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DISCUSSIONS ABOUT GaN (gallium nitride) transistors and diodes have focused upon the potential of the new material to provide better devices in the future. This usually began with a comparison of specific on-resistance versus breakdown voltage (a static figure of merit) to illustrate the potential to use smaller devices fabricated with GaN material. In the absence of compelling performance, this was useful to build interest in the new technology. Now there are devices available that demonstrate significant performance improvement using GaN/AlGaN fabricated devices that are suitable for mass production. Fig 1 shows the breakdown vs. specific on-resistance characteristics of silicon, GaN and SiC (siicon carbide) semiconductorsFig. 2 shows the typical GaN/AlGaN device structure including the two degree electron gas area (2DEG) at the junction of the AlGaN — GaN boundary in which the electrons have extremely high mobility that produces the low on resistance of the Al GaN/GaN transistors. It is also the reason these devices are called high electron mobility transistors (HEMTs) rather than FETs. Although the basic device structure used by most power device designers is the same, the performance of these 600 V AlGaN/GaN devices are the first to deliver better efficiency than similarly-rated silicon super junction devices. This demonstrated the maturity of GaN development in general and of the EZ-GaNTM technology developed by Transphorm Inc.
SMALLER POWER CONVERSION IS NOW POSSIBLE
In power conversion, smaller size has driven product development for the past 30 years. To meet this challenge, it has been necessary to sacrifice efficiency to achieve higher density in the front end ac-dc power converter until now, when 600 V GaN diodes and transistors with high efficiency at high frequency have become a reality.
The potential of GaN/AlGaN devices to improve the density/efficiency trade-off dilemma depends upon reducing the switching losses of equivalent RDS(ON) devices of the same voltage rating. To achieve this level of performance, it is not sufficient to simply design a AlGaN/GaN device that has low reverse leakage current and a low nominal resistance under static conditions. Devices must be designed to minimize the charging capacitances and facilitate fast switching. In-circuit testing is the simplest and most accurate way to validate that all the above issues have been resolved.
Although research for high voltage GaN devices has resulted in numerous publications1, no GaN devices were previously shown to outperform the mature Si counterparts at 400V and above. At APEC 2011, we focused on delivering highly efficient power conversion solutions with GaN, which demonstrated what is now possible with the first 600 V Total-GaNTM solution2. The circuit used to demonstrate the performance of high voltage GaN devices was a simple dc to dc boost circuit3 (Fig. 3), operating with 220 V dc on the input.
The boosted voltage achieved was 400 V at a power of 760 W. The circuit is not a new topology but rather is so simple (no gate resistor, no snubber and even no insulation shim between the tab and heat-sink) with the patent pending package that it immediately illustrates any weakness in the devices being used. Fig. 4 compares the efficiency of a GaN converter and silicon converter operating with a 100kHz PWM.
The two devices used in the circuit are a 600 V diode rated at 2 A and a 600 V high electron mobility transistor (HEMT) with a typical on resistance of 250 mΩ (maximum 310 mΩ) each housed in a TO-220 style package outline.
The power level of the circuit was chosen to allow it to be used in an exhibition booth so engineers could view the oscilloscope and confirm that the waveforms and therefore the performance is real as shown in Fig. 5 left and 5 right. In fact, the most common explanation heard during the show about this performance was textbook; “Such waveforms are usually only seen in a text book, not in a boost circuit switching at 100 kHz, and definitely not with a TO-220 package!”. It is this fact that defines EZ-GaN, so simple to use, no special driver requirements, no special packages; just superior performance achieved with attention to minimizing circuit parasitics beginning internal to the device and following through to the package as demonstrated by the pin layout being changed from G-D-S to G-S-D on the TO-220, which is a very simple change that enables big improvement in circuit performance.
The device design is a cascode connection shown in Fig. 6 that was selected to take advantage of the strengths of both silicon and GaN materials. Silicon was selected as the gate interface to allow easy interface to existing integrated drivers and controllers due to the well defined threshold voltage and the proven ability to withstand ± 20 V breakdown. Because of the absence of an insulating oxide that is compatible with GaN, the gate threshold voltage and gate dielectric strength of GaN devices subjected to positive voltages are limited to + 6 V at this time 4. GaN was selected for the high voltage section due to its low charge, fast switching, and lower R × A product that combine to enable higher efficiency with less switching losses or smaller size when the frequency is increased, giving power conversion designers more options to reduce system size or in the case of motor drives to offer wider bandwidth for improved system performance.
Once the device layout and package layout have solved the internal switching inhibitors, a tight circuit layout is necessary to achieve the highest overall performance. Luckily this is now easy and can be done in a manufacturable manner using two- to four- layer PCBs. An inefficient layout can only degrade device performance; it can't enhance what is not already present.
IS GALLIUM NITRIDE MATURE ENOUGH?
Low voltage GaN devices have been available for the last 10-15 years such as RF HEMT devices (2004) and Light emitting diodes (early-mid 90s5). As these products moved into high volume production, they paved the way for the lower voltage power devices that were first introduced in 2010. Early GaN devices suffered from problems that are unique to GaN/AlGaN devices, such as increased conduction losses (dispersion or dynamic on resistance6) when switching as well as poor switching characteristics in general, even in the 30-200 volt range. The high efficiency demonstrated at APEC confirms these problems can be resolved, even at high voltage (600V device) by the Transphorm solution.
Accelerated life-testing of these devices confirm both the design reliability and repeatability of the EZ-GaN™ technology. EPI deposition equipment previously developed to support the rapidly growing GaN LED market ensures high volume low cost manufacturing is readily available. Recent system performance demonstrates this technology has finally moved out of the lab and into real applications such as high density power conversion for data centers, improved efficiency for solar inverters and small, high efficiency VFD (Variable Frequency Drive) motor controllers with integrated filters.
BRIGHT FUTURE
GaN is no longer a promise but a reality, as we can now discuss these devices in terms of their system performance benefits. But, the 600 V devices from Transphorm represent only the first generation of this exciting new technology. Having observed the rapid evolution of silicon MOSFET technology, one can easily envision the rapid improvements in device performance, pricing, and availability, that can be expected as GaN devices are adopted into advanced electronics systems that drive the volume and maturity of the technology.
Note: Additional information about Gallium Nitride devices from Transphorm can be obtained by contacting the article's author, Carl Blake, at 805-456-1300 or via email at [email protected].
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REFERENCES
- U.K. Mishra, P. Parikh, Y.F. Yu, “AlGaN/GaN HEMTs, “An overview of device operation and applications,” UCSB.edu, Proceeding of the IEEE, 2002.
- Y.F. Wu, R.Coffie, Y. Dora, C.S. Suh, L. Shen, P. Parikh, and UK Mishra, “Total GaN Solution to Power Conversion Applications,” IEEE Device Research Conference, June 20-21, 2011 (to be published).
- Y. F. Wu, “Transphorm Exhibitor Seminar,” APEC 2011
- Y. Dora, “Understanding Material and Process Limits for High Breakdown Voltage ALGaN/GaN,” PhD Dissertation, UCSB 2006.
- U.K. Mishra, “Presentation to PSMA,” March 2011.
- R. Coffie, “Characterizing and Suppressing DC-to-RF Dispersion in AlGaN/GaN High Electron Mobility Transistors,” PhD Dissertation, UCSB 2003.