IF YOU ASK DR. ALEX LIDOW about the future of silicon MOSFETs, this 30-year veteran of the electronics industry will tell you that he sees the technology reaching the “end of the road.”
“We've made incremental changes in the silicon devices, but we're now getting diminishing returns,” he notes. “The theoretical limit of silicon MOSFETs is before us; we have to go to another semiconductor material. It will be disruptive to the MOSFET industry, but in the long run it will usher in new devices that are superior to the present state of the art.”
Lidow's solution is gallium nitride (GaN). He says GaN is young in its life cycle, and will certainly see significant improvements in the years to come. Lidow is backing up this prediction with a new company he formed called Efficient Power Conversion Corporation (EPC). The company is already sampling several of its GaN transistors and officially launched its venture in March 2010.
Lidow touts GaN for the following reasons:
- GaN offers superior performance compared with silicon and silicon carbide (SiC)
- Device-grade GaN can be grown on top of silicon wafers
- GaN-on-silicon offers self-isolation; therfore, efficient monolithic power ICs can be fabricated economically
- EPC has developed proprietary technology for the industry's first enhancement-mode GaN devices
EPC produces GaN on silicon wafers using standard MOS processing equipment. GaN's exceptionally high electron mobility and low temperature coefficient enables a very low RDS(ON), while its lateral device structure and majority carrier diode provide exceptionally low QG (total gate charge) and zero QRR (source-drain recovery charge). As a result, GaN devices can handle very high switching speeds.
Initially, GaN-on-silicon transistors were depletion-mode types: they operated like a normally on power switch that required a negative voltage to turn them off. The ideal mode for designers is an enhancement-mode transistor that is normally non-conducting and requires a positive voltage to turn it on, like today's silicon-only MOSFETs.
EPC produces an enhancement-mode GaN transistor using a proprietary process with a GaN-on-silicon structure (Fig. 1). In operation, a positive gate voltage turns on the enhancement-mode GaN transistor.
An advantage of the GaN transistor is that its blocking-voltage rating depends on the distance between the drain and gate: the longer the distance, the higher the voltage rating. Another advantage is its very low resistance. Fig. 2 plots the theoretical resistance-times-die-area limits of GaN-silicon, versus silicon-only, versus voltage.
GaN transistors are fully enhanced at 5 V and have a threshold voltage around 1.5 V. Fig. 3 shows the transfer characteristics curve for the EPC1001, a 100-V, 5.6-mΩ transistor. A negative relationship between current and temperature provides excellent sharing in the linear region and in diode conduction.
GaN's RDS(ON) versus VGS curves are similar to MOSFETs that operate with a 5-V drive (Fig. 4). There is negligible gate-drive-loss penalty, so GaN transistors can be driven with up to 5 V. The GaN transistor's temperature coefficient for RDS(ON) is positive, and lower than that of a silicon-only MOSFET.
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CAPACITANCE
Besides low RDS(ON), the GaN transistor's lateral structure also makes it a very-low-charge device, giving GaN the ability to switch hundreds of volts in nanoseconds, reaching multiple megahertz frequencies. Fig. 5 shows the capacitances associated with the EPC1001 GaN MOSFET.
The most important factor in switching is the CGD, gate-drain capacitance. An extremely low CGD leads to very rapid voltage switching. Gate-source capacitance, CGS, is large compared with CGD — giving GaN transistors excellent dV/dt immunity — but still small compared with silicon-only MOSFETs. This gives GaN very short delay times, and excellent controllability in low duty-cycle applications. GaN transistors have significantly lower output capacitance (COSS) than silicon-only MOSFETs with a similar RDS(ON).
Series gate resistance (RG) limits how quickly FET capacitance can be charged or discharged. With GaN transistors, metal gates have resistances of a couple tenths of an ohm, which aids dV/dt immunity.
Total gate charge (QG) is the integral of CGS plus CGD over voltage. QG is one of the factors in the GaN transistor's figure of merit, which is the product of RDS(ON) and QG. Fig. 6a illustrates the figure of merit for GaN transistors versus best-in-class silicon-only MOSFETs at 100 V, and Fig. 6b for 200-V devices.
Similar to silicon, GaN transistors have an intrinsic body diode. However, due to being a majority carrier-only device, the body diode for GaN transistors has a QRR (source-drain recovery charge) of zero.
GaN PACKAGE
EPC's GaN transistors are insulated from the substrate, allowing monolithic fabrication of multiple transistors in any configuration and efficient, common heat-sinking without the need for an insulating layer between the device and the heat sink. It also forces the current for both drain and source to be collected on one side of the die. Paths are kept short to minimize resistance in the layers that collect the current.
Fig. 7 shows the pin layout for the EPC1001, EPC1010, and EPC1015 devices. Pin arrangement is in the form of a line grid array with alternating source and drain lines. The lines on the EPC1010 are further apart because it has a higher voltage rating than the other two parts. This GaN packaging approach eliminates most of the parasitic effects inherent in conventional transistor packages. The number of source and drain lines determine the transistor's current-handling capacity. Fig. 8 is a side view of an EPC GaN MOSFET mounted on a PC board.
Using multiple drain and source lines spreads the heat produced by the device, and the insulated top allows a heat sink to be placed directly on that surface. This requires only thermal grease or another thermally conducting compound between the heat sink and the top.
THE FUTURE
A few years ago, Professor Umesh K. Mishra of the University of California, Santa Barbara used a baseball metaphor to describe the future of GaN.
“I am a believer in grand slams,” Mishra said. “First we have to get the bases loaded, and then we have to hit a home run. Hitting a homer with the bases loaded is the kind of development that propels technology forward. The bases are now loaded.
“Now we need to hit that home run. That's where we think we are now. If not, we strike out. That's part of the game, too. But I am an optimist.”
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REFERENCES
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Application note, “Single-Stage 48 V - 1 V DC-DC Conversion Simplifies Power Distribution While Significantly Boosting Conversion Efficiency,” Edgar Abdoulin and Alex Lidow, Ph.D., Efficient Power Corporation, 2009
ABOUT EFFICIENT POWER CONVERSION CORP.
- Located in the Los Angeles, CA area
- Dr. Lidow is now assembling his staff
- EPC has applied for patents on its processing techniques for GaN
- Tested GaN MOSFET prototypes have switched at up to 9 GHz
- The company employs a CMOS foundry to produce its devices
- At the present time, only n-channel transistors are possible with this technology
- Dr. Lidow says there will be a modest price premium for GaN transistors compared with silicon MOSFETs
- Besides discrete transistors, EPC is looking at mixed-signal ICs that can take advantage of GaN's capabilities
- The company has signed Digi-Key to handle distribution
- The Web site is http://www.epc-co.com
Is GaN THE NEXT POWER SEMICONDUCTOR STAR?
Some industry experts feel that gallium nitride (GaN) is the most important semiconductor material since silicon. GaN possesses some unique material properties which would make it possible to create new devices with high breakdown voltage and high power density. And, GaN's excellent thermal properties make it ideal for high-power, high-temperature, extreme-environment applications.
Research on GaN goes back to the 1930s, but work focusing on semiconductors began in earnest during the 1960s. This work intended to take advantage of GaN's wide bandgap between its valence and conduction bands. The larger the bandgap, the more voltage it can safely handle. This GaN characteristic enables transistors that can handle high voltage, as well as high current.
Another advantage is that GaN transistors can look like transistors made with other semiconductors, but GaN performs at a higher level. This aids the adoption of new technology, because you don't have to reeducate people on how to use it. The box may look the same, but what's inside performs much better.