The New Power Brokers: Freescale Semiconductor on LDMOS
Leonard Pelletier of Freescale Semiconductor talks to Microwave Journal about their LDMOS technology, the markets and the competition.
MWJ: What are the most significant benefits of LDMOS?
• LDMOS has the lowest cost per watt ($/W) ratio of any of the high power RF amplifier technologies.
• LDMOS has some of the best Class-AB linearity, gain, reliability, and thermal resistance numbers of any of the popular RF power technologies.
MWJ: How does this compare to other high power devices?
Freescale: A rough rule of thumb is that for high volume cellular base station applications, LDMOS prices are on the order of $0.30/W, GaAs is about $0.9/W and GaN is about $1/W.
At 2 GHz, the latest LDMOS has about 18 dB gain, a 4000 year MTTF, and is 65% efficient at the p1dB compression point. At a 6 dB back off point, the devices are typically -45 dBc W-CMDA ACPR and 30% efficient. GaN offers slightly higher efficiency at 6dB OBO. GaAs suffers from low gain performance.
MWJ: What applications are most applicable to these benefits?
Freescale: LDMOS excels in high volume, medium to high power, linear systems with very stringent cost goals and very high reliability requirements. LDMOS performance is excellent up to 3.8GHz but degrades rapidly beyond 4 GHz
MWJ: What are the most substantial system-related issues to consider when working with LDMOS?
Freescale: Creating higher linearity through digital predistortion or feed forward error correction systems. With improvements in the correction capability of the architectures, the RF devices can be operated closer to their P1dB compression point, where the DC to RF conversion efficiencies are at their highest. Improved linearity translates to improved efficiency and lower overall system costs.
LDMOS, with its relatively low power density, has relatively low terminal impedance as well, so the bandwidth capability of the RF matching structures is limited to an octave or less. High power LDMOS is not very applicable for decade wide, broadband system designs.
MWJ: What is the potential of LDMOS technology in mobile radio communication systems?
Freescale: It is the market leader now and its’ cost effectiveness, high RF performance and long term reliability bodes well for future market dominance.
MWJ: How well does LDMOS address military and government system requirements and application in military systems, specifically radar and electronic warfare?
Freescale: For narrow band applications, LDMOS can provide the most cost effective solution for all of the typical RF performance parameters of gain, efficiency and high power. The improved dynamic range linearity of LDMOS is perfectly suited for the newer radar system applications with pulse shaping and wide range power output control functions.
MWJ: What is the status and trends of non-linear modeling of LDMOS RF devices?
Freescale: LDMOS models are very highly developed and also very accurate. Freescale ha partnered with all the leading providers of simulation modeling software packages and we provide validated models for our most popular devices. Models are downloadable off of the Freescale web site and they provide a very accurate simulation tool for predicting the device’s RF performance capability under a wide range of test conditions and load impedances.
MWJ: What type of circuit architectures are best suited for LDMOS, such as Doherty, push-pull, cascode, distributed, etc.
Freescale: LDMOS is well suited for single ended, push-pull, quadrature combining and Doherty applications. The high off impedance of the LMDOS device makes it well suited for the peaking section of a Doherty amplifier and for Distributed amplifier applications. The relatively small output capacitance also lends itself towards drain modulation applications. Very good performance has also been reported in drain modulation systems
MWJ: Any specific recommendations for Class of operation? Any guide based on application?
Freescale: Most LDMOS is used in Class AB, Class B or Class C modes. Freescale has specifically altered the operational range of our ESD protection circuits to allow for a wider range of Class C bias conditions. Class AB works for the greatest majority of the linear applications but peaking amplifiers in the Doherty configuration are typically Class C as are most radar, heating and lighting applications. Class F and Class D are useful modes at the lower frequencies.
MWJ: What is the status on the use of LDMOS devices in MMICs.
Freescale: Very highly developed and extensively used in the market place as multi-stage RF drivers and gain blocks. Freescale has a very extensive internal library of on-chip silicon passive components so we design a wide variety of internally matched, temperature compensated, multi-stage RF MMIC devices with power levels ranging from 10 to 100 W.
MWJ: What’s the relationship between LDMOS Frequency Figure of Merit and Temperature? What is the state of efficient heat removal?
Freescale: All of Freescale’s LDMOS devices are specifically designed to accommodate the very high reliability requirements of a cellular system. When operating the device with a typical maximum die temperature in the 150 to 175 degree C range, the devices typically have MTTF’s between 2000 to 8000 years. The die structure, package materials, assembly methodology and manufacturing process are all designed to achieve the optimum heat transfer of the total assembly in order to reach this specific MTTF design goal. The maximum operating temperature is 200 deg C and is backed up with 15 years of reliability data
MWJ: What are the commercial and military markets concern about LDMOS and reliability? Are there issues or history of issues?
Freescale: In the mid 90’s LDMOS did have an issue with Hot Carrier Injection that translated to a biasing shift over time in the 30% range over 20 years of operation. Freescale devices developed since late 1997 all have incorporated changes in the active area of the die that limit this bias shift value to under 5% across the same 20 years. Class AB devices can tolerate a bias shift of about 25% before there is any significantly change in RF performance so this bias shift is no longer an issue.
MWJ: What separates your company’s LDMOS devices from others producing similar technology?
Just a few Freescale First’s….
Freescale was the first to market with 2 GHz RF power LDMOS devices. We were first to deliver 50 V LDMOS and the first to deliver over-molded plastic packaging.
Freescale has the broadest product portfolio with over 235 various LDMOS devices in production now. This portfolio ranges in frequency from 1 to 3800 MHz and ranges in power output from 4 W up to 1000 W for a single device.
Freescale also has the highest manufacturing economies of scale. Freescale has the highest run rates in the RF power market and we have an extensive line of devices available in the cost effective, over-molded plastic package options. We are adding manufacturing capacity to accommodate future growth and market share increases.
MWJ: What are the pros and cons behind GaN silicon carbide versus silicon substrates?
Freescale: SiC provides superior thermal conductivity than Si. Si loses its semi-insulating property at high temperature which significantly degrades the performance of the device. Because of all these issues, GaN on Si can not typically be operated at 48V like GaN on SiC transistors and therefore, have reduced power density, gain and impedance figures.
MWJ: Could you discuss up front costs, added expenses to implement, maintain, etc. compared to other power transistor technologies?
Freescale: LDMOS is the market leader with the most extensive depth of modeling expertise, the largest amount of reference designs and the longest manufacture track record of any amplifier technology. LDMOS has the lowest initial costs and the longest lifespan projections in the RF power industry.