GaN Switching Research Articles

A Need for Control [Book\/Software Reviews

No matter what kind of system we build, we need control components. Switches, attenuators, phase shifters, and limiters are often realized with devices used as variable resistors or on/off switches. Inder Bahl has been a prolific writer of microwave books. This book on control components may be the author's smallest in terms of number of pages, but it is packed with useful information on GaAs, Si, and GaN switches, attenuators, phase shifters, and limiters. PIN, Schottky, and varactor diodes are also discussed. And each chapter has a large set of references, ranging from about 50 to 160, in this well researched book. Readers will appreciate the history and overview perspectives the book offers, as well as the many practical details. [One appendix, for example, covers Cooke’s thermal analysis of a GaAs field effect transfer (FET) structure, which is very important for power handling decisions.] Control Components Using Si, CaAs, and GaN Technologies is arranged by chapters on each type of component. Individual chapters discuss the variety of technologies used to realize the component, along with the advantages and disadvantages of the technologies.

(Invited) Role of GaN-Based Devices in Medium and High Power Conversion

All power converters rely on a switch that is predominantly based on Si technology today. Power conversion techniques largely served by Si transistors and diodes, are facing a roadblock where any further improvement in performance is now incremental. It is well recognized that high efficiency and high-speed compact power converters will offer more versatility, value and create platforms for newer applications. New functionality at the system level requires switches that can operate faster and at higher temperature without adding to the system losses. Studies show that the overall system performance over the system cost drives the technology to market rather than its component’s cost. Therefore it is crucial to evaluate the impact of any new device technology at the system level. The evaluation of the effect of a more efficient GaN switch, as a replacement for Si-IGBT, on the overall system, has been possible due to the recently commercialized lateral 600V HEMT technology [1]. GaN-on-Si technology now offers a scalable and cost-effective solution for power switches. A 600V lateral HEMT, where the gate to drain distance absorbs the off-state high voltage with very low leakage current, can switch 1.5 times higher operating current and at a frequency which is at least 10 times or higher compared to the state-of-the art Si-based devices without compromising the efficiency of the converter. GaN switched converters can be designed to operate at higher frequencies (100s of kHz), with reduced size of the passive components leading to an overall reduction in the form-factor. In addition to performing at high frequencies, GaN-based devices can operate at higher temperature (>150oC) thereby requiring smaller or no heat sinks; further reducing the form-factor of the converter. A 40% reduction in volume of a Photovoltaic inverter over the state-of-the art Si based inverter was realized by Yaskawa Electric Corporation using Transphorm’s GaN switches, due to reduction in size of the output filter and the heat sink. Thus Lateral GaN technology has effectively reduced the electrical and mechanical footprint in a converter by reducing the size of passive components and heat sinks, respectively [2]. Although well suited for the medium power applications, lateral GaN HEMTs become unattractive, both in switching performance and cost, for high power (10kW-1MW) application space since the chip area becomes large. A vertical GaN switch offers more attractive solution at these higher power ratings, where the required blocking voltages exceeds 1kV. Early results from CAVETs have established the fact that bulk GaN devices offer breakdown electric field almost 3 times higher compared to lateral HEMTs. In a CAVET or a CAVET-like design, the peak electric field is buried into the bulk of the material, enabling higher breakdown electric field, which renders lower Ron for the same rated voltage, compared to HEMTs. With a buried electric field far from the surface, one expects and CAVETs have demonstrated dispersion less output current without the need of complex field plate structures (an integral part of the lateral HEMT design). One challenge that is yet to be overcome in lateral HEMTs is single chip normally off operation. Our recent research has identified vertical device designs, using bulk GaN, in the form of a vertical MOSFET that warranties single chip normally-off devices. Single chip normally-off operation will further improve switching performance by eliminating bond wire and PCB trace related inductances. Recent results on GaN substrates with defect densities lower that 104cm-2 and allowing electron mobility >1100cm2V-1s-1 [3] create a very encouraging picture for bulk GaN power devices. Development of implantation technology with high activation efficiency is an ongoing activity that has lately shown some promising diode operation achieved with [Mg] implanted GaN/n GaN structures [4]. In conclusion, Lateral GaN has made a remarkable progress over the past 5 years making it into a product that can be designed into various low and medium power converters. The same trend is expected to repeat for vertical GaN for higher power applications, which is less matured than its lateral counterpart. Vertical GaN, owing to its promising cost roadmap and high quality of material allowing high electron mobility is in an advantageous position over its competitors. Reference: [[1] S.Chowdhury and U.K Mishra, IEEE Transaction on Electron Devices, 60 3060(2013) [2] Yaskawa Annual Report (2013) http://www.yaskawa.co.jp/ir/ir_document/annualreport/2013/en/ar2013e.pdf[3] P Kruszewski et al. The International Workshop on Nitride Semiconductor (2014) [4] T. J. Anderson et al, Electronics Letts. , 50, 197(2014) Figure 1

(Invited) Low Loss Power Conversion with Gallium Nitride Based Devices

Power converters are an integral part of energy conversion and energy storage systems. High efficiency and high-speed compact power converters are an ever-increasing need to offer more versatility, value and create new platforms. All power converters rely on a switch that is typically a solid-state device. Si-based devices have enabled solid-state power conversion leading to several advances so far, but any further improvement in performance using Si-based transistors is now incremental. New functionality at the system level requires switches that can switch faster and at higher temperature without adding to the system losses. It is now well agreed that the overall performance and cost of the system drives the technology over its component’s cost and hence it is crucial to evaluate the potential of any new technology at the system level. Current trends in GaN based power conversion system show that the impact of a high efficiency switch can result in superior system performance, reduced form-factor, and lower system’s cost by reducing number of active components (transistor and diodes), shrinking the size of the passive components (capacitors and inductors), and eliminating or reducing heat sinks. GaN-on-Si technology has been developed for providing scalable and cost-effective power conversion switches in the form of HEMTs. 600V lateral HEMTs, where the gate to drain distance absorbs the off-state high voltage allowing very low leakage current, can switch 1.5 times higher operating current and at a frequency which is at least 10 times or higher compared to the state-of-the art Si-based devices without compromising the efficiency of the converter [1]. GaN switched converters can be designed to operate at higher frequencies (100s of kHz), with reduced size of the passive components leading to an overall reduction in the form-factor. In addition to performing at high frequencies, GaN-based devices can operate at higher temperature thereby requiring smaller or no heat sinks and hence further reducing the form-factor of the converter. A 40% reduction in volume of a Photovoltaic inverter over the state-of-the art Si based inverter was realized by Yaskawa Electric Corporation using Transphorm’s GaN switches, due to reduction in size of the output filter and the heat sink. Electromechanical efficiency of a motor increases when unwanted harmonics are screened off from the input of the motor. A 100kHz switching frequency enabled a compact output filter that filtered out the harmonics demonstrating an overall increase of the electromechanical and hence the system efficiency by 5% at 50% load [2]. Very stable 1.2kV class AlGaN/GaN HEMTs clearly gives GaN a leading edge in medium power (up to 10kW) market. Lateral GaN HEMTs although well suited for the medium power market, cannot address high power (10kW-1MW) application space since the chip area becomes large and hence uneconomical in terms of both cost and switching performance. Vertical GaN switches enabled by high quality bulk GaN substrates showing a promising roadmap towards a scalable bulk GaN technology is capable of delivering a sustainable solution for the high power market. Early results from CAVETs have established the fact that bulk GaN devices offer breakdown electric field almost 3 times higher compared to lateral HEMTs. CAVETs have also demonstrated dispersion less output current without the need of complex field plate structures (an integral part of the lateral HEMT design) [3]. Our recent research has identified vertical device designs that warranties single chip normally-off devices, which will further improve switching performance by eliminating bond wire and PCB trace related inductances. Simplicity of device design plays a key role in reducing the device cost and is favoring the vertical GaN-based device to penetrate the high power application space. GaN substrates with defect densities lower that 104cm-2 and allowing electron mobility >1100cm2V-1s-1create a very encouraging picture for bulk GaN power devices. Next generation power conversion will rely on cost and reliability just like the present generation, but functionality and form-factor will play bigger roles than ever and GaN has the portfolio to address it all. Reference: [1] S.Chowdhury and U.K Mishra, Lateral and Vertical transistors using the AlGaN/GaN heterostructure, IEEE Transaction on Electron Devices, vol. 60 (2013) (7pp). [2] J.Honea and J. Kang, "High-Speed GaN Switches for Motor Drives", Power Electronics Europe, 3 (2012) 38-41. [3] S. Chowdhury, B.L Swenson, M.H. Wong and U.K. Mishra, "Current status and scope of gallium nitride-based vertical transistors for high-power electronics application", Semicond. Sci. Technol. vol. 28 (2013) (8pp). Figure 1

Performance characterization of gallium nitride HEMT cascode switch for power conditioning applications

Abstract A hybrid cascoded GaN switch configuration is demonstrated in power conversion applications. A novel metal package is proposed for the packaging of a D-mode GaN MIS-HEMT cascoded with an integrated power MOSFET and a SBD. The normally-off cascode circuit provides a maximum drain current of 14.6 A and a blocking capability of 600 V. Analysis of 200 V/1 A power conversion characteristics are discussed and show the excellent switching performance in load circuits. Switching characteristics of the integral SiC SBD are also demonstrated. Finally, a 48-to-96 V boost converter is used to evaluate the benefit of GaN cascode switches. These results show that high-voltage GaN-HEMTs can be switching devices for an ultralow-loss converter circuit.

Design and test of a pulse-width modulator and driver for space-borne GaN switch mode power amplifiers in P-band

In this paper, the design and test of a single-chip RF pulse-width modulator and driver (PWMD) aimed at exciting a high-power class-E GaN high-power stage at 435 MHz is described. For the required buffer size, avoiding potential ringing of the pulses within the buffer structure presents a major challenge in the design process. Therefore, a smaller test chip capable of driving capacitive loads of up to 5 pF was initially designed, fabricated, and tested. An approach based on three-dimensional electromagnetic simulations was used to validate the test results and offers excellent simulation accuracy. Based on the results obtained for test chip an enlarged PWMD chip capable of driving a 40 W high-power stage has been designed and tested on passive loads representing the targeted final stage.

GaN HEMT with AlGaN back barrier for high power MMIC switch application

0.25 μm GaN HEMT with AlGaN back barrier for high power switch application has been presented. By introducing AlGaN back barrier, the buffer layer breakdown voltage for the metal-organic chemical vapor deposited AlGaN/GaN hetero-structure on 3-inch SiC substrate showed a considerable increment, which was nearly 4× and 2× of that for the conventional GaN buffer layer and GaN buffer layer with Fe doped, respectively. GaN switch HEMTs with source to drain spacing of 2, 2.5, 3, 3.5 and 4 μm were fabricated on the AlGaN/GaN epitaxial material with AlGaN back barrier and estimated off state power handling for the GaN switch HEMTs were 25.0, 46.2, 64.0, 79.2, and 88.4 W, respectively. A demonstrator DC–12 GHz GaN SPDT MMIC switch was designed in reflective series-shunt-shunt configuration based on the GaN HEMT, with a source to drain spacing of 2.5 μm. The developed SPDT MMIC switch showed a maximum insertion loss of 1.0 dB and a minimum isolation of 30 dB at a frequency range of DC–12 GHz. A power handling capability of 44.1 dBm was achieved at 10 GHz for the MMIC switch with continuous wave power compression measurement.

Characterization of GaN Switches for Solar Invertors

To improve the efficiency of a solar cell application, two areas of focus are necessary. The first is to improve the quality of the solar panels; and the second is to increase the inverter efficiency, which can be accomplished by using high frequency switches with low on-state and switching losses. GaN has a significantly bigger bandgap compared to Si material, resulting in higher breakdown voltage. Also, the channel lengths of GaN MOSFETs are smaller and, therefore, have smaller gate-to-source and drain-to-source capacitance. In this work, different GaN switches are characterized through their steady state and switching characteristics. Also, the use of GaN switches in some Solantro Semiconductor Corp. applications and a comparison of the total power losses of Si and SiC switches are also presented.

Characterization of GaN Switches for Solar Invertors

To improve the coefficient of efficiency of a solar cell application two directions are necessary. The first one is to improve the quality of the solar panels and the second direction is to increase the inverter efficiency. The second one can be obtained by using high frequency switches with low on and switching losses. GaN has significantly bigger bandgap compared to Si and SiC materials, resulting in higher breaking voltage. Also the channel lengths of GaN MOSFETs are smaller and therefore having smaller gate to source and drain to source capacitance. In this work, different GaN switches are characterized through their steady state and switching characteristics. Also, using GaN switches in some Solantro Semiconductor Corp. applications and comparing the total power losses of Si and SiC switches are also presented.

Filter Design Methodology and Application of GaN HEMTs in High-Frequency DC/DC Converter

ABSTRACTOptimization of the radio frequency (RF) transmitter performance in envelope tracking (ET) and envelope elimination and restoration (EER) techniques directly implies optimization of the envelope amplifier (EA) design. Since EA should have fast dynamic response together with high efficiency, synchronous buck is a suitable topology for this kind of application. The key issues for optimization of the converter design are the characteristics of the switching devices, the LC filter design, and the driving circuit for the switches. In this paper, design optimization of the LC filter for sinusoidal output voltage together with new technological solution for switching devices based on gallium nitride is proposed, while the applied drivers are commercially available ones, suitable for aforementioned GaN switches. Design of the output filter in synchronous buck converter for high-frequency application is important from two points of view: the first one is sufficient attenuation of the unwanted harmonics of the switching frequency so that envelope reference has minimum level of distortion and the second one is the overall impact of the filter design on the converter's efficiency. In order to increase the efficiency, the LC filter should be designed in a way to obtain compromise between the conduction and switching losses of the buck converter. In the case of sinusoidal output voltage, filter design parameter has been defined as the ratio between the maximum values of the inductor current and load current and power losses dependence on this parameter has been obtained. Theoretically obtained optimum design has been confirmed through experimental measurements, where four different designs were built and tested. On the other hand, application of new switching devices based on gallium nitride provided significant efficiency enhancement in comparison to Si MOSFETs and laterally diffused MOS transistors, due to the lower values of the gate charge and on-resistance. Tests with 64QAM and wideband code division multiple access (WCDMA) RF signals showed that these devices are the best choice for this kind of application, where the main challenge is to increase the efficiency of the signal transmission simultaneously with the bandwidth.

Breakdown and dynamic effects in GaN power switching devices

AbstractApproaches towards fast switching GaN devices for applications in power electronics are presented. First, an overview on breakdown mechanisms in GaN power devices is given. Suitable techniques towards devices with high breakdown strength are then discussed and set into relation to their specific dynamic switching properties. It has been found that dynamic switching properties of GaN power devices may be significantly compromised at high drain bias levels. The dynamic effects depend on technological parameters such as epitaxial layer design and crystalline quality of the buffer layer, passivation layers and the lateral device designs (e.g. field plate arrangements). GaN high voltage switching devices therefore require a balanced trade‐off between all these influencing quantities. This paper concentrates on dependencies related to the epitaxial design of the buffer layer(s). (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

(Invited) Technological Approaches Towards High Voltage, Fast Switching GaN Power Transistors

Approaches towards fast switching GaN devices for applications in power electronics are discussed. First, an overview on the most prominent techniques towards increased breakdown voltage will be given for devices grown on SiC and Si substrates. These tech-niques will then be set in relation to dynamic switching properties at different bias voltage levels. Dynamic switching properties are still lagging behind competing devices, especially if switching at high bias levels is considered. The switching properties are usually expressed by the so-called dynamic on-state resistance. It signifi-cantly depends on technological parameters such as epitaxial layer design and crystalline quality of the buffer layer, passivation layers and, last but not least the lateral device designs (e.g. field plate ar-rangements). A balanced trade-off between all these influencing quantities is shown and correlated to current state of GaN high voltage switching devices.

Recent advances in GaN transistors for future emerging applications

AbstractIn this paper, recent advances in GaN‐based transistors for power switching and millimeter wave communications are reviewed. These two applications are emerging in addition to the widely developed power amplifiers at microwave frequencies mainly for cellular base stations. Reduction of the fabrication cost is strongly required for power switching GaN transistors, which is enabled by our epitaxial growth technology on large area Si substrates. Another requisite for the application is to achieve normally‐off operation and our novel device structure called Gate Injection Transistors (GIT) enables it with low enough specific on‐resistance (Ron · A) and high drain current. Here, we also present the world highest breakdown voltage of 10400 V in AlGaN/GaN HFETs, extracting full advantage of the high breakdown strength of GaN. The used poly AlN passivation works as a surface heat spreader with its high thermal conductivity, which effectively relieves the channel temperature resulting in lower thermal resistances. The GaN device for future millimeter wave communication consists of MIS‐type gate using so‐called “in‐situ” deposited SiN as a gate insulator, which exhibits high fmax of 203 GHz and low noise figure of 1.4 dB at 28 GHz. The presented GaN transistors are promising for the two future emerging applications demonstrating high enough potential of the material systems. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN TRANSISTORS FOR POWER SWITCHING AND MILLIMETER-WAVE APPLICATIONS

We review our state-of-the-art GaN -based device technologies for power switching at low frequencies and for future millimeter-wave communication systems. These two applications are emerging in addition to the power amplifiers at microwave frequencies which have been already commercialized for cellular base stations. Technical issues of the power switching GaN device include lowering the fabrication cost, normally-off operation and further increase of the breakdown voltages extracting full potential of GaN -based materials. We establish flat and crack-free epitaxial growth of GaN on Si which can reduce the chip cost. Our novel device structure called Gate Injection Transistor (GIT) achieves normally-off operation with high enough drain current utilizing conductivity modulation. Here we also present the world highest breakdown voltage of 10400V in AlGaN / GaN HFETs. In this paper, we also present high frequency GaN -based devices for millimeter-wave applications. Short-gate MIS-HFETs using in-situ SiN as gate insulators achieve high fmax up to 203GHz. Successful integration of low-loss microstrip lines with via-holes onto sapphire enables compact 3-stage K -band amplifier MMIC of which the small-signal gain is as high as 22dB at 26GHz. The presented devices are promising for the two future emerging applications demonstrating high enough potential of GaN -based transistors.