SiC Transistors Fire UHF Radars

Solid-state devices still have ground to gain in terms of output power at high frequencies compared to vacuum devices, such as traveling-wave tubes. But the new model 0405SC-2200M siliconcarbide (SiC) transistor from Microsemi Corp. represents progress for the cause of solid-state power, with 2.2 kW pulsed output power for ultrahigh-frequency (UHF) signals from 406 to 450 MHz.

Devices formed of SiC epitaxial materials have long held promise for high-power, high-temperature applications. In avionics and airborne radar systems, for example, the high-temperature capabilities of SiCbased semiconductors can reduce packaging requirements and the need for additional cooling devices. The high-temperature capabilities can also improve the performance of sensor-based systems, where system sensitivity can be improved by optimum sensor placement, even when close to heat sources. Packaged SiC transistors have been constructed for operating temperatures as high as +350C, with considerably higher temperatures achieved for unpackaged die.

SiC is a wide-bandgap epitaxial semiconductor material, as defi ned by the amount of energy needed to make the material carry current (SiC has an energy bandgap of about 3.0 to 3.2 eV). The material offers a high maximum electron velocity, which supports its use in RF and microwave device applications. Sic enables the fabrication of high-breakdown-voltage devices, for use at the higher operating voltages needed to produce high RF outputpower levels.

Compared to other semiconductor materials, including silicon (Si) and gallium arsenide (GaAs), it is relatively immune to external factors, including radiation and electromagnetic-pulse (EMP) effects. This characteristic makes SiC attractive for use in radiationhardened electronics, including power-supply switching transistors and power-amplifi er transistors. SiC, which emits blue light and can be used to form light-emitting diodes (LEDs), can also operate at higher power densities than similar-sized silicon and GaAs transistors.

Improvements in producing defect-free SiC wafer materials have supported higher yields of devices, and have made the fabrication of SiC LEDs commercially viable and (larger) high-power RF transistors feasible. A number of raw materials suppliers now offer SiC epitaxial wafers in various diameters for use in LEDs, switching transistors, and RF transistors, including Dow Corning, Semiconductor Wafer, Inc., University Wafe, and SiCrystal AG.

Microsemi has been a long-time proponent of SiC carbide technology, and has made tremendous strides in achieving vacuum-tube-like power levels from SiC transistors. For example, earlier this year, the company's RF Integrated Solutions group introduced its model 0405SC- 1500M SiC power transistor, capable of 1500 W pulsed output power with ultrahigh-frequency (UHF) signals from 406 to 450 MHz. The packaged transistor is about one-half the size of Si bipolar transistors with the same output-power rating, and features a simple single-ended design for ease of combining multiple devices for higher output powers.

The common-gate, Class AB device is designed for pulsed UHF weather radars and over-the-horizon radar applications. It is constructed with 100-percent gold metallization and gold bond wires in a hermetic package for high reliability. The rugged depletionmode static-induction transistor (SIT) is built to last, and can operate into a termination equivalent to a 5.0:1 VSWR without damage. The device, which is geared for medium-duration pulses of about 300 s at a 6-percent duty cycle achieves 8-dB typical power gain across its bandwidth. Its drain effi ciency is 45 percent at 450 MHz. The transistor makes full use of SiC's high-voltage capabilities by running at a supply voltage of +125 VDC.

Building on the performance achieved with this earlier SiC device, the company most recently announced its model 0405SC-2200M SiC device, capable of 2200 W pulsed output power, also for UHF radar systems operating from 406 to 450 MHz. It is also supplied in a hermetic, singleended package for common-gate Class AB amplifi er designs. It uses high-temperature gold metallization and gold bond wires for high reliability. Like the company's 1500-W UHF SiC transistor, the 0405SC-2200M is designed to provide its rated output power at 1-dB compression and with better than 8 dB typical power gain. It offers somewhat improved drain effi ciency than the 1500-W device, at about 55 percent at 450 MHz (minimum of 50 percent), and is also designed for a supply voltage of +125 VDC. Both transistors exhibit thermal resistance of 0.15 W/C; demo units, data sheets, and S-parameters are available for both models from the company.

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