Enhancement Mode Gallium Nitride MOSFET Delivers Impressive Performance

Mar 1, 2010 12:00 PM
Sam Davis, Editor in Chief

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 R_DS(ON), while its lateral device structure and majority carrier diode provide exceptionally low Q_G (total gate charge) and zero Q_RR (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 R_DS(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 R_DS(ON) is positive, and lower than that of a silicon-only MOSFET.

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