Gallium nitride based power conversion raises density bar
23 April 2009
GaN-on-silicon based High Electron Mobility Transistors (HEMTs) are poised to spark a new trend in power conversion solutions. This article highlights the merits of the new devices and examines the impact of multi-MHz switching on power density and analogue regulation.
While MOSFETs have adequately served power electronics for more than 30 years, its time to look beyond silicon, as the power device has approached maturity. As a result, any further incremental improvements in silicon FETs will not be cost competitive. Consequently, new materials and transistor structures are needed to fill this performance void.
Well, IR started the MOSFET revolution three decades ago with the introduction of commercially viable power HEXFETs in 1978. Now, thirty years later to fulfill new performance demands, scientists and engineers at IR have developed a novel gallium nitride (GaN) based power device technology platform that has the potential to stimulate a new paradigm in power conversion. More than five years of device R&D has gone into this effort, which has resulted in a proprietary gallium nitride-onsilicon (GaN-on-Si) epitaxial process and device design and fabrication platform. Referred to as GaNpowIR, it promises to deliver power conversion solutions at much higher frequency, higher density and higher efficiency at a cost that is unprecedented. In short, the novel GaN-on-Si based power devices are expected to offer performance that is at least 10x better than current state-ofthe- art silicon MOSFETs to rekindle another revolution in power electronics.
Due to the intrinsic mismatch in lattice constants and thermal expansion coefficients between the substrate and the epitaxial film, accomplishing a uniform reliable silicon based hetero-epitaxial process has not been easy. Subsequently, significant engineering efforts were made to resolve these problems. The end result is a GaN-on-Si technology platform that offers excellent epitaxial film uniformity, lower defect levels, higher device reliability, as well as a CMOS compatible device manufacturing process. Thus enabling high volume deposition of GaN on low cost silicon wafers costing about 100 times less than silicon carbide (SiC) and offering larger diameter substrates.
Raising the bar
The basic GaN-on-Si power device 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. An inherent blend of high conduction electron density, high electron mobility and higher bandgap, GaN based HEMTs provide a significant reduction in device specific on-resistance RDS(on) for a given reverse hold-off voltage. Figure 1 compares published measured results for SiC and silicon FETs, including highly compensated superjunction (SJ) and bipolar
(IGBT) structures in silicon, with results from early stage GaNpowIR technology developments at IR (IR GaN). From this figure it is evident that more than a 10x improvement in specific on-resistance can be achieved for GaN based devices over silicon counterparts, even at the early stage of GaN power technology development. In fact, Figure 1 shows that in the 600 to 1200 V application range, GaN based devices have the potential of improving RDS(on) by a factor of 100 over silicon MOSFETs.
Additionally, GaN based power devices also offer much lower gate capacitance to realise dramatic improvements in the device switching Figure Of Merit (FOM) RDS(on)*Qg (RQ). Simulation results using device models based on early fabricated prototypes indicate that the first generation GaN-on-Si based power HEMTs, slated for release in late 2009, are expected to deliver about 33% improvement over the state-of-the-art silicon MOSFETs. As illustrated in Figure 2, continuous enhancements in the device switching FOM will result in an order of magnitude reduction in RDS(on)*Qg within five years of introduction of GaN-on-Si based power HEMTs. This figure shows that RDS(on)*Qg for 30 V GaN HEMTs is expected to be as low as 13 mΩ-nC by 2011, representing more than a 50% improvement over GaN based devices introduced in 2009. It is projected to go below 5 mΩ-nC by 2014, a ten fold improvement over the new generation MOSFETs released in 2009.
As a result, the operating frequency of GaN based power devices has been pushed to new heights. Internal studies suggest that GaN-on-Si based power devices have the potential to switch efficiently as high as 60 MHz. The impact of efficiently switching at higher frequencies is depicted in Figure 3. As shown, current state-ofthe- art multiphase converters using silicon MOSFETs perform 12 V to 1.2 V conversion efficiently up to about 2 MHz per phase. By comparison, GaNpowIR technology devices will permit power conversion to greater than 50 MHz per phase. Switching at such high frequencies cuts external component count as well as the undesired distance between the converter and the load to curb parasitic related losses. The result is an unprecedented achievement of high density, higher efficiency and lower system cost.
Interestingly, new generation CPUs with transient current requirements of 1000 A/μs are demanding Point-Of-Load (POL) DC-DC converters that can respond to such fast step current functions. To meet such stringent no load to full load requirements, the converters must switch at very high frequencies. However, switching losses for silicon MOSFETs will increase significantly at higher frequencies and therefore, are not recommended at very high frequencies. On the other hand, GaN-on- Si based power devices have the ability to efficiently switch above 10 MHz and up to 60 MHz to eliminate board parasitics and wasted space. And, thereby, achieve smaller size and higher density of DC-DC converters that will permit power stage function to become part of the CPU socket. Several prototypes have been built to demonstrate the distinct advantages of the new GaN technology. One such prototype is a low voltage POL converter with a 12 V input and 1.2 V output and a 10 A load current. Operating at 5 MHz, the GaN-on-Si based POL delivers efficiency that is comparable to a commercially available silicon solution switching at 1 MHz, but at less than one third the size (Figure 4). Both the solutions integrate the controller/driver IC and the output inductor within the power stage package.
Pushing analogue regulation
Lately, a lot of attention has been paid to the proposition that the closed loop regulation of power conversion can best be achieved through digital control implemented in digital CMOS. While the communication, control and configurability of power regulation using digital circuits has been well established in the last few years, achieving required regulation precision at much higher bandwidth presents a fundamental challenge to the adoption of digital circuits. In the last few years, it has become clear that the application of digital regulation for power conversion requires advanced analogue/mixed-signal design talent to provide the required high resolution Analogue-to-Digital Converters (ADCs). This strains the precision*frequency*cost FOM. As a result, it diminishes the value proposition of power conversion using digital control schemes.
For instance, to achieve required regulation performance with absolute accuracy of <0.5% of Vout and switching frequency of over 20 MHz, an effective resolution of some 12 bits is needed for an Analogue-to-Digital Converter, clocking at 100’s of MHz. This will translate into a larger die area for the digital controller, as well as require high-speed design skills in deep submicron CMOS, e.g. 45 to 90 nm platforms. Such an approach will substantially increase the cost of the regulator solution and reduce the dynamic headroom of the regulator, making it susceptible to noise induced errors.
However, with the advent of IR’s commercially viable GaN based power device technology platform, the density*efficiency/cost FOM for power conversion is significantly better than silicon alternatives. Unlike digital, it has the ability to provide the precision at much higher frequencies than currently used (e.g. 10’s of MHz) at a reasonable cost. Consequently, the proposition of digital regulation will not be compelling for applications that will require cost effective high density and high efficiency power conversion in the not too distant future.
MICHAEL A. BRIERE is an Executive Consultant at ACOO Enterprises LLC, under contract to International Rectifier Corp.
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