Come of Age
The revolutionary power, efficiency and bandwidth performance improvements demonstrated by Raytheon’s gallium nitride (GaN) technology are now being realized in state-of-the-art microwave power amplifiers, enabling the next generation of radar systems. Raytheon’s large development effort leveraged extensive gallium arsenide (GaAs) development experience, strategic partnerships with universities and the government, and long-term investment commitments.
High-power semiconductors play an important role in radar performance. In a phased array radar, the RF energy is distributed to each element, phase shifted and then amplified before being radiated. The final amplification of the RF signal at each element is performed by the power amplifier. Traditionally, GaAs has been the semiconductor of choice for efficiently amplifying this signal, creating the desired output power.
Throughout the 1990s, Raytheon was a pioneer in inserting GaAs-based monolithic microwave integrated circuits (MMICs) into phased array radars. As the performance requirements of these military systems have increased to meet ever-growing threats, so too have the power and efficiency requirements on the power amplifiers. Over that time, GaAs performance was stretched from the unit power density of 0.5 watt per millimeter of transistor periphery to 1.5 W/mm by increasing the drain voltage from 5V to 24V. GaN, however, continued to make dramatic performance improvements, quickly surpassing GaAs capability (see table).
Today, with Raytheon’s development phase nearing completion, the power, efficiency and bandwidth performance of GaN-based MMICs is unsurpassed — revolutionizing the design of radars by creating not only higher performance but also lower system cost. With over 5 W/mm of power density, GaN RF amplifiers can provide more than 5X the power per element of GaAs in the same footprint. Fewer high-power GaN MMICs could be used to replace many low-power GaAs MMICs, or alternatively, equal-power GaN chips can be made dramatically smaller. Both approaches reduce overall system costs while enabling size-constrained systems. The higher drain current that GaN offers makes the broadband matching of high-power MMICs simpler and more efficient than GaAs, while the seven to eight times improvement in the thermal conductivity enables amplifier cooling. Finally, the wide band gap intrinsic to GaN material provides large critical breakdown fields and voltages, making a more robust amplifier and easing system implementation
Development History of GaN
GaN semiconductors were first studied more than 30 years ago, and even then they were considered ideal for high-power microwave devices based on their high theoretical breakdown field and high saturated electron velocity. But at that time, the gallium nitride material quality was insufficient to produce microwave transistors. This all began to change in the early 1990s when researchers used gallium nitride to fabricate the world’s first green, blue, violet and white light-emitting diodes (LEDs). This breakthrough drove forward a rapid improvement in GaN material quality. Now, these LEDs can be found all around us: on traffic lights, scoreboards, billboards and flashlights.
Another obstacle to the development of GaN transistors was the lack of an inexpensive substrate material. Traditionally the substrate material of the transistor is the same material as the transistor itself, but, to date, researchers have been unable to grow large area GaN substrates. Instead, researchers originally turned to growing GaN transistors on sapphire substrates, and in 1996 demonstrated the first microwave GaN power transistors. The sapphire substrates are low cost and widely available. However, their poor thermal conductivity and non-ideal lattice match to GaN limited the performance of the transistors. Researchers then turned to growing the GaN devices on semi-insulating silicon carbide (SiC) substrates. Silicon carbide has a good lattice match to GaN and is an excellent thermal conductor. The only drawback was that silicon carbide substrates were only available in small sizes (50 mm diameter) and were very expensive (100 times the price of GaAs) in the late 1990s. The last 10 years have seen a rapid improvement in the size, quality and cost of the silicon carbide substrates. Today, Raytheon’s production GaN process uses 100 mm (4-inch) diameter SiC substrates.
Raytheon has been researching GaN since 1999, fabricating its first gallium nitride transistors in 2000. MMIC demonstrations quickly followed. While the demonstrated performance of the GaN transistors was excellent, it took a number of years to improve the reliability and yield of the transistors to the present state, where the technology is ready to meet the stringent needs of defense systems. This development was funded through both Raytheon internal research and development investments and external government contracts from Ballistic Missile Defense Office, Office of Naval Research and Defense Advanced Research Projects Agency (DARPA).
As shown in Figure 1, Raytheon’s long-term commitment to the development of GaN technology began nearly 10 years ago, and the company has leveraged its long history of GaAs semiconductor work, as well as partnerships with industry, university and government. Raytheon’s development history with GaAs provided the needed infrastructure and lessons-learned experience to accelerate GaN’s development. This included the growth of starting material, the modeling of transistors’ RF performance, the semiconductor fabrication facility, the microwave and module design, and the RF testing capabilities. Through early strategic partnering with Cree, the University of California Santa Barbara and U.S. government labs, the team shortened the cycles of learning and shared findings to more quickly advance the state of GaN transistors. Focused Raytheon-funded university research at Cornell, Georgia Tech and MIT continues to push the performance envelope of GaN to higher frequencies.
Raytheon’s focus on early reliability demonstrations and transition to 4-inch wafers, to leverage the existing manufacturing facility, has resulted in industry-leading maturity. Raytheon’s 4-inch microwave GaN process was moved into a production environment two years ago and today is completing final production validation. Many hundreds of wafers have been processed, resulting in increased process maturity and lower system insertion costs. The capabilities of this process provide not only the performance benefits of GaN, but also the assurance of supply and the capture of early system insertion opportunities.
The high maturity of Raytheon’s GaN technology, signaled with its transition to production, has provided Raytheon the ability to quickly scale the technology to millimeter-wave frequencies (> 30 GHz). With a nominal voltage of 20V and similar currents levels, millimeter-wave GaN gives the same five times performance improvement over existing high frequency GaAs technology as microwave GaN technology does.
The ability of GaN transistors to operate at very high voltage and current enables them to produce very high output power, high-efficiency amplifiers. To realize these high-performance designs requires accurate modeling of the transistor’s harmonic performance. The maturity of Raytheon’s GaN technology has enabled us to obtain consistent, repeatable performance and the models needed to obtain the high-efficiency amplifiers. We have demonstrated amplifiers with record combinations of power and efficiency amplifiers at L-band, S-band and X-band. The higher voltage and load impedance of GaN also makes it especially well suited for the broadband amplifiers required for future multifunction systems. Raytheon has demonstrated high power broadband amplifiers with bandwidths greater than 4:1.
Raytheon is also leading the way for extending the performance of GaN to millimeter-wave frequencies and higher. Raytheon has recently demonstrated state-of-the-art power performance for MMICs operating at 35 GHz and at 95 GHz. Raytheon’s 95 GHz MMIC work has been funded in part by the Joint Non Lethal Weapons Directorate to produce a solid-state version of the company’s Active Denial System. Raytheon’s Active Denial System is designed to use millimeter wave technology to repel individuals without causing injury
The high power handling capability of GaN transistors also makes it an ideal choice for other types of circuits. For example, Raytheon has demonstrated GaN low-noise amplifiers with record survivability and GaN microwave switches with record power handling.
As customer requirements increase beyond GaAs capabilities, there has been a strong pull to mature GaN for system insertion. In collaboration with the government, universities and small businesses, Raytheon has matured GaN from 2-inch wafers with transistors measured with hours of lifetime, to 4-inch wafers and transistors with the millions of hours of reliability today needed to transition into U.S. Department of Defense system. Ultimately, GaN will become the power amplification standard for all new radars, communication and weapon systems, where cost-effective, high RF power is needed.
Colin Whelan, Nick Kolias