The New Power Brokers: NXP on LDMOS
Steve Theeuwen from NXP talks to Microwave Journal about their LDMOS technology, the markets and the competition.
MWJ: What is the potential of GaN/LDMOS/HVFET technology in mobile radio communication systems. What are the most significant benefits of GaN/LDMOS/HVFET?
NXP: NXP is the company that develops both LDMOS and GaN technology. LDMOS is today’s technology of choice for base station, broadcast and radar applications. Being technology leader, NXP commits to further develop new generations in this established technology and push the LDMOS performance to its limits realizing superb power and efficiency levels at ever increasing frequencies. In parallel research and development efforts are put into GaN technology. GaN has outstanding material parameters and is therefore seen by NXP as the RF Power technology of the future. GaN is already used in niche applications and may gradually develop as the technology to support advanced high efficiency PA concepts.
NXP’s LDMOS has been derived from mainstream CMOS technology and has proven to be the technology of choice for base station, broadcast and radar applications for more than one decade. LDMOS is very reliable, rugged and cost effective in combination with best-in-class RF performance. NXP has 3 LDMOS technology flavors optimized for operation at supply voltages of 28V, 42V and 50V respectively. For power applications below 1.5 GHz (VHF, UHF, ISM, Broadcast) the 42V and 50V LDMOS are the best choices; 42V LDMOS gives the highest broadband efficiency while 50V LDMOS gives highest power, both in combination with extremely good ruggedness. The 28V LDMOS is applied in products for frequencies up to 4 GHz and is mostly used for GSM-EDGE, W-CDMA, WiMax and S-band radar applications. NXP continues to bring new generations on the market (currently NXP is rolling out the 7th generation) with best in class RF performance. Efficiency is the predominant technology roadmap driver. Furthermore, the maximum operating frequency of LDMOS will continuously be increased, allowing more and more applications (e.g. the recently introduced WiMax bands) to be addressed with LDMOS.
GaN is a wide band gap material and has superb material parameters, but still needs to mature as technology like CMOS to become well accepted and establish a reliable alternative. In its GaN development program, NXP reuses proven technology features and application knowledge of Si LDMOS technology to achieve reliable and rugged GaN devices. NXP has set up a GaN program together with IAF and UMS, forming a European platform for wide band gap expertise. GaN’s large band gap enables power densities greater than for Si LDMOS technologies. The maximum operation frequency also exceeds LDMOS allowing operation far above 4 GHz. These properties make GaN an attractive technology to support the use of advanced high efficiency amplifier concepts.
An overview of the material parameters for Si, GaAs, SiC and GaN is given in the table below. Based on the Johnson’s Figure of Merit it is evident that GaN is the most likely candidate to become the preferred high frequency power technology for tomorrow’s applications (or devices).
MWJ: What applications and what types of circuit architectures are best suited for GaN/LDMOS/HVFET?
NXP: LDMOS is today’s technology of choice for 3G applications, LTE, WiMax Base stations, Broadcast applications, ISM, and radar applications. LDMOS devices are mostly used for class AB and (n-way) Doherty architectures. Doherty architectures are suited to realize high efficiency levels for modern modulated signal types with high peak to average ratios. NXP 7th generation LDMOS is optimized for Doherty operation and efficiency levels of 50% have been realized for W-CDMA signals. Digital Pre-Distortion equipment is used to meet the system linearity requirements. LDMOS devices are very well suited to achieve the tough linearity requirements due to their stable frequency and temperature behavior. Mismatch conditions can be tolerated for LDMOS without degradation of performance (i.e. excellent ruggedness).
GaN technology can also be used for class AB and Doherty architectures, but GaN is considered by NXP as a disruptive technology to support the use of advanced switch mode, very high efficiency amplifier concepts. In such applications the full benefits of GaN with its high maximum operation frequency, high power density and broadband capabilities can be exploited. A single switch mode GaN power amplifier can handle various signal types and address many frequency bands. NXP is developing both the GaN technology with his European partners and the switch mode concepts to achieve more flexible and higher efficient amplifier solutions.
MWJ: How well does GaN/LDMOS/HVFET address military and government system requirements and application in military systems, specifically radar and electronic warfare? What are the commercial and military markets concern about GaN/LDMOS/HVFET and reliability? Are there issues or history of issues?
NXP: Power amplifiers are used in several military and government systems. For radar systems, several bands are used: L-band, S-band, and X-band being the most common. Both L- and S-band amplifiers are covered by LDMOS technology, while GaN and occasionally GaAs are used for X-band. Both LDMOS and GaN are employed in electronic warfare equipment. For military application, reliability is the key parameter apart from overall performance. LDMOS can leverage the high volume CMOS reliability experience and technology to achieve the required high quality and reliability levels. This has resulted in a reduction of degradation, e.g. in case of the important quiescent current degradation shown in the figure below. NXP is using its experience with LDMOS in the GaN program to develop and sustain the same stringent reliability and ruggedness criteria.
MWJ: What is the status and trends of non-linear modeling of GaN/LDMOS/HVFET rf devices? What is the status on the use of LDMOS devices in MMICs?
NXP: For LDMOS several best in class NXP home built models are available for large signal simulation. These models can be used for a large range of device sizes. A thermal node is included in these models as well. Also for GaN, models are being developed by NXP. The world standard PSP model (successor of BSIM) is rebuilt and validated for GaN. LDMOS MMIC devices have been on the market for many years. NXP offers a whole MMIC family with extra functionality like bias and temperature control. In general, for each transistor generation an equivalent MMIC product generation is released by NXP. In each subsequent generation both the transistor performance and the passive components are improved. NXP leverages its 5 layer metal CMOS back-end LDMOS process to produce high quality passives. Also MMIC technology is readily available for GaN, e.g. for X-band applications.
MWJ: What is the state of efficient heat removal for GaN/LDMOS/HVFET? What are the pros and cons behind GaN silicon carbide versus silicon substrates?
NXP: Good heat removal is essential for power devices since it can limit the power level of the device and can badly affect reliability. LDMOS makes use of the good conducting Si substrates, while NXP’s GaN makes use of the excellent SiC substrates. Silicon carbide substrates offer a better heat removal than Si substrates giving better RF performance. This goes at the cost of a higher wafer prize. Both technologies make use of thinning of substrates, improved thermal flange materials, optimized die attach and thermally optimized layouts.
MWJ: What separates your company’s LDMOS devices from others producing similar technology?
NXP: NXP is the only company that delivers both LDMOS and GaN technology, for optimum trade-off between performance and cost. NXP is the technology leader for LDMOS transistors. NXP’s LDMOS devices show record-value efficiencies and are used in applications up to 4 GHz. NXP holds a reputation with respect to very stringent reliability tests and best in class ruggedness and reliability. NXP’s LDMOS technology is produced in a deep sub-micron fab realizing gate lengths down to 300 nm, utilizing advanced CMOS equipment. NXP fabricates 3 LDMOS technologies for operation at supply voltages of 28V, 42V and 50V, respectively. For power applications below 1.5 GHz (VHF, UHF, ISM, Broadcast) the 42V and 50V LDMOS are the best choices. The 28V LDMOS is applied in products for frequencies up to 4 GHz and is mostly used for GSM-EDGE, W-CDMA, WiMax and S-band radar applications. NXP continues to bring new generations to the market (currently NXP is rolling out the 7th generation) with best in class RF performance. Efficiency is the predominant technology roadmap driver.
Furthermore, the maximum frequency operation of LDMOS is continuously expanded, allowing more and more applications (e.g. the recently introduced WiMax bands) to be addressed with LDMOS. NXP develops GaN technology together with IAF and UMS forming a European based platform for wide band gap expertise. NXP’s GaN devices have a factor 4 greater power density than Si LDMOS. This technology is optimized for operation in switched mode high efficiency concepts. NXP reuses reliability and application knowledge of their Si LDMOS technology to achieve reliable and very rugged GaN devices.
MWJ: Could you discuss up front costs, added expenses to implement, maintain, etc. compared to other power transistor technologies?
NXP: NXP’s LDMOS makes use of 8 inch CMOS fab facilities suited for high quality mass production. LDMOS benefits from the continuously improving CMOS cost structure using well maintainable fab equipment. NXP’s GaN is currently produced in a 4 inch III-V fab facility. NXP expects GaN-based transistors to always remain more expensive than Si-based transistors, given the missing economy of scale.
MWJ: Are there any efforts you have made for “Green” initiatives to reduce power consumption in high power, wide use applications.
NXP: Generally speaking, all the developments for particularly base stations and broadcast applications are driven by an ever increasing demand for higher efficiency. In turn this means less lost power, which means "greener" amplifiers. Higher efficiency does not only mean better use of RF Power, but also reduces the energy necessary to remove the dissipated energy (cooling systems). Hence, improved efficiency helps twice, on amplifier- and system level, to reduce overall power consumption. NXP is a world leader in delivering high efficient transistors and high efficiency concepts: It developed world's first fully integrated Doherty amplifier and the highest efficiency, discrete Doherty setup to date.