Availability of cost-effective, high-quality, robust, high-performance GaN-based power devices will enable truly innovative improvements in power electronic density, efficiencies and costs in the coming years.
Find a downloadable version of this story in pdf format at the end of the story.As microelectronic loads proliferate and the desire for high performance as well as mobile computing accelerates, there is an increasing demand for high-density power conversion solutions. At the same time, economic, political and social pressures are mounting to increase the power delivery efficiency. Of course, these two performance metrics, efficiency and density, are in conflict. As silicon based technology is reaching maturity, a truly revolutionary change in this performance trade off requires that a fundamentally new power device technology platform be introduced.
The trade-off between density and efficiency is largely a question of switching frequency. As the switching frequency increases, losses are compounded in the power converter from three main sources: namely the driving losses, the current×voltage overlap of the power devices and the capacitive losses of the device output impedance. In addition there are core losses in the magnetic element of the output filter inductance. To achieve improved efficiency, it is therefore imperative that improvements in the power device behavior, particularly the requisite input and output charge levels involved in the device switching between on and off states be achieved for a given current handling capability.
As has been discussed previously, GaN based power devices represent such an opportunity. Since the discovery by M. Asif Kahn in 1991 of the spontaneous formation of a high mobility electron sheet at the interface of AlGaN and GaN, significant efforts have been made to bring the inherent capabilities of this exciting material system to bear in practical semiconductor power devices. The combination of high breakdown field strength due to the wide band gap of the III-nitrides, high electron mobility, as well as, an unusually high channel electron density, yields a remarkably compelling drift resistance. Such devices also benefit from the reduced gate charge requirements involved in switching the devices on and off. Probably the most exciting attribute of the system involves the easily isolating nature of the inherently lateral devices, permitting unprecedented monolithic integration of power systems.
The GaN based power device program at International Rectifier, referred to as GaNpowIR®, has involved a long, costly and intense effort to turn, what has been to a large extent, an intellectual curiosity into a practical, commercially viable technology platform broadly applicable to the power electronic community from 20 to 1200 V. This required significantly reducing the cost as well as achieving best in class performance for these new device structures.
The first issue to resolve involved the choice of substrate. A cursory review of the alternatives will show that only silicon substrates provide the necessary combination of cost, size, quality and volume of supply needed to support the broad power electronics market. The technological challenge of growing uniform high quality AlInGaN epitaxial layers on silicon substrates, made especially complex due to significant mismatches in both the lattice constants and their thermal coefficient of expansion, has been met. The requisite control of film thickness and compositional uniformities, epitaxial defects and macroscopic wafer warpage has been achieved on 150-mm silicon wafers through significant engineering efforts.
SILICON PROCESSING COMPATIBILITY
Next, the device design and fabrication process required additional significant attention. Here, the first principle applied is that the fabrication process must be compatible with large volume silicon wafer manufacturing lines. This allows for the use of modern, highly capable equipment, as well as an overhead cost infrastructure competitive with the incumbent silicon based devices, a pre-requisite for widespread adoption.
This requirement, often abbreviated as “CMOS compatibility”, requires that such III-V device stalwarts such as gold metallurgy, e-beam lithography and lift-off processes be abandoned. The device design and construction must not compromise on performance, quality or reliability metrics established by the incumbent silicon based technologies. This requires that off-state leakage be kept below 1 µA/mm gate length, significantly less than the 1mA/mm often employed in the GaN based device field to date. This, in turn requires the use of low leakage epitaxial layers, as well as insulated gates, replacing the much more common Schottky metal-semiconductor structures. Fig. 1 shows the reverse leakage characteristics of prototype 600 V GaNpowIR® devices.
As can be seen, drain leakage is less than 0.1 µA/mm at the rated breakdown voltage, achieving an on to off ratio for such devices of greater than 1 million. Together with the fact that the leakage is between drain and source terminals and that the gate leakage remains at pA/mm levels throughout the applied voltage range, this represents best-in-class performance for GaN based power devices.
MAINTAINING DEVICE RUGGEDNESS
Device ruggedness in application conditions must also remain un-compromised with respect to expectations established by the incumbent silicon based technologies. Large forward biased safe operating area is an important indication of such robustness. Such performance has been demonstrated and reported elsewhere. Device stability under accelerated stress conditions for extended periods of time is essential for acceptance in the power electronic community.
To date, IR has collected over 10,000,000 device hours of reliability data on 30V devices released in 2010, with up to 10,000 hours per device. No intrinsic premature device failures have been found to date and parametric stability has been excellent. Commonly reported trapping related instability phenomenon such as current collapse or dynamic RDS(ON)  must likewise be minimized beyond concern. Fig. 2 shows the R DS(ON) measured within 1µs of applying varying reverse bias conditions for early 600 V GaNpowIR® prototypes. As can be seen, the dynamic RDS(ON) effects have been effectively minimized in this platform. This accomplishment has required optimization of all facets of the GaN based device, including epitaxial layers, device design and wafer fabrication processes.
It is with great satisfaction that these issues have been addressed sufficiently to support the targeted release of the first commercially available 600 V rated GaN based power devices by the end of 2011, in agreement with the schedule announced in September 2008.
This technology platform strongly supports the objective to enable lower system costs to promote the adoption of efficient systems that significantly reduce worldwide power consumption. As data processing now consumes some 16 % of all electricity generation in the U.S., it is imperative that improvements in efficiency both of the load and the power conversion electronics be realized. At the same time, there are economic forces driving the density of power conversion significantly higher. Both of these objectives, as well as the constant demand for lower overall system costs must be met at the same time. This simultaneous requirement for higher efficiency, density and cost effectiveness requires solutions beyond the capability of silicon based power conversion electronics.
MORE EFFICIENT POWER CONVERSION
This is where GaN devices shine. Fig. 3 shows the significant advantages of GaN based power stages, using first generation low voltage GaNpowIR devices, available commercially since February 2010, in high frequency, dense dc-dc converters designed to support electronic loads such as microprocessors. As shown in the figure, efficient conversion from 12V to POL has been demonstrated at 5 and 10 MHz. It is expected that the performance of such 20-30 V rated GaNpowIR® device can be improved by a factor of 10 over the next 5 years. This will enable efficient (e.g. 88 %) power conversion form 12 V to 1.2 V at frequencies of 60 to 100 MHz, providing otherwise unrealizable solutions for electronic power supplies, such as energy efficient many core microprocessors .
Lowering the cost of high performance AC-DC power supplies using power factor correction (PFC) boost circuitry is another application where GaN based power devices provides an unprecedented combination of efficiency, switching speed and cost effectiveness. Cascoded GaN based rectifiers provide essentially the same performance as high cost SiC Schottky diodes. This will allow wide adoption of high efficiency PFC circuits. In addition, the availability of lower cost, high performance wide band gap semiconductor based switches and rectifiers will promote wide spread adoption of efficient converters, such as inverters for distributed solar power generation, saving 2-3 % in energy loss, representing a full decade of improvement in solar cell efficiency.
POWERING MAGNETIC MOTORS
One of the greatest opportunities for worldwide energy conservation involves the use of efficient permanent magnet motors driven by inverters in appliances, such as air conditioners, refrigerators and clothes washers. In addition, the increasing electrification of transportation drive systems will require improved inverter electronics for both the primary and auxiliary electronic systems. The incumbent technology for these motor drive applications is silicon-based trench IGBTs. Fig. 4 shows a comparison between state of the art 600 V rated silicon trench IGBTs and prototype first generation 600 V rated GaNpowIR® devices in terms of conduction × switching loss figure of merit. As can be seen the GaN based devices perform remarkably better. What is truly exciting is that an order of magnitude further improvement in performance for GaN based power devices is potential over the coming decade.
From more efficient solar panel based inverters to higher density efficient permanent magnet based motion control systems to lighter weight and denser inverters for electrified vehicles, as well as next generation integrated dc-dc power supplies for electronics, GaN based power devices will revolutionize the industry. The intrinsically integrated nature of the lateral GaN based device platform will allow integration without comprising device performance, a feat not possible in traditional vertical silicon based power device technology.
As GaN power devices gain in usage, they will give design engineers an entirely new level of performance, density, robustness and cost effectiveness for current- and next-generation electronic circuits.
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