The New Power Brokers: RFMD on GaN
Dave Aichele from RFMD 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?
RFMD: GaN based amplifiers provide the following significant benefits
1) Higher efficiency for pulsed, CW saturated and linear applications which improve thermal requirements and energy usage. GaN is showing 5 to 10% improvement vs LDMOS and Si bipolar.
2) Ability to provide efficient power over octave and decade bandwidths. GaN based amplifiers are available to cover 500MHz up to 3GHz of bandwidth. These devices provide higher impedance simplifying integration and lower capacitance enabling broadband match vs LDMOS and Si bipolar.
3) Higher power density which enable smaller form factor and/or greater power in similar footprint as LDMOS or Si bipolar.
MWJ: What applications are most applicable to these benefits?
1) Software Defined Radio for Military Communications & Public
2) Civilian & Military Pulsed Radar (Ground, Shipborn and Airborn)
3) Cellular Base Stations for WCDMA, LTE & WiMAX
4) IED Jamming/Electronic Warfare
5) Emerging markets – RF Lighting, RF Thermal
MWJ: What are the most substantial system-related issues to consider when working with GaN/LDMOS/HVFET?
RFMD: Overall system performance and cost needs to be considered. What is the efficiency of the RF chain? Are there benefits in bandwidth and design resources? Can the system be simplified due to the technology of use (easier DPD algorithms, power setting, etc).
System design may also be simplified by utilizing the technology which offers the greatest bandwidth at acceptable power and efficiency points - thereby allowing a reduction in the number of transmit chains required to cover a family of operational bands. Such a reduction not only reduces BOM costs, but also the physical dimensions of the transmitter and the RF losses incurred in switching networks.
MWJ: What is the potential of GaN/LDMOS/HVFET technology in mobile radio communication systems?
RFMD: GaN based amplifiers have high potential for implementation in applications such as cellular base station and public mobile radios. These devices offer high video bandwidth and offer excellent performance when utilized in linear architectures such as DPD, Doherty, Envelope Tracking. Well suited for Multi-band GSM, W-CDMA, WiMAX and LTE applications.
PMR & Military communications are developing SDR platforms that will enable interoperability between multiple users. Both portable and mobile systems need high efficiency power amplifiers to cover 1GHz to 2GHz of BW and GaN-based amplifiers offer the best performance for these applications.
MWJ: How well does GaN/LDMOS/HVFET address military and government system requirements and application in military systems, specifically radar and electronic warfare?
RFMD: GaN based amplifiers are very well suited for military and government systems. There is strong interest from the radar community on implementing GaN to displace LDMOS and eventually TWT for L-band and S-band radars based on efficiency/bw/power density (displacing LDMOS) and reliability with similar power capabilities (displacing TWT)
The Electronic warfare or IED jamming is one of the early adopters of GaN-based amplifiers based on bandwidth capabilities with efficient power.
MWJ: What is the status and trends of non-linear modeling of GaN/LDMOS/HVFET rf devices?
RFMD: High Power GaN benefits from the work pioneered by LDMOS suppliers. The published material showing EM modeling of packages, wired, and feed structures, fundamental device modeling, and issues related to scaling are all directly applicable. Modifications of the device models are then made based on the physical device characteristics and the same measured vs. modeled metrics are easily applied.
MWJ: What type of circuit architectures are best suited for GaN/LDMOS/HVFET, such as Doherty, push-pull, cascode, distributed, etc.
RFMD: The high terminal impedances of high voltage GaN devices make the amplifiers suitable for higher bandwidth class-AB amplifiers as well as applications that attempt to improve efficiency such as Doherty, linearity such as push-pull, and extreme bandwidth such as distributed techniques.
Thanks to the high impedances (and low parasitics) of GaN HEMT’s, these higher bandwidth amplifiers can utilize most any standard circuit architecture with enhanced simultaneous bandwidth/efficiency/power performance.
MWJ: Any specific recommendations for Class of operation? Any guide based on application?
RFMD: High Power GaN can easily be applied for any Class of operation. There aren’t any additional issues to consider compared to other technologies
MWJ: What is the status on the use of GaN/LDMOS/HVFET devices in MMICs
RFMD: GaN MMIC technologies are currently available with passive components very similar in performance to existing GaAs bases processes. Some components, such as MIM capacitors are undergoing design enhancements to allow their use in circuits nodes operating at the higher voltages and RF swings supported by GaN devices.
MWJ: What’s the relationship between GaN/LDMOS/HVFET Frequency Figure of Merit and Temperature? What is the state of efficient heat removal?
RFMD: GaN is capable of operation at extraordinarily high power density. As such, it is imperative that amplifier designs make use of the high efficiency possible with GaN devices and that heat be removed in an efficient manner. GaN on SiC represents the best production-ready heat removal option, as SiC thermal conductivities rival that of Cu the substrate acts as an efficient heat spreader. Other existing GaN systems such as GaN on Si suffer from much worse thermal performance. Exotic experimental substrates, such as GaN on diamond, suggest possible improvements for the future.
Breakdown voltages, carrier mobilities and saturation velocities degrade at high temperature for both wideband gap technologies and Si based technologies. However the degradation of these properties with increasing temperature is much less severe in GaN based RF power devices compared to Si LDMOS or HVFET.
MWJ: What are the commercial and military markets concern about GaN/LDMOS/HVFET and reliability? Are there issues or history of issues?
RFMD: Biggest concern for conservative markets is lack of field data and minimal reliability data based on life history of the technology. GaN based manufacturers are addressing this by continuing to perform accelerated temperature DC and RF testing and
MWJ: What separates your company’s GaN/LDMOS/HVFET devices from others producing similar technology?
RFMD: RFMD advantages our based on best in class high power density GaN process manufactured on thermally efficiency SiC. In addition, our ability to offer fully engineered solutions and referenced sub-system expertise enabling customers to maximize end system performance with increased engineering efficiency.
MWJ: What are the pros and cons behind GaN silicon carbide versus silicon substrates?
- greater than 3x better thermal conductivity of SiC vs Si (4.9W/cmK vs 1.5 W/cmK)
- epitaxial growth of GaN is better on SiC due to smaller lattice mismatch (3.4% on SiC vs 17% on Si)
- lower TEC mismatch on SiC (+25% for SiC vs +100% on Si)
- See #10: Thermals
- cost is higher for SiC, although this factor is expected to decrease with increasing volume of SiC substrates manufactured over time)
- this factor should be considered within the context of the overall cost of the product/technology
- diameters available (SiC is commercially available in 3” and 4” vs 12” for Si)
MWJ: Could you discuss up front costs, added expenses to implement, maintain, etc. compared to other power transistor technologies?
RFMD: RFMD has an advantage in this respect – there is minimal significant upfront manufacturing cost to implement and maintain the GaN technology. RFMD GaN wafers are manufactured in the same plant and similar equipment as our high volume GaAs wafers.