover GaN device technology starting with the basics of GaN materials and device structures. It will show how the step-function improvement in materials and device capability with GaN translates to step-function advances in system capability. It will then give examples of GaN power amplifier applications for present day 4G wireless communications applications. Along the way, the presentation will address industry progress that has enabled GaN technology to become commercially viable. Finally, the presentation will preview opportunities for GaN technology in future 5G communications applications.

Class-X—Harmonically Tuned Power Amplifiers With Maximally Flat Waveforms Suitable for Over One-Octave Bandwidth Designs

This paper proposes a new theory for harmonically tuned power amplifiers (PAs) with maximally flat waveforms, Class-X PAs. This class has been developed to achieve maximally flat waveforms. In this class, the relationship between the fundamental load impedance and the independently set second- and third-harmonic load impedances is established to maintain constant output power with constant efficiency. This relationship sets an optimum fundamental load design space when the second- and third-harmonic impedances are varied simultaneously and independently of each other. By varying the second- and third-harmonic impedances independently, higher flexibility in the design of broadband PAs is obtained. The theory of Class-X PA and its broadband characteristics were validated with an experimental design example using a Cree’s 10-W Gallium Nitride (GaN) device. This design showed a measured drain efficiency higher than 70% and output power within 3-dB variation over more than one-octave bandwidth.

Mechanical stress effects on electrical breakdown of freestanding GaN thin films

Gallium Nitride (GaN) devices are intended to operate at high temperature and mechanical stress conditions, rendering reliability a major concern. To investigate the effects of temperature and stress on electrical breakdown they were applied individually and simultaneously, on free standing epitaxially grown GaN specimens after release from the growth substrate. Both temperature and mechanical stress were seen to degrade the material by decreasing the breakdown voltage. Up to 60% decrease was observed at around 870MPa. It is hypothesized that stress-generated defects climb to the free surfaces, creating localized leakage current instability or 'ringing' effects. This study also captures the synergy of temperature and stress, which can be translated to breakdown of buffer layers in GaN devices or in designing harsh environment sensors.

$X$ -band DC–DC Power Converter for High-Speed Bit-Stream Modulation

This letter proposes an X-band power converter for high-speed bit-stream modulation. The converter consists of a power amplifier (PA) with pulsed load modulation, which performs highly efficient dc–ac conversion for a wide range of output powers, and a switching rectifier. Power conversion was performed in the X-band, and the potential for broad bandwidth and profile minimization was demonstrated. The PA was fabricated using a discrete gallium-nitride HEMT device and it delivered 39-dBm RF output power with 57.4% drain efficiency (DE) at 9.2 GHz under class-B conditions. At 6-dB output power back-off, 38.2% DE was measured for a 4 Gb/s data rate. The switching rectifier is designed using a class-B PA strategy to demonstrate the proposed concept. The ac–dc conversion efficiency was measured to be 44.6%, and 3.05-W dc power was delivered.

Epitaxial Stress‐Free Growth of High Crystallinity Ferroelectric PbZr0.52Ti0.48O3 on GaN/AlGaN/Si(111) Substrate

AbstractDue to its physical properties gallium‐nitride (GaN) is gaining a lot of attention as an emerging semiconductor material in the field of high‐power and high‐frequency electronics applications. Therefore, the improvement in the performance and/or perhaps even extension in functionality of GaN based devices would be highly desirable. The integration of ferroelectric materials such as lead–zirconate–titanate (PbZrxTi1‐xO3) with GaN has a strong potential to offer such an improvement. However, the large lattice mismatch between PZT and GaN makes the epitaxial growth of Pb(Zr1‐xTix)O3 on GaN a formidable challenge. This work discusses a novel strain relaxation mechanism observed when MgO is used as a buffer layer, with thicknesses down to a single unit cell, inducing epitaxial growth of high crystallinity Pb(Zr0.52Ti0.48)O3 (PZT) thin films. The epitaxial PZT films exhibit good ferroelectric properties, showing great promise for future GaN device applications.

Flexible Gallium Nitride for High-Performance, Strainable Radio-Frequency Devices.

Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio-frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high-frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state-of-the-art values for electrical performance, with electron mobility exceeding 2000 cm2 V-1 s-1 and sheet carrier density above 1.07 × 1013 cm-2 . The influence of strain on the RF performance of flexible GaN high-electron-mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.

A Multifunctional Double Pulse Tester for Cascode GaN Devices

Gallium Nitride (GaN) power devices with low switching and conduction losses can lead to superior power density in numerous power conversion applications. Their fast switching speeds, however, pose challenges in dynamic device characterization. The large parasitic inductances and contact impedances in conventional double pulse testers (DPTs) meant for Si devices, make them unsuitable for GaN characterization. GaN devices are directly soldered on to testers for minimizing parasitic effects. Furthermore, currently different testers are used for different types of device characterization, requiring the device under test to be repeatedly soldered to different boards. This paper proposes a multifunctional tester well suited for GaN devices and capable of completing all the dynamic characterization on the same board. The proposed tester is able to characterize: device turn on and off transition under hard and soft switching, dynamic Rdson, diode reverse recovery, and device reverse conduction voltage drop. The proposed tester has been implemented in hardware and the functions are validated with tests on a cascode GaN device. Some special properties of the cascode GaN device seen from these tests are highlighted. Detailed design procedures for selecting the critical components of the DPT are presented.

Experimental Demonstration of a 98.8% Efficient Isolated DC–DC GaN Converter

This paper demonstrates the utilization of gallium nitride (GaN) devices to achieve an ultrahigh efficiency in an isolated dc–dc converter. This paper presents the design and implementation of high-efficiency magnetics necessary to realize such an ultrahigh efficiency converter. Synchronous rectification is also implemented to further improve the efficiency. Compared to an equivalent silicon MOSFET, GaN devices have low output charge, very low gate drive losses, and zero reverse recovery losses, this makes GaN devices more suitable for power converters to achieve high efficiency. A complete analytical loss modeling for an isolated full bridge dc–dc converter is discussed in this paper. Furthermore, the experimental demonstration of ultrahigh efficiency in a 2.4-kW isolated GaN converter is presented. Ultrahigh conversion efficiency allows the realization of a very compact dc–dc converter with a very small or no heat sink. The prototype converter has a power density of 7 kW/L and the measured efficiency is above 98.5% over a wide range of output power. The maximum measured efficiency of the converter is 98.8% at 50% of full load.

Suppression of the self-heating effect in GaN HEMT by few-layer graphene heat spreading elements

Self-heating has an adverse effect on characteristics of gallium nitride (GaN) high electron mobility transistors (HEMTs). Various solutions to the problem have been proposed, however, a temperature rise due to dissipated electrical power still hinders the production of high power and high speed GaN devices. In this paper, thermal management of GaN HEMT via few-layer graphene (FLG) heat spreading elements is investigated. It is shown that integration of the FLG elements on top of the device structure considerably reduces the maximum temperature and improves the DC and small signal AC performance.

Comparative Analysis on Conducted CM EMI Emission of Motor Drives: WBG Versus Si Devices

Silicon carbide (SiC) MOSFETs and gallium nitride (GaN) high-electron mobility transistors are perceived as future replacements for Si IGBTs and MOSFETs in medium- and low-voltage drives due to their low conduction and switching losses. However, it is widely believed that the already significant conducted common-mode (CM) electromagnetic interference (EMI) emission of motor drives will be further exacerbated by the high-speed switching operation of these new devices. Hence, this paper investigates and quantifies the increase in the conducted CM EMI emission of a pulse width modulation inverter-based motor drive when SiC and GaN devices are adopted. Through an analytical approach, the results reveal that the influence of dv/dt on the conducted CM emission is generally limited. On the other hand, the influence of switching frequency is more significant. Lab tests are also conducted to verify the analysis.