2009 RF Power Device Status
MWJ: What are the most significant benefits of GaN/LDMOS/HVFET? How does this compare to other high power devices?
Cree: GaN-on-SiC HEMTs have high power densities – at 28 volts ~ 4watts/mm of gate periphery; at 48 volts over 8 watts/mm. GaN HEMTs have high breakdown voltages typically 100 to 200 volts. The transistors have low capacitance per watt of RF power.
The transistors can have high ft’s depending on gate length anywhere from 20 GHz to >150 GHz
MWJ: What applications are most applicable to these benefits?
• Efficient power amplifiers such as waveform engineered amplifiers because of high ft – important for both military and commercial applications
• Very broadband amplifiers at power levels in excess of 100 watts because of low capacitance
• Small footprint amplifier applications
MWJ: What are the most substantial system-related issues to consider when working with GaN/LDMOS/HVFET?
Cree: High RF power density is also related to high heat density unless very high DC to RF conversion efficiencies are maintained. Thermal management is important.
MWJ: What is the potential of GaN/LDMOS/HVFET technology in mobile radio communication systems?
Cree: Because of high efficiencies and high operating voltages, GaN-on-SiC HEMTs are already finding applications in mobile telecommunications such as software defined radios, cognitive systems, manpack and mobile mounted multi-standard radios.
MWJ: How well does GaN/LDMOS/HVFET address military and government system requirements and application in military systems, specifically radar and electronic warfare?
Cree: GaN-on-SiC HEMTs have found their first major system insertions in wide band EW applications. Properly designed power amplifiers can be very rugged and withstand field abuse because of high breakdown voltages and robust reliability. The higher operating voltage and efficiency make GaN very attractive for radar system applications.
MWJ: What is the status and trends of non-linear modeling of GaN/LDMOS/HVFET rf devices?
Cree: Cree prides itself on its accurate non-linear transistor models which were primarily developed for MMIC applications but now have extensive application in all Cree’s discrete transistor products. Models are available for Agilent’s ADS and AWR’s Microwave Office. We have many examples of first time design pass success.
MWJ: What type of circuit architectures are best suited for GaN/LDMOS/HVFET, such as Doherty, push-pull, cascode, distributed, etc.
Cree: For hybrid, discrete designs GaN HEMTs are most suited to Class A/B, Doherty, push-pull, feedback and lossy match etc. We have also seen examples of distributed (traveling-wave) amplifiers being designed up to a few 10’s of watts in chip and wire approaches. There has also been much success designing very high efficiency Class E, F, inverse F, J, etc PA’s. For MMICs, a range of amplifier techniques ranging from distributed, through multi-stage and cascode implementations have been proven and many are now in production or pre-production.
MWJ: Any specific recommendations for Class of operation? Any guide based on application?
Cree: Doherty (Class A/B/C) with DPD for a range of telecommunications applications. We today achieve efficiencies > 50% at peak power levels as high as 500 watts. PA’s associated with Envelope Tracking (ET) where the high breakdown aspects of GaN allow drain voltage operation anywhere from 20 to 65 volts again with efficiencies exceeding 50%. ET is particularly promising for multi-band telecom applications. Class E, F and inverse F for very high efficiency needs
MWJ: What is the status on the use of GaN/LDMOS/HVFET devices in MMICs
Cree: Cree has worked on GaN-on-SiC MMICs for many years over a range of frequencies anywhere from the 10’s of MHz to millimeter-wave depending on gate length. MMICs are not constrained to power amplifiers but also cover all the usual circuit needs such as robust LNA’s, attenuators, switches, phase shifters etc. Cree was the first wide bandgap semiconductor company to make COTS PA MMICs available in late 2007. We also offer GaN-on-SiC MMIC Foundry services.
MWJ: What’s the relationship between GaN/LDMOS/HVFET Frequency Figure of Merit and Temperature? What is the state of efficient heat removal?
Cree: Today’s commercially available GaN-on-SiC HEMTs usually use either copper-moly-copper or copper-tungsten flanged packages. In some special cases more elaborate heat-sinking is required using advanced materials, such as those containing diamond, but they are less mature and tend to be expensive. In some critical applications where liquid cooling may not be available, that option may be acceptable. The transistors can also be “spread out” to reduce heat density, but this can have an impact on the frequency of operation of the devices as well as the number of parts that can be produced on each wafer.
MWJ: What are the commercial and military markets concern about GaN/LDMOS/HVFET and reliability? Are there issues or history of issues?
Cree: As the number of fielded systems containing GaN-on-SiC continues to increase without issue, both the military and commercial markets are accepting the use of the technology more widely.
MWJ: What separates your company’s GaN/LDMOS/HVFET devices from others producing similar technology?
Cree: Based on customer feedback, Cree offers a better combination of power density, efficiency, and high frequency performance using our GaN-on-SiC technology. Our highly accurate non-linear models appear to be the best in the industry. Cree is also the only company to offer a line of catalog GaN-on-SiC MMIC products.
Cree has many years experience in fabricating its own substrates – we are totally a vertically integrated company. In the wide bandgap arena, we have the largest portfolio of available products.
MWJ: What are the pros and cons behind GaN silicon carbide versus silicon substrates?
• GaN on SiC can operate at much higher junction temperatures which reduces system complexity and weight
• GaN on SiC offers higher ft devices because of lower parasitic capacitances
• Easier to process than GaN on Si since there is no need to make the substrate ultra thin to aid in thermal management
MWJ: Could you discuss up front costs, added expenses to implement, maintain, etc. compared to other power transistor technologies?
Cree: GaN-on-SiC technology is successfully being produced and purchased in high volumes across a wide variety of price competitive markets, including optoelectonics (e.g. LEDs). Once the fabrication process is adequately developed, the recurring cost of producing GaN-on-SiC HEMTs is very similar to other III-IV technologies.