Over the past 40 years, the biggest advances in power conversion efficiency have come from improving conversion technique, although important enhancements have also been made to the characteristics of the silicon used.
At the same time, a short trr enables faster switching, meaning smaller passives. Lower power losses mean the cooling system (heat sink and/or fan) can be smaller, so replacing a silicon fast recovery diode with a SiC diode in a switched mode power supply contributes to end product miniaturisation. In addition, SiC diodes are more stable with temperature than silicon based devices (see fig 2). A constant forward voltage over the temperature range simplifies the design of parallel connections and prevents thermal runaway. A constant trr enables high temperature operation without increasing switching losses. Switching speed and temperature stability are important mosfet characteristics and SiC devices are inherently superior on both counts than silicon and manufacturers such as Cree and Rohm are offering 600V and 1200V SiC mosfets. For example, Cree's first 1200V power SiC mosfet, the CMF20120D, has a maximum Rds(on) of 130mO over the whole temperature range and a typical gate charge of less than 100nC. It should not be surprising that 1200V SiC mosfets have appeared: they compete with igbts, which have dramatically higher switching and on-state losses. Rohm is in production with the first 1200V 'Full SiC' power module, which combines a half bridge SiC mosfet with parallel SiC Schottky diodes for a rated current of 120A. This module, half the volume of a conventional silicon module, can operate safely in temperatures up to 200°C. Implementing GaN GaN, meanwhile, has some favourable attributes when compared to silicon, but the main problem is the economics of production: pure GaN is only available on 2in wafers. In addition, the production processes for high quality epi-ready GaN substrates are less mature than those for growing SiC substrates. While an SiC epitaxy is grown on a SiC substrate, the trend is to grow a GaN epitaxy on silicon. Because of differences between the two crystal lattices, a buffer layer has to be added between them (see fig 3). This complex process results in a higher cost per die for GaN than for a silicon equivalent. Nevertheless, GaN-on-Si devices appear to have found a sweet spot: dc/dc power conversion at less than 100V. Because a GaN dc/dc converter can operate at several MegaHertz, it enables higher power density since it requires smaller associated components. Another advantage is that GaN fets do not exhibit body diode characteristics, which means there is no reverse recovery loss. This increases conversion efficiency, which in turn reduces the amount of waste heat generated. With a smaller requirement for heat sinking, system size can be reduced. International Rectifier (IR) was the first company to offer GaN-on-Si power switching devices, introducing a complete output stage (high and low side drivers and GaN fets) for use in point of load dc/dc converters. The iP2010/11 features an input voltage range of 7V to 13.2V and an output voltage range of 0.6V to 5.5V. Output current is up to 30A with frequency up to 5MHz. A discrete GaN transistor is also available from Efficient Power Conversion. The EPC2010FET is a 200V GaN FET with on resistance of 25mO at 5V and a pulsed current rating of 60A. While most GaN products available today are suited to low voltage dc/dc conversion, several manufacturers have announced products that support drain to source voltages as high as 600V. One of these, IR, will offer a discrete switching device that pairs a low voltage silicon fet in series with a 600V normally on GaN HEMT (high electron mobility transistor, see fig 4) in a cascade configuration. This means the device can be driven as if it were a classic normally off mosfet, since it allows the use of standard gate drivers that require no special consideration of voltage limitation, over voltage limits or reliability.
Overcoming the last barriers For a wide bandgap material to gain widespread adoption, a sustainable cost of $3/cm² for the substrate and epitaxy layers must be achieved, with wafers of at least 150mm in diameter. To hit this target, a manufacturer must be in high volume production using a high yield fabrication process which is scalable and flexible. This explains why IR has bet on a heterogeneous GaN on Si epitaxy technology: processing can be performed on a standard silicon manufacturing line with little modification to either equipment or process. So, while SiC devices have been on the market longer than GaN parts and currently enjoy a higher level of sales, GaN seems to be better equipped to meet the economic conditions necessary for commercialisation of a new wide bandgap material. Antoine Descazot is a power and analogue field application engineer with Future Electronics (France).