The New Power Brokers: Nitronex on GaN
Ray Campton from Nitronex talks to Microwave Journal about their GaN technology, the markets and the competition.
MWJ: What are the most significant benefits of GaN/LDMOS/HVFET? How does this compare to other high power devices?
Nitronex: The most significant advantages of GaN over other power technologies are is its power density and high frequency response. Having several times higher power density results in lower parasitics, lower loss and less complicated on-chip combining structures. The benefit is a device that is easier to match, leading to higher efficiency and higher power in broadband applications. The native frequency response of GaN HEMTs allows designers to realize these power density benefits to frequencies above even that of GaAs.
MWJ: What applications are most applicable to these benefits?
Nitronex: GaN has initially emerged as a preferred technology in broadband power amplifiers and amplifiers that have stringent robustness requirements.
MWJ: What are the most substantial system-related issues to consider when working with GaN/LDMOS/HVFET?
Nitronex: GaN allows more power to be delivered from a single packaged device than other technologies so the system may need fewer combined devices to achieve power targets, shrinking overall size. Its higher efficiency lowers total system power dissipation and therefore cooling requirements are reduced. GaN HEMTs are robust devices so isolators, diodes and other protection components may be eliminated, further reducing design complexity and cost.
MWJ: What is the potential of GaN/LDMOS/HVFET technology in mobile radio communication systems?
Nitronex: GaN helps with size weight, and power (SWAP) goals of communications systems. GaN can shrink overall solution size and weight by reducing matching circuit size and package area, eliminating protection circuitry and reducing cooling requirements. GaN-based mobile radios have higher efficiency power amplifiers and consume less power, resulting in longer life on a battery charge. These benefits have driven many mobile communications system designers to switch to GaN over the last 2 years.
MWJ: How well does GaN/LDMOS/HVFET address military and government system requirements and application in military systems, specifically radar and electronic warfare?
Nitronex: Electronic warfare amplifiers require highly robust devices that can deliver industry-leading efficiency across broad bandwidths as well as high frequencies. GaN has significant efficiency and robustness requirements over Si LDMOS and GaAs resulting in GaN being the preferred technology in modern electronic warfare systems.
Radar applications require pulsed power in the hundreds of watts up to very high frequencies. This fits well because GaN devices offer improved robustness, higher efficiency, higher power density, higher gain, and faster switching speed than competing technologies for today’s broadband radar systems. GaN also offers the ability to replace TWTs with more reliable and robust devices.
MWJ: What is the status and trends of non-linear modeling of GaN/LDMOS/HVFET rf devices?
Nitronex: Nonlinear models for GaN devices are relatively immature compared to LDMOS, GaAs, and other established technologies. Models have been improving and will continue to improve over the next several years. Experienced power amplifier designers typically rely on both models and measured data such as load-pull contours and s-parameters when designing with any technology but they lean more heavily on empirical data when designing with GaN.
MWJ: What type of circuit architectures are best suited for GaN/LDMOS/HVFET, such as Doherty, push-pull, cascode, distributed, etc.
Nitronex: Like other power technologies GaN is being used in all common circuit architectures including single-ended, push-pull, quadrature combined, distributed, cascade, Doherty, switch-mode, and others. Specific GaN devices and products may be optimized for a particular architecture but there is not an inherent bias at the core technology level.
MWJ: Any specific recommendations for Class of operation? Any guide based on application?
Nitronex: GaN devices have been used successfully in all classes of operation – class A, AB, B, C, D, E, and F. One often overlooked technique is to scale the drain voltage to improve efficiency. For example if a device data sheet shows 50W at 28V in class AB operation, a linear amplifier designer may get more optimal results at 20V class A to achieve a target of 20W. Operating class A improves linearity while reducing drain voltage improves power dissipation.
MWJ: What is the status on the use of GaN/LDMOS/HVFET devices in MMICs?
Nitronex: GaN MMICs are in production today. The biggest challenges in developing a GaN MMIC process is making reliable high voltage capacitors and achieving acceptable yields. Nitronex has developed a complete GaN MMIC process with high voltage capacitors using a high resistivity Si substrate. This substrate is defect-free, has high thermal conductivity, and good frequency response above 20GHz. This allows us to develop large, high power MMICs without the yield problems of exotic substrates such as SiC.
Most GaN MMICs to date have been custom developments for specific customers. In upcoming years the industry will see a more broad catalog offering that will make this technology widely available.
MWJ: What’s the relationship between GaN/LDMOS/HVFET Frequency Figure of Merit and Temperature? What is the state of efficient heat removal?
Nitronex: Nitronex GaN device temperature characteristics are similar to other technologies. We see roughly 1dB of peak saturated power and 10 points of peak efficiency slope at the device level over 100°C junction temperature change.
Power removal is always a difficult task in power devices and GaN device designers are continually improving thermal design. The higher efficiency of GaN over other technologies results in less total power that needs to be removed from a system. The higher power density requires system designers to pay closer attention to thermal management in the proximity of the power devices themselves.
MWJ: What are the commercial and military markets concern about GaN/LDMOS/HVFET and reliability? Are there issues or history of issues?
Nitronex: Nitronex has publicly published its reliability data and qualification reports for review by the industry. We see relatively little concern from customers about the reliability of our devices. The GaN industry in general has not offered qualification and reliability results to their customer base, causing an unnecessary level of general concern in the design community.
MWJ: What separates your company’s GaN/LDMOS/HVFET devices from others producing similar technology?
Nitronex: We routinely find that our products offer the best tradeoff between linearity, output power, and efficiency in broadband applications compared to other GaN vendors. We also find that being on a silicon substrate helps us more readily prove that our quality and reliability meets customer requirements. Finally, Nitronex is 100% focused on RF products. This builds confidence by our customers that partnering with us is likely to be beneficial from both short and long term perspectives.
MWJ: What are the pros and cons behind GaN silicon carbide versus silicon substrates?
Nitronex: Nitronex has fundamental patents covering growth of GaN on Si substrates so we are the only company currently using this approach. We start with a very low cost, defect-free industry standard silicon substrate. This gives us significant advantages over expensive and exotic substrates such as silicon carbide. We can make very large area die with high yield, so complicated MMICs with large combining structures can be produced in volume. We use a high resistivity substrate so we can design well over 20GHz, and thermal conductivity is as good as other Si technologies such as LDMOS and much better than GaAs.
Just as important are the supply chain advantages of silicon. Aside from epitaxial growth, the entire supply chain from wafer procurement, wafer processing, die attach and packaging can be second sourced by several existing well respected companies to avoid single thread manufacturing steps as well as the ability to scale production to support high volume requirements. In addition silicon wafers are widely available in high quality from many sources. As volumes continue to increase this will continue become a strong differentiator for silicon-based devices.
MWJ: Could you discuss up front costs, added expenses to implement, maintain, etc. compared to other power transistor technologies?
Nitronex: The fastest approach to a new design is to reuse as much as possible from an existing, proven design. From an electrical design perspective any new power transistor requires additional work and a new technology such as GaN is no different. If doing a design essentially from the ground up this is much less of an issue. Mechanical designs can also be very expensive and time consuming. In some cases GaN’s higher efficiency has been used to allow a power upgrade to be made using an existing mechanical design.
Production costs may be reduced compared to other technologies. GaN devices can result in fewer power transistors and reduced components required for protection. Higher device impedances and less parasitic capacitance variations compared to an incumbent device may result in less tuning on the production line.
Sustaining costs may be reduced due to GaN’s robustness to overdrive, output mismatch, and other environmental conditions.
Overall the cost versus performance benefit of GaN has to be weighed on a case by case basis.