Some 30 years ago, International Rectifier introduced the first commercially viable silicon MOSFETs, called HEXFETs, to enable rapid adoption of switch mode power supplies (SMPS) over then dominant linear supplies which used bipolar devices. Since then, this Si-based technology platform has continued to evolve to satisfactorily serve myriad markets. In fact, without the widespread use of compact efficient SMPSes, the current communications infrastructure—high-speed networks, mobile electronics and ubiquitous computing, etc.—would not likely have been practically feasible.
However, silicon MOSFETs now have approached a performance plateau, and cost of advancements has increased dramatically. At the same time, next generation and emerging applications are demanding further substantial leaps in power conversion performance.
Hence, to meet new requirements of forthcoming applications, new materials and transistor structures are needed to fill this gap. Although, silicon carbide (SiC) FETs have emerged on the scene in the past 10 years to address these issues, they suffer from significant cost premiums due to limited quality material supply, as well as the intrinsic cost structure of the material. Additionally, SiC based technology is not highly scalable in substrate size, epitaxial deposition equipment throughput, material supply and device fabrication manufacturing platforms.
Envisioning such a need, IR scientists and engineers have developed a revolutionary gallium nitride (GaN)-based power device technology platform that promises to deliver performance that is at least ten times better than existing silicon devices to enable dramatic reductions in energy consumption in end applications in a variety of market segments, such as computing and communications, consumer appliances and automotive.
In fact, over five years of device R&D has resulted in a proprietary GaN-on-silicon epitaxial process and device fabrication technology platform that heralds a new era in power conversion. IR expects the potential impact of the new GaN based device technology platform to be at least as significant as the introduction of HEXFETs some three decades ago. Referred to as GaNpowIR, the company plans to offer a broad range of commercially viable products (20V to 1.2kV) supporting discrete as well as circuit solutions (modules and chipsets) for a variety of DC/DC and AC/DC converters, lighting, class D audio and motor drives.
BENEFITS OF GANStructurally, bulk GaN substrates have been prohibitively high-priced, requiring the use of hetero-epitaxial films. However, major substrates used for GaN epitaxy until now, such as SiC or sapphire, have also been relatively expensive. While silicon was a very attractive low cost alternative, it remained difficult because of defects and deformations due to intrinsic mismatch in lattice constants and thermal expansion coefficients. Recently, solutions to these epitaxial process difficulties have been developed. This program has leveraged the extensive experience in GaN epitaxy and devices that has been achieved through the efforts of a wide community of investigators, focused mainly in the fields of GaN RF devices and LEDs. This hetero-epitaxial process allows for volume deposition of GaN based material on low cost silicon wafers, costing about 100 times less than SiC. For instance, a device quality 4in SiC wafer costs more than $4,000 as opposed to the $20 cost for 4in silicon wafer. Silicon wafers also provide much larger diameter substrates and in higher volumes than are available with either sapphire or SiC, including 6in, 8in and 12in diameters.
As a result, excellent epitaxial film uniformity, lower defect levels, higher device reliability and a competitive process cost structure have been achieved to permit fabrication of GaN-on-Si based devices for widespread use in power electronics.
It is useful to note that commercial RF high electron mobility transistors (HEMTs) using GaN-on-Si technology have already been in production by Nitronex for the past several years. Meanwhile, combined with low cost GaN-on-Si epitaxy, IR’s GaNpowIR platform is compatible with established high volume CMOS manufacturing processes, providing the power conversion circuit designer community with a commercially viable manufacturing platform for GaN based power devices.
As shown in Figure 1, the basic current GaN-on-Si based device structure is an HEMT, based on the presence of a two dimensional electron gas ( 2 DEG) 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. It is clear then that the native nature of this device structure is a JFET with a high electron mobility channel and conducts in the absence of applied voltage (normally on). There are several techniques which have been developed to provide a built in modification of the 2DEG under the gated region, providing for normally off behavior.
Internal studies show that GaN based power devices can offer performance that is comparable to SiC but at much lower cost. Prototype tests conducted show that reverse recovery (Qrr) characteristics for high voltage GaN diode function is same as for commercially available SiC diodes, both being significantly better than state of the art silicon fast recovery diodes (FRED) (Figure 2). This is due to the essential absence of holes in the GaN HEMT structure, eliminating the minority carrier effect in reverse recovery charge Qrr. This results in quiet switching behavior, eliminating the otherwise needed filtering (snubber) circuitry. This reduces system size, cost and weight and has been used extensively in power factor correction (PFC) circuitry in AC/DC converters using expensive SiC diodes. This naturally fast and quite switching behavior also provides for significantly reduced switching losses when anti-parallel diode functions are required (such as with IGBTs in motor drive circuitry).
A combination of high electron mobility and higher bandgap provides GaN with a significant reduction on device specific on-resistance RDS(on) for a given reverse hold off voltage capability than both SiC and silicon devices, as shown in the calculated material limit curves for (non-highly compensated) unipolar devices in Figure 3. Also shown are representative, best case, published measured results for the three materials, as well as for highly compensated (SJ) and bipolar (IGBT) device structures in Si. Results from the early stage development of the GaNpowIR technology platform at International Rectifier are also shown (IR GaN). It is clear that an order of magnitude 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 (less than 10 years), as compared to the mature Si (more than 30 years) or SiC (more than 20 years) technologies.
Whereas both the Si and SiC based unipolar devices have essentially reached their theoretical limits, the GaN devices are still several orders of magnitude higher than the calculated potential performance. It is expected that engineering efforts over the next 10 to 20 years will result in a significant reduction in this performance gap. Though the absence of a comparable 2DEG device for SiC based devices, together with ohmic contact resistance, make low voltage SiC devices non-competitive with GaN HEMTs, bulk SiC substrates enabled homo-epitaxy provide for the very thick films required for voltages above about 1.5kV. In contrast, it is much more difficult to achieve these higher voltages for GaN hetero-epitaxy on silicon substrates. Thick GaN hetero-epitaxial films are easier on sapphire substrates, since the mismatch in thermal coefficient of expansion is small, but insulating substrates restrict the power handling capability of the device due to self heating constraints. It is therefore expected that SiC will remain an attractive choice for high voltage switches (>1.5kV) in the future.
HIGHER FREQUENCY SWITCHINGSince GaN based power devices achieve a combination low gate capacitance and low on-resistance, it permits much higher frequency switching converters than competing silicon transistors. Results based on device modeling indicate that R(on)*Qg figure of merit (FOM) for first generation GaNpowIR HEMTs is 33 percent lower than that of state of the art silicon MOSFETs. On-going engineering efforts are expected to provide further significant improvements in the next few years. Figure 4 shows that R(on)*Qg for GaNpowIR devices is expected to be as low as 13mΩ-nC by 2011, representing more than a 50 percent improvement over GaN-based devices introduced in 2009. By 2014, the R(on)*Qg FOM for GaNpowIR is expected to be less than 5mΩ-nC, an order of magnitude improvement over state of the art Si-based devices available in 2009.
Figure 5 depicts the expected effect of the improvements in R(on)*Qg FOM of the power switch on the size and efficiency of a DC/DC converter, including the output filter. Current state of the art converters perform 12V to 1V conversion efficiently up to 2MHz. The GaNpowIR technology platform is expected to enable efficient power conversion to greater than 50MHz in the near future. As can be seen, the improvements in the power switch FOM enable a corresponding increase in operating frequency and a corresponding decrease in converter size, without a reduction in power conversion efficiency. In fact, when the frequency is high enough (20MHz to 60MHz), as to eliminate the need for significant external components and wasteful distance between the converter and the load, a significant reduction in parasitic related power loss is achieved. This then provides for a revolutionary simultaneous achievement of high density, higher efficiency and lower system cost.
IR’s GaN based power device roadmap anticipates that initial prototypes will switch efficiently at 4MHz to 5MHz, with commercial products introduced over the next few years supporting switching frequencies of 10MHz to 60MHz.
At IR, the new GaN-based products are not viewed simply as replacements for existing silicon devices; rather the focus is to enable new functional blocks that take distinctive advantage of performance attributes of GaN-based devices compared to those based on silicon. The GaNpowIR platform from International Rectifier has been designed from its inception to be commercially viable, using high volume, large diameter silicon substrate-based epitaxy and CMOS compatible device processing, representing a low risk to volume production ramp. Based on the experience with the introduction of HEXFET power Si MOSFET 30 years ago, it is expected that the GaN-based power devices will be used in about 10 percent of the all power conversion application between 20V and 1.2kV within five to seven years of commercial introduction.
ABOUT THE AUTHOR
The article is written by Michael A. Briere, Executive Scientific Consultant, ACOO Enterprises LLC, under exclusive contract to International Rectifier Corp.
CAPTIONS
Figure 1: Basic GaN based device structure is HEMT.
Figure 2: Reverse recovery (Qrr) performance for GaN device is same as Sic diode.
Figure 3: Comparing specific on-resistance of IR’s GaN-on-Si-based HEMTs with silicon and Sic power MOSFETs.
Figure 4: Potential evolution of R(on)*Qg FOM for low voltage GaNpowIR HEMTs.
Figure 5: Projected evolution of size and power conversion efficiency for a 100A, 12V to 1V converter (including output filter), corresponding to improvements of the power switch FOM, R(on)*Qg.
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