New GaN FETs, Amplifiers and Switches Offer System Engineers a Way to Reduce RF Board Space and System Prime Power

Transcription

1 New GaN FETs, Amplifiers and Switches Offer System Engineers a Way to Reduce RF Board Space and System Prime Power By: TriQuint Semiconductor, Inc. Dean White, Defense Products & Foundry Services, Business Development Grant Wilcox, Defense Products & Foundry Services, Program Manager, Product Solutions Abstract Ever-increasing demands for faster product launch times, higher performance, lower part counts and reduced costs create RF system design challenges. This paper focuses on how the latest gallium nitride (GaN) products, specifically discrete transistors, amplifiers and switches, offer design engineers enhanced flexibility and performance options by reducing RF board space and system prime power requirements. Introduction For applications such as defense and commercial radar systems, communications and electronic warfare (EW), existing technologies such as GaAs, LDMOS and TWT simply cannot keep pace with the evolving demands for size, reliability, linearity, power density and energy efficiency. Today's GaN-based discrete transistors, amplifiers and switches are rising to the challenge, providing RF system engineers with the flexibility to achieve significantly higher power and efficiency, with lower part count, board space and resultant cost. General Advantages of GaN SWaP For today's RF engineers, reducing system size, weight and power, collectively referred to as SWaP, is critical in addressing evolving operational needs and cost targets. Extending operational life, reducing form factor, reducing prime power, switching faster and decreasing weight must all be achieved at competitive costs. GaN technology is suited to meet these evolving SWaP demands better than older technologies like GaAs, LDMOS and TWT (traveling-wave tubes) given the following technology benefits: Higher power densities leading to reduced combining losses for a given power target Increased efficiency over frequency Ability to maintain high performance over wide bandwidths Higher thermal conductivity / lower thermal resistance (GaN on SiC) Specific Advantages of GaN Over GaAs and TWT To appreciate the benefits of GaN-based FETs, amplifiers and switches, and how they reduce RF board space and system prime power, we can compare it to earlier technologies like GaAs and TWTs. GaN vs. GaAs. Generally speaking, and with respect to the specific products referenced later in this paper, GaN technology is capable of achieving up to five-times the power and in many cases a 10-30% increase in PAE over the same bandwidth as GaAs. Given the much higher power density, one can achieve a given power target with much less combining. In the example shown in Figure 1, it would take combining four 6.5W Ku-band amplifiers to achieve the same Page 1 of 6

2 20W power target that a single GaN amplifier can achieve. The reduced combining leads to reduced board space and lower system losses, which in turn leads to higher efficiencies and reduced prime power requirements. This advantage over GaAs and other technologies expands as you go higher in frequency and increased bandwidth. Figure 1 Although power and efficiency advantages offer tremendous flexibility in system design, thermal management of increased power dissipations have to be considered. Inherently, GaN on SiC bears some of that responsibility. GaN has better thermal properties than competing GaAs technologies. Thermal conductivity for SiC is roughly 4x that of GaAs which allows heat to move more quickly through the substrate and away from the active area. An added benefit is that GaN can support the million hour MTTF reliability benchmark at a junction temperature of 200 C or higher versus 150 C for GaAs. These thermal advantages do not solve the thermal problem at the system level; however, they bring the thermal management concern down to a reasonable design trade-off for the system engineer. GaN vs. TWTs. While TWTs are quite efficient, GaN's primary advantages over TWTs are increased reliability, reduced weight, a smaller footprint and lower long-term costs. Standard GaN Products Available from TriQuint With the maturing of the GaN process, TriQuint's product portfolio is also maturing. A sampling of the available GaN portfolio is shown in Table 1, below. For a more complete offering, please reference TriQuint s website at Page 2 of 6

3 Table 1 GaN Switches. GaN switches achieve high levels of power handling in a small form factor, particularly versus insertion loss. For example, a 2 or 3 watt GaAs switch may have 2 db insertion loss, whereas a 20 or 30 watt GaN switch may have only a 1 db insertion loss. Additionally, GaN switches require very low current measured in micro-amps (μa) as opposed to milliamps or even amps for pin switches. And because GaN essentially brings more power per mm 2 to the table, small but higher power-handling components are needed to switch that level of power. The TGS2351-SM, for example, can switch 40W, as compared to GaAs FET-based switches which can typically switch between 3 and 10 watts in a similar board space. Table 2: Comparison of GaAs versus GaN switch FET characteristics Page 3 of 6

4 GaN FETs. Bare die and packaged GaN transistors provide flexible access to the power levels and bandwidths that GaN achieves, as well as the increased efficiencies for a variety of broadband applications, including military (EW, radar, communications), commercial (cable, VSAT, point-to-point) and instrumentation. Figure 2: Photograph of the GaN hybrid X-band power amplifier using two of the TGF transistors Figure 3: Example of an X-band GaN hybrid power amplifier; 40 dbm input, pulsed 100μs, 10% duty, Vd =30V, 25 C GaN Amplifiers. Wideband and narrow-band GaN amplifiers achieve unprecedented levels of saturated power and PAE for the same broad range of applications. TriQuint s product portfolio growth extends from DC through Ka-band. These types of GaN amplifiers offer greater ease of use compared to transistors since they are fully matched for 50 ohms and contain on-chip DC blocking capacitors. Industries and Applications Benefiting from GaN FETs, Amplifiers & Switches To date, the defense industry has benefitted most prominently from advances in GaN technology. Applications include radar, EW and communications all of which require the output power versus size advantage that is only available through GaN. Page 4 of 6

5 While EW and low frequency communications have already seen considerable improvements with GaN products working in the field (e.g., low frequency broadband communications was a natural fit for GaN where LDMOS could not perform), there are still areas in development. For example, TriQuint is working on GaN solutions for several higher frequency communications bands that are currently addressed with TWTs. Radar has been a slightly slower adopter since these are typically larger, more established systems. However, there is plenty of GaN development in the works for this market for defense as well as commercial systems like weather and marine radar, customers are seeing the benefits of using GaN rather than tubes, magnetrons or GaAs-based amplifiers. For these commercial applications, cost is a bigger driver than it is on the defense applications side. But as the cost of GaN is coming down, it is certainly more of an option today than it was just two years ago. Another cost consideration is the cost per level of performance instead of area. In the past, GaN has been wrongly judged on cost because the performance aspect was taken out of the equation in many cases. Even today, it can be said that GaN offers cost benefits over other technologies when viewed in terms of dollars-per-watt, as opposed to the standard dollars-per-square-millimeter comparison. As the frequency increases from S-band and X-band to Ku-band, GaN's dollars-per-watt cost offers a markedly better value than GaAs and other existing technologies, both now and in the years ahead. Further Development of TriQuint GaN GaN product suppliers originally focused on opportunities up to 6 GHz, supported by numerous μm (micron) processes. TriQuint introduced their 0.25μm GaN on SiC that offered operation up to 18 GHz. This 0.25μm GaN process supports the same 6 GHz and below products, but provides higher efficiency and higher gain advantages than the 0.5μm technologies. However, it also provides outstanding performance up through X-band as well as Ku-band. To address the Ka-band market, TriQuint is developing a 0.15μm gate technology that will not only support Ka-band power, but also wideband low noise amplifiers (LNAs). They are not stopping there. Research is underway to extend frequency coverage through W-band and to 500 GHz for high-speed logic applications. On the commercial side, opportunities in base stations will increase as GaN cost continues to drop. Similarly, opportunities in Ku and Ka-band VSAT are growing as are ones in point-to-point communications and CATV. From a product standpoint, GaN is driving thermal management activities. In terms of watts per millimeter 2, GaN products generate four to five times more power than a correspondingly-sized GaAs product. And although, as mentioned earlier in this paper, efficiency and thermal properties are inherently better with GaN, there is still heat dissipation that must be addressed, which is creating a need for thermally improved packages. This is being addressed on several fronts including TriQuint R&D and vendor-related development, as well as government-funded activities. From an overall cost perspective, TriQuint's vendors continue to make advancements in processes (e.g., substrates) while TriQuint continues to optimize yields. This overall maturing of the technology is driving cost downward. This, in turn, will key the interest of high-volume commercial applications, further adding to the downward pressure. Page 5 of 6

6 Much of TriQuint's technology push for GaN is through government contracts and government-sponsored programs, with a focus on both low-frequency applications and high-frequency opportunities. TriQuint's participation in these programs includes: Defense Production Act Title III a manufacturing readiness program that seeks to expedite the maturity of GaN and increase the supply base to support high-volume production. Near Junction Thermal Transport (NJTT) a Defense Advanced Research Projects Agency (DARPA) effort to improve thermal management of GaN's high-power dissipation levels. Nitride Electronic NeXt-Generation Technology (NEXT) a DARPA initiative aimed at supporting high-speed logic applications. TriQuint just completed Phase 2, which allowed greater than 200 GHz of operation, and is now engaged in Phase 3, which aims to increase to 500 GHz of operation. Microscale Power Conversion (MPC) TriQuint is leading this $12.3M DARPA program focused on ultra-fast GaN switch technology for DC-DC power converters. As far as new applications, it could be said that the sky is the limit with GaN. This is certainly the case with the higher frequency efforts already mentioned here, but also the high-power switching that the MPC program supports which creates possibilities outside RF power handling applications. Research is actively engaged in developing GaN DC-DC power converters for use in automobile hybrids and all-electric vehicles. The inherent power handling and density advantages GaN offers are expected to reduce the size and weight of DC-DC converters compared to the current technology that requires active heat mitigation to cool battery and electric power systems. Combining GaN advantages with continuing developments in next-generation battery technology, researchers have projected it will be possible to reduce vehicle weight by hundreds of pounds while improving performance and overall efficiency. Conclusions As seen in this paper, the advantages of GaN-based FETs, amplifiers and switches are numerous, with RF system designers benefitting in terms of more design flexibility, lower board space, lower system power requirements, better power handling, lower part combining requirements and better dollar-per-watt costs. GaN's reach and applications are expanding rapidly far beyond the initial power focus. With that, TriQuint GaN continues to lead the industry in power amplifiers and switches and is now moving into LNAs while it researches DC-DC devices for next-generation power handling requirements. With the technology in its infancy, the potential is tremendous. Learn more and purchase TriQuint's GaN FETs, Amplifiers and Switches online at TriQuint s GaN Tech Hub brought to you by Richardson RFPD: Page 6 of 6