Out of the frying pan, into the fire? No problem for silicon carbide ICs

Engineers familiar with silicon-based semiconductors sometimes have a hard time wrapping their heads around silicon carbide electronics. The reason: It is difficult to conceive of a semiconductor device able to work just fine while glowing red hot. That high thermal conductivity, of course, is one of the big attractions of SiC devices.

After years of research and development in the lab, the compound semiconductor material SiC is taking a bigger role in handling electrical power. Though not quite yet a main-stream technology, it is carving out a niche in applications that demand an ability to work at high-voltages and temperatures while demonstrating high efficiency. Several suppliers now provide Schottky barrier diodes based on the technology, with about 40 times lower reverse leakage current than that of silicon Schottky diodes. Also available are a host of junction field effect, bipolar junction, MOS field effect and insulated-gate field-effect transistors (JFETs, BJTs, MOSFETs and IGBTs). Big names in the field include Cree, United Silicon Carbide, Rohm, GE, SemiSouth, Fairchild (formerly Transic) and Infineon Technologies. Mitsubishi, Toshiba and Denso are known to have developed SiC devices for motor drivers and automotive uses.

To be sure, other compound semiconductor materials like gallium nitride (GaN), as well as ordinary silicon, are also improving. However SiC holds the most promise for future wide-band-gap materials as silicon approaches its theoretical limits for handling power. It is safe to say that future power applications demanding voltages upwards of 1,200 V or more will be largely filled by SiC devices, which have bandgaps 3x greater than other compound semiconductor ICs.

This is not surprising given the many useful qualities of SiC materials, not the least of which is ruggedness. This makes them prime candidates for providing the long lifetimes needed for electric vehicles and solar/photovoltaic inverters. In the lab, SiC devices have worked at red hot temperatures of 650°C or more. In actual field applications, operating temperatures of 250°C are becoming the norm. Higher efficiencies also mean smaller size and less need for heat sinking. Compared with ordinary silicon semiconductors, SiC semiconductors feature higher dielectric breakdown voltages (by 10x), lower EMI emissions, faster recovery times and lower forward-voltage drops than diodes.

In the beginning

The first commercially available SiC devices surfaced about a decade ago. The renewed recent interest in them has been driven by advances that include lower production costs, a push for greener energy, and more efficient power conversion. IMS Research senior analyst Richard Eden thinks the global market for SiC devices will reach $100 million this year.

The move to bigger SiC wafers is likely to hava big impact on photovoltaic inverters, among other things, says the market research firm Yole Developpement.

But for now, the technology will remain a niche player. Eden says SiC is unlikely to take over from silicon and gallium-nitride power devices because SiC devices are considerably more costly than both of them. Furthermore, the latest silicon super-junction MOSFETs are pushing toward 600-V operation. Silicon IGBTs are already widely used for 900-V applications and above. And GaN shows similar energy efficiency improvements at voltages of around 200 V. But Eden does see a brighter future for SiC as lower wafer costs push down device pricing.

There's a large market for SiC devices in solar inverters. The market research firm Yole Développement has called a recent move to four-inch SiC wafers significant for these inverters and sees more fabs moving to six-inch wafers. It also sees more SiC diodes and transistors for photovoltaic inverters in the next few years.

In 2005, Japan's Denso Silicon employed SiC devices in power control units (PCUs) for the Lexus LS 600h and the Lexus LS600hL hybrid EVs. The SiC components let these PCUs handle higher power using a smaller package than PCUs using ordinary silicon devices. The Denso PCU consists of a boost converter that raises the main battery voltage (288 V) to the maximum system voltage (650 V), and two inverters that convert dc into ac to drive the main traction motors.

For the PCU, Denso developed a special cooling structure for the power devices. As a result, the SiC-equipped PCU can output about 60% more power per unit volume than that of Denso's conventional technology. A SiC PCU designed to produce as much output as conventional technology can be just 30% of the size and about 20% of the volume of ordinary PCUs.

Large firms like General Electric are also active in SiC production. This year GE Global Research developed a new line of SiC-based power-conversion devices for air, land and sea-going craft. “With SiC technology, we have the potential to reduce the weight on an aircraft by more than 200 lbs. while also delivering higher performance and freeing up precious cargo space,” explains GE Aviation Systems' president of Electrical Power Vic Bonneau.

SiC technology also has its share of academic research efforts. One such program is at the University of Warwick in the UK which houses a special laboratory for materials physics and fabrication technology on SiC devices. The lab gets funding from a regional development agency called Advantage West Midland and an EU funding entity called the European Regional Development fund.