Although, silicon MOSFETs have dominated the power supply market for more than 30 years, the technology has now approached a performance plateau, meaning any incremental improvement will result in costs that are commercially uneconomical. While SiC-based FETs have emerged to address such performance limitations, there are significant cost premiums due to limited quality material supply, as well as the intrinsic cost structure of the material. In addition, SiC-based power transistors are not highly scalable in substrate size and device manufacturing platforms.
Envisioning a need for new materials and transistor structures, International Rectifier (IR) has developed a family of gallium nitride (GaN)-based power switches that, together with the cost effectiveness of the technology platform, have the potential to change the game in power conversion electronics. In fact, this proprietary new power technology is the result of over five years of device R&D and more than a $50 million investment.
To meet new challenges of forthcoming applications, IR scientists and engineers have developed a revolutionary cost-effective CMOS compatible gallium nitride (GaN)-on-silicon- (GaN-on-Si) based power device technology platform that promises to deliver figure of merit (FOM) performance that is an order of magnitude better than existing state-of-the-art silicon and SiC MOSFETs within the next five years.
In fact, to fully realize the benefits of such GaN-on-Si based switches, the technology platform, referred to as GaNpowIR, includes the development of optimized gate drivers, controller ICs, novel topologies and advanced packaging solutions. Truly, the advent of commercially viable GaN-on-Si based power devices signal a new paradigm in high frequency, high density, highly efficient cost effective power conversion solutions.
Novel power technologySince bulk GaN substrates are prohibitively high-priced with small sizes (≤2in diameter) and limited availability, IR developers were prompted to explore the use of hetero-epitaxial films for GaN based power devices. Traditionally, substrates of SiC or sapphire have been used for this purpose. Unfortunately, both are relatively expensive, with sapphire having the added detriment of being a thermally poor conductor. Though silicon is an obviously attractive low-cost alternative substrate for GaN hetero-epitaxy, it presented many process challenges. Defects and deformations due to significant intrinsic mismatches in lattice constants and thermal expansion coefficients of the silicon substrate and epitaxial films proved challenging hurdles to overcome in a large scale production process.
However, via rigorous process design and control methodologies, the developers have achieved the necessary uniformity for both epitaxial properties and device characteristics across 150mm silicon wafers. In addition, significant engineering efforts have been made to improve the device performance figures of merit (FOMs), as well as device initial quality and long term reliability.
Designing the device fabrication process to be CMOS compatible also required significant engineering. Traditionally, III-V semiconductor device processing has relied on relatively low throughput, expensive techniques like gold interconnects, liftoff and e-beam lithography. Unlike the traditional compound semiconductor suppliers, IR has crafted a platform that is compatible with silicon manufacturing facilities, offering a high volume commercially viable manufacturing platform for GaN based power devices.
As illustrated in Figure 1, the basic GaN-on-Si structure is a high electron mobility transistor (HEMT), based on the presence of a two dimensional electron gas (2DEG) spontaneously formed by the intimacy of a thin layer of AlGaN on a high quality GaN surface. Ohmic contacts are made to the 2DEG, typically using Ti/Al based metallurgy. An insulated or rectifying metal gate structure is formed between the ohmic contacts and provides for the field induced modulation of the 2DEG. The native device structure is a HFET with a high electron mobility channel that conducts in the absence of an applied voltage (normally-on). However, to offer a normally-off behavior, several techniques have been implemented that provide a built-in modification of the 2DEG under the gated region.
A combination of high electron mobility and higher bandgap provides GaN based HEMT devices with a significant reduction in device specific on-resistance RDS(ON) for a given reverse hold-off voltage capability compared to SiC and silicon devices, as shown in the calculated material limit curves for (non-highly compensated) unipolar devices in Figure 2. Measured results from the early stage development of the GaNpowIR technology platform (IR GaN) are also presented in this figure. It is clear that a significant improvement in specific on-resistance can be achieved for GaN based devices over silicon counterparts, even at the early stages of power device development of GaN technology (<10 years), as compared to the mature Si (>30 years) or SiC (>20 years) technologies.
Besides low on-resistance, GaN-based power devices also offer low gate capacitance. This combination of low gate capacitance and on-resistance permits much higher switching frequencies than competing silicon transistors. Results based on device modeling, extrapolated from early measured data, indicate that RDS(ON)*Qg FOM for first generation 30V GaNpowIR HEMTs is 33 percent lower than that of the state-of-the-art silicon MOSFETs. Figure 3 shows that by 2014, the RDS(on)*Qg FOM for GaNpowIR is expected to be less than 5mΩ-nC, an order of magnitude improvement over current state-of-the-art silicon MOSFETs.
The potential GaN power stage roadmap in Figure 4 depicts the expected impact of the improvements in RDS(on)*Qsw FOM of the power switch on the size and efficiency of a 12V to 1.2V, 100A DC/DC converter, which includes the output filter. Current state-of-the-art multiphase silicon-based solutions perform 12V to 1.2V conversion efficiently (>85 percent) up to 2MHz per phase. The GaNpowIR technology platform is expected to enable efficient power conversion to greater than 50MHz per phase in the near future. As can be seen, the improvements in the power switch FOM enable a corresponding increase in the operating frequency and a subsequent decrease in the converter size, without compromising power conversion efficiency. The switching frequency is selected to provide a constant conversion efficiency of 85 percent. In fact, when the frequency is high enough (>10MHz), the need for external components and the wasteful distance between the converter and the load is decreased substantially, thereby significantly reducing parasitic related power loss. The result is a revolutionary simultaneous achievement of higher density, higher efficiency and lower system cost.
Consequently, the dominant power conversion application FOM: efficiency*density/cost will be enhanced using GaN based power conversion solutions. Due to improvements in the device RQ FOM, together with improved packaging and drive technologies, it is expected that an order of magnitude improvement will be achieved in this FOM within the first five years of commercial introduction of the GaNpowIR platform.
To demonstrate the distinct advantages of the new GaN-on-Si based power devices, several prototypes have been built. One such prototype is a low voltage point-of-load (POL) converter designed for a 12V input to 1.2V output at 10A load current. This GaN-based POL converter runs at 5MHz to deliver efficiency that is comparable to the state-of-the-art silicon solution running at 1MHz , but at less than one third the size.
In conclusion, with dramatic improvements in specific on-resistance RDS(ON) and power device FOM RDS(ON)*Qg, commercially viable GaN-on-Si based power devices will drive a revolution in power conversion electronics due to consequent advances in the efficiency*density/cost performance.
Author InformationMichael A. Briere is the Executive Scientific Consultant for ACOO Enterprises LLC. He wrote this article under contract to International Rectifier Corp.
Captions
Figure 1: Schematic cross section of a basic GaN-on-Si-based HEMT.
Figure 2: Comparing specific on-resistance of IR’s GaN-on-Si-based HEMTs with silicon and SiC power FETs. (Reference: Ikeda et. al. ISPSD 2008, p. 289)
Figure 3: Projected evolution of RDS(ON)*Qg FOM for low-voltage GaN-on-Si HEMTs indicate a tenfold reduction in RQ FOM within five years.
Figure 4: GaN power stage roadmap illustrates the impact of switching at higher frequencies on the size and effective power conversion efficiency of a multiphase 100A, 12V to 1.2V DC/DC converter (includes output filter).
