With radical improvements in R(ON) x Qsw figure of merit (FOM), leading to an application value proposition - efficiency x density/cost - that is an order of magnitude better than state-of-the-art silicon, gallium nitride (GaN)-based power devices promise a revolution in high efficiency, high density, cost effective power conversion solutions.
By enabling rapid adoption of switch mode power supplies (SMPS), silicon power MOSFETs have been at the forefront of power conversion for last three decades.
From planar HEXFETs, introduced in 1978 by IR, to TrenchFETs and superjunction (SJ) FETs, power MOSFETs have given bipolars a run for their money for nearly 30 years. But now this silicon power device has approached a performance plateau. That means, going forward it does not have the juice to deliver performance/cost ratio demanded by next -generation applications. Consequently, any performance increment will result in unwarranted excessive expenses.
New materials and transistor structures are therefore needed to fill this power conversion performance gap. Even though silicon carbide (SiC) developers have been tackling these issues in the past ten years, it has not made any dent in this market because of cost. Besides the intrinsic cost structure of SiC, the limited supply of quality material also makes this technology very expensive, adding to the non-scalability of the substrate size and the expitaxial deposition throughput shortcomings.
Anticipating a need to look for solutions beyond silicon, scientists and engineers have been researching new technologies. One of these is the proprietary GaN-based power device technology platform developed by IR, which delivers a FOM performance at least 10x better than existing silicon MOSFETs.
GaN-on-Si Benefits
Since bulk GaN substrates are uneconomical, developers have taken the hetero-epitaxial route for building GaN-based power devices. However, until now, major substrates used for GaN epitaxy have been SiC or sapphire. But, both are relatively expensive propositions.
Although silicon was an attractive low-cost alternative, it remained difficult because of defects and deformations due to intrinsic mismatch in lattice constants and thermal expansion coefficients. Leveraging the extensive industry experience in GaN epitaxy and devices, significant engineering efforts have been made to resolve these issues. As a result, GaN-on-silicon technology platform has been developed to offer high epitaxial film uniformity, lower defect levels and higher device reliability. In addition, the device manufacturing process is CMOS compatible, thus allowing high volume deposition of GaN-based material on low-cost silicon wafers with larger diameter substrates. This novel GaN-on-Si design and fabrication technology platform is referred to as GaNpowIR.
As shown in the Fig, the basic GaN-on-Si-based 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. As this device structure is a HFET with a high electron mobility channel that conducts in the absence of applied voltage (normally on), several techniques have been developed to provide a built-in modification of the 2DEG under the gated region that permits normally off behavior.
In essence, this novel power device delivers dramatic improvements in three basic FOMs, namely specific ON resistance, device switching and power conversion application. Fundamental physics indicates that GaN-based HEMTs can achieve a factor of 10 improvement in RDS(ON) over silicon MOSFETs in the 100 to 300V application range.
With on-going improvements in the GaN power devices, it will offer a 10x improvement in RDS(ON) over current state-of-the-art silicon MOSFETs within five years.
Likewise, in the 600 to 1,200V application range, the calculated material limit curves for unipolar devices show that GaN-based power devices have the potential of further reducing RDS(ON) by a factor of 100 over silicon MOSFETs. Results from early stage development of GaNpowIR devices are compared with silicon and SiC devices in this figure.
Besides enhancing specific ON resistance, GaN-based power devices also offer a significant boost in the device switching FOM RDS(ON) x Qg(RQ). Simulated results from early prototypes fabricated indicate that the first generation of GaN-on-Si-based power HEMTs are expected to realize about a 33% improvement over the state-of-the-art silicon MOSFETs. As this momentum continues, it is estimated that GaN-based power devices will further cut RQ FOM by an order of magnitude within five years of introduction of GaNpowIR technology platform in 2009.
Paradigm Shift
The combination of low gate capacitance and low ON resistance permits switching at much higher frequencies. Internal simulations show that GaNpowIR devices have the potential to switch at frequencies above 50MHz, far beyond the capabilities of silicon MOSFETs. Consequently, DC-DC converters using GaN-on-Si-based HEMTs will achieve much higher power density without compromising conversion efficiency. Current state-of-the-art multi-phase silicon-based solutions perform 12V to 1.2V conversion efficiently up to about 2MHz per phase. By comparison, GaN-on-Si-based power devices will enable efficient power conversion to greater than 50MHz per phase. That translates into fewer external components and lower parasitic-related power losses. The end result is the achievement of a high density, high efficiency and low cost system.
Analog vs Digital Regulation
In order to take advantage of this fundamental improvement in basic value proposition of power conversion, the regulation scheme must provide the required precision at a much higher bandwidth than that which is currently deployed. This presents a fundamental challenge to the adoption of digital circuits to provide the regulation function, as the precision frequency cost FOM is strained. Switching at frequencies above 20MHz presents a real challenge to digital regulation.
In order to achieve the required regulation performance, including absolute accuracy of <0.5% of VOUT and switching at >20MHz, an effective resolution of some 12 bits is needed for analog-to-digital converters (ADC) clocking at hundreds of MHz. This results in larger die area, coupled with high speed designs using deep sub-micron CMOS technologies. Such solutions substantially increase the cost of the regulator and reduce the dynamic headroom of the regulator, making it much more susceptible to noise-induced error.
Consequently, the benefits of digital regulation (non-linear control, self compensation, etc) are second order to the fundamental value proposition presented by the ability to switch at high frequency. Hence, it is likely that the proposition of digital regulation will not be compelling for the most demanding applications, which will require cost effective high density and high efficiency power conversion in the near future.
By achieving dramatic improvements in specific ON resistance, device switching FOM R(ON) x Qsw and power conversion application FOM efficiency x density/cost, a commercially viable GaN-on-Si-based power technology platform is all set to stimulate a new revolution in high frequency, high density, highly efficient cost effective power conversion solutions. As a result, new conversion architectures and control schemes will be developed to take full advantage of the capabilities of GaN-on-Si-based power HEMTs.
Fuente:
Gerald Soto, CRF 2010-1.
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