While silicon carbide (SiC) was once well known as the technology used to implement the first blue light-emitting diodes (LEDs), and is now used as a substrate for high-power LEDs, the technology is also enabling performance advances in discrete power components.
For example, SiC versions of the diode, the simplest semiconductor device, have been available for several years. Another relatively mature device, the SiC enhancement JFET, functions as a power switch. The characteristics of this device can reduce the switching losses generated by conventional MOSFETs in switching power supplies. There is also the SiC MESFET, a fairly exotic component that has found a niche in radio-frequency transmitters. All of these applications illustrate how SiC technology is providing viable alternatives to well-established silicon devices.
In keeping with this trend, Cree Semiconductor has recently announced the availability of a power-device combination pack (co-pack), which combines IGBT devices from International Rectifier with SiC Schottky diodes from Cree. In this module, the SiC device serves as the freewheeling diode and is connected in reverse parallel across the emitter and collector contacts of the IGBT.
The first part released in this product family is the CID150660 (Fig. 1). This is a 15-A IGBT co-packaged with a 6-A SiC diode in a TO-220 case. The SiC device replaces a 15-A silicon rectifier. Michael O'Neill, an applications engineer with Cree, explained that the SiC device is rated for 6 A at 150°C case temperature, but at a case temperature of 100°C (which is where the silicon device is rated), it is capable of supporting a steady-state current of 13 A.
Because the SiC diode is a unipolar device, reverse-recovery currents associated with minority carrier recombination in conventional silicon devices are eliminated; the only recovery mechanism is the charge associated with the junction capacitance of the device. According to O'Neill, for the SiC devices in the co-pack this is roughly 1 A sustained for 20 ns, or approximately 1% of the switching loss of a conventional silicon diode. It translates into the elimination of diode switching losses and a significant reduction in IGBT turn-on loss when used in a half-bridge configuration.
Cree's co-pack is rated for 600 V. This is the first device in a family that will support this same voltage for a variety of current ratings. However, Cree's main focus is with 300-V, 600-V and 1200-V SiC Schottky diodes, the main application for these devices being PFC converters. Cree has field reliability data showing more than 31 billion device hours of reliable operation for these components. For more information, see www.cree.com.
Infineon, another major developer of SiC technology, is now in production with its second-generation SiC diodes, the ThinQ!2G series. This technology employs a structure referred to as the merged junction. This is a configuration where small p-n junctions are interleaved within the rectifying Schottky metal-SiC interface. The p-n structures contribute to the surge robustness of the device as well as the reduction of reverse-bias leakage currents. Beyond a certain forward voltage, the operation of the device transitions from Schottky mode to bipolar mode, which provides surge-current capabilities comparable to silicon diodes. For Infineon's ThinQ!2G 4-A SiC device, this threshold is 4 V, beyond which the current slope is approximately linear and temperature independent. The application of an 8-V forward voltage across this part produces a current on the order of 45 A.
All second-generation SiC diodes from Infineon are rated for 600 V. The device family includes the IDT04S60C, IDT05S60C, IDT06S60C, IDT08S60C, IDT10S60C, IDT12S60C and IDT16S60C, all packaged in the TO-220 case style (Fig. 2), and the IDD04S60C, which uses the DPAK package.
Infineon's philosophy in the use of SiC devices in power design is to offset higher component costs with system-level gains enabled by SiC technology. In a PFC stage, for example, the elimination of reverse-recovery current in a SiC boost diode and the resulting increase in switching speed enables the use of a smaller switching device and inductor, respectively. For more information, see www.infineon.com.
SiC can contribute indirectly to power savings in switching devices by eliminating reverse-recovery current. It also can contribute directly in the form of SiC JFETs. Infineon has chosen to develop the SiC JFET because it promises device ruggedness and high performance at a reasonable cost.
Another company, SemiSouth, has fully developed SiC JFETs as well as circuit designs for driving the gate of these devices. SemiSouth's focus is on devices with vertical channels and corresponding very high-channel packing density (Fig. 3). This feature produces very low, specific on-resistance, which is one factor in the race to reduce cost. One consideration for these SiC JFETs is that an enhancement-mode option is available, which means a slight forward bias must be applied to the gate of the device. While this may be a radical departure from conventional wisdom with respect to the JFET, the voltage swing is lower than it would be for a MOSFET (6 V wide versus 15 V for a MOSFET). For more information, visit www.semisouth.com.
The future of SiC technology is promising. Especially bright is the potential of SiC FETs once the various technical challenges to developing a cost-effective device have been overcome. However, another challenge of equal significance will be changing established power-design practices to accommodate the electrical and thermal advantages of SiC devices as they continue to evolve.