Recent Developments in RF Power Device Technology

TECHNOLOGY REPORT

evelopers of new RF power devices have differ- Companies like RF Micro Devices, WJ Communi- ent goals, depending on the frequency, power cations (through its acquisition of EiC), Hittite Dlevel, linearity, cost, duty cycle, efficiency, and Microwave, Sirenza Microdevices and others provide other environmental and performance factors. Recent GaAs HBT power amplifiers to handset makers. years have seen rapid changes in the technologies Silicon-germanium technology has also been devel- used for RF power, as new applications present new oped for handset applications, but presently has a requirements. much smaller share of the market than GaAs HBT. SiGe has done much better in 2.4 GHz WLAN and LDMOS is Dominant for Linear Applications other unlicensed device applications. In the frequency range of 500 MHz to 2 GHz, silicon LDMOS is the overwhelming choice for high power Silicon Bipolar Still Has Applications linear applications such as wireless base stations and Silicon bipolar has current density characteristics digital television transmitters. Earlier, silicon bipolar that have not been matched by other technologies. was used in non-linear and mildly-linear systems (e.g. This makes them suitable for short-pulse, high power AMPS), while GaAs power devices gained the early applications such as TACAN and IFF, plus other avion- lead in linear systems. Neither of these technologies ics applications and some electronic countermeasures had the combination of cost and performance required systems. Long-pulse applications have used derated for more advanced systems (e.g. CDMA and DTV). bipolar transistors, but LDMOS devices are now gain- The capabilities of LDMOS for providing the neces- ing ground in these applications in military and avion- sary performance using low-cost silicon die were ics systems. known, but practical issues in fabrication, thermal Silicon bipolar is also used, along with other device effects and aging required significant development types, in driver applications and low-level amplifica- effort to overcome. As reported in this and the previous tion. The low cost of fabrication and the relative unim- issue of this magazine, accurate modeling and simula- portance of achieving high efficiency makes this tech- tion of LDMOS devices is essential. The model devel- nology useful. There are many bipolar and BiCMOS oped for the Cree Microwave devices is a good example devices being produced. of the depth of the work required to obtain maximum performance for demanding applications RF Power at HF through VHF Now, LDMOS power devices are being produced in Both legacy and modern communications systems very large quantities by large and small companies, are in operation at frequencies from the low-kHz range including not only Cree Microwave, but Freescale through VHF. For these applications, several power Semiconductor (formerly Motorola Semiconductor), device technologies are used. Silicon bipolar is still in Infineon, Advanced Power Technology, PolyFet RF use over all of these frequencies, but in ever-smaller Devices and others. quantities. Newer technologies are now used in differ- ent portions of this segment of the spectrum. Handset PA Device Evolution At frequencies up to a few MHz, power FETs with As a high-volume application, the handset market very low ON-resistance and high current handling are has driven development of RFIC power amplifiers. used in broadcast and navigation radio transmitters, Similar to LDMOS in the power arena, these lower- and in switching power supplies. Extremely high effi- power amplifiers experienced an evolution from silicon ciencies can be achieved (90% and better) at these fre- bipolar to GaAs MESFETs and now to GaAs HBT and quencies in non-linear (e.g. CW and pulse-width mod- its variations, mainly InGaP. ulated) applications. Class D/E and other variations on

switch-mode amplifiers are the dominant circuit types, of power FETs made it difficult to use them above 1 or and have been continually improved since their intro- 2 MHz. Successful efforts to refine the structure have duction in the mid-1970s. created a new family of switch-mode power FETs that In the HF and low-VHF range (roughly 2 to 150 are optimized for applications into the tens of MHz MHz), RF power FETS using various forms of gate and higher. Two companies dominate this niche: trench fabrication have been dominant. Certainly the Advanced Power Technology and IXYS. best known of these devices is the venerable “Motorola MRF-150,” which has been copied, modified, adapted In Development and improved by several other companies. There are several new technologies being explored With single devices providing up to 150 watts RF for new applications. We’ve chosen two for special men- power output, and with parallel development of trans- tion: wide band gap devices and silicon carbide. mission-line combining and transformer technology, Gallium arsenide was the first wide band gap amplifiers with power output of several kilowatts are material to be explored, and it has been highly suc- common. The development of magnetic resonance cessful, but only after a significant period of develop- imaging (MRI) systems gave this technology a huge ment and market “shakeout.” Recently, InGaP and InP boost in the 1980s, which continues today. have reached a similar level of successful development. Gallium nitride and several other materials are ISM-Band Industrial RF Power being studied intensely for emerging applications at Industrial RF for heating, plasma generation, laser frequencies above 5 GHz. driving and other non-communication uses is a big Silicon carbide is still early in its development, but application area. Classic HF/VHF power FETs like it has the promise of higher power density, which can those noted previously are still used in many systems, mean either smaller power devices, or lower die tem- but there has been a clear evolution to a less-expensive peratures for longer life and higher reliability. FET derived from the common lower-frequency power Thermal issues are extremely important in power FETs. Previously, the high gate and drain capacitance devices, some of which may be solved by SiC.

High Frequency Electronics