To detect and track fast-moving threats, military radars need to transmit radio signals at high frequencies. The workhorse material that enables the amplification of those signals may yield its crown to a new semiconductor that promises leaps in power capability and performance.
Researchers have been developing gallium nitride semiconductors for nearly two decades. When grown on silicon carbide substrates, the gallium nitride transistors can operate at higher power than the gallium arsenide circuits currently found in a number of U.S. weapons, including phased-array radars aboard platforms ranging from fighter jets to ground-based missile defense systems.
Gallium arsenide chips today are embedded inside the radars’ transmit and receive modules. Several semiconductor chips per module help to boost RF signals that are radiated from the antenna, bounced off targets and terrain and then received by the system and filtered through low-noise amplifiers for processing.
“Gallium nitride gives us that flexibility of not only dramatically improving the performance of power output, but also simultaneously allowing us to shrink the sizes of the components, which allows us to reduce the cost of the system,” said Colin Whelan, technical director of advanced technology at Raytheon Co.’s Integrated Defense Systems.
Raytheon in 2000 began investing in the technology, which had gained material quality improvements in part from the use of gallium nitride in green, blue, violet and white light-emitting diodes, or LEDs. Scientists fabricated gallium nitride semiconductor wafers in increasingly larger sizes, from 2-inch diameter silicon carbide substrates to 4-inch wafers to match the existing gallium arsenide material standard. Raytheon in 2009 established a process for fabricating the 4-inch gallium nitride wafers at its Andover, Mass., foundry, where it has been producing gallium arsenide chips for about 20 years. Engineers there are now building gallium nitride wafers and incorporating them into modules for defense applications.
“We have all the process design tools and the statistical models in place to build these circuits and predict performance even before we build,” said Whelan.
Gallium nitride is a semiconductor that has a “wide band gap,” meaning that it can operate at high voltage to generate power. It produces five times the power of gallium arsenide. If it replaced gallium arsenide devices in existing radars, the chip would give troops the ability to search the horizon for threats more quickly and to track objects about 50 times farther away. If the size of the radar were a more pressing issue than its performance, using gallium nitride chips would shrink the system by 50 percent while maintaining the performance characteristics.
“That offers tremendous cost savings and also allows us to fit radars into spaces where we previously couldn’t,” Whelan said. If the technology were used in an electronic warfare context, the high-power amplifier could cover a wide frequency range, he added.
Because the gallium nitride chips have high thermal conductivity, they efficiently convert the direct-current power into radio frequency power. Residual heat generated from amplifying the RF signals can be removed with ease.
“Ultimately you need to physically remove the heat either by air cooling or using a coolant flow through a metal base, but the gallium nitride material itself, and the substrate it sits on, spreads the heat,” making the cooling process more efficient, Whelan explained.
Scientists are working on versions of gallium nitride that will operate at higher frequencies.
“We have gallium nitride in full-scale production to support really everything through 30 Gigahertz. Then we have a higher performance version that will reach into even higher frequencies, and that’s marching down to high levels of maturity, too,” said Whelan.
Extending the gallium nitride chips into millimeter-wave frequencies so far has achieved successful demonstrations in monolithic microwave integrated circuits operating at 35 Gigahertz and at 95 Gigahertz. Researchers are readying the fabrication process for mass production.
In the meantime, the company is focused on demonstrating the necessary reliability of the microwave gallium nitride technology in radar systems. Engineers have shown the semiconductor’s ability to achieve a 30-year lifetime of 1 million hours operating at 28 volts and 150 degrees Celsius. They also have demonstrated that the technology can withstand high temperatures of 300 to 400 degrees Celsius.
The use of gallium nitride in the commercial world is gaining ground. An ability to emit and absorb light at very short wavelengths has popularized gallium nitride’s use as the blue lasers found in Blu-ray players. The lasers can densely pack information onto discs.
The telecommunications industry, too, is eyeing gallium nitride semiconductors as a potential candidate for use in cell phone towers that receive and transmit signals from portable electronic devices.