At Freescale™ we have been making low frequency, 48 Volt LDMOS devices since 2007 when the company rolled out the first MRF6V2300N plastic parts. These parts were designed for the 2 to 600 MHz, Industrial, Scientific, and Medical (ISM) market applications where the devices’ linearity was not a required trait. The main benefits of 48V operation were three fold. The gain of these 48V devices is significantly higher, by as much as 6 dB or about a third higher than that of the 28V parts. Secondly, the power density of the 48 Volt technologies also is about 50 percent greater than that of the 28V LDMOS die. The standard OM780 plastic package is currently limited to 200W at 900 MHz under 28V technology and the same package can house up to 350W with some versions of the 48V LDMOS die. Lastly, the terminal impedances of these higher voltage parts is significantly higher, due to the die’s lower Capacitance, Drain-to-Source (Cds) and higher power density, allowing for devices with a wider bandwidth capability and simpler external matching circuits.
These are all lovely benefits, but there are many more RF power amplifier applications in radio communications, where a highly linear output is required. What would it take to have all the benefits of a 48V LDMOS die technology with the added requirement of superior LDMOS linearity? All it takes is the desire, and good design engineering.
In order to achieve higher linearity out of a LDMOS design, all one has to do is to make the appropriate engineering trade-offs in the design of the core active channel area of the die and emphasize all options in the direction of a superior linearity requirement above all other factors. Higher silicon doping concentrations in the active channel area produce the required low Resistance, Drain to Source (Rdson) needed for high linearity yet the bulk resistance of the substrate must be high enough to support the required 120V plus Breakdown, Voltage, Drain to Source, with the gate shorted to the source (BVdss). A long drain finger geometry is great for higher power density but it adds to the distortion products of the device due to the variable phase length feeding of the active area of the die. Short drain fingers create a tightly packed active area, which reduces the distortion products and the Cds of the die and increase the power density but also increases the thermal resistances of the device, causing it to run hotter and have a lower Mean Time to Failure (MTTF).
The packaging and mounting of the die must be optimized in order to maintain low die temperatures and meet the expectation of a minimum 200+ years MTTF. There also are linearity optimizations of both the gate length and the field plate dimensions, which can all be tweaked in the direction of a highly linear device, operating at 48 Volts, while still maintaining the three key benefits stated above We recently released the first 48V LDMOS device designed for a multichannel, GSM Doherty application in the cellular base station market space. The part belongs to the new Airfast™ RF Power solution family of devices. This family of products has been engineered to deliver the industry’s leading RF performance numbers with regards to power density, signal bandwidth, linearity, gain and efficiency, all in cost effective standard packages. Rather than focus solely on linearity and efficiency, this family of devices delivers a complete system level approach to superior RF performance focused on ease of use along with the new, advanced RF performance requirements. The part also utilizes the first example of an in-package symmetrical Doherty device configuration in a cost effective, four leaded, OM780 overmolded package style. This first 48V cellular device is an AFT09VP350N and you can look up the RF performance numbers on the data sheet on the web site.
The main point of this blog is to point out that there is a major shift in LDMOS technology for cellular applications and the device operating voltage is changing from the current 28V range up into the 48V region. There are very compelling RF performance advantages that can be achieved with this higher voltage operation and, when following the proper engineering considerations, better, more compact and cost effective designs can be created using this new 48 Volt technology. This one AFT09VP350N device is just the first in a complete family of both 28V and 48V Freescale Airfast devices that we will be rolling out during the course of this year. These new devices promise to deliver the industry’s leading, highest level of RF system performance capability that has ever been created.
About the Author Leonard Pelletier is lead RF applications engineer at Freescale Semiconductor and is a regular guest contributor on RF topics for RF DesignLine. He can be reached for questions and suggestions for future topics by commenting in the box below. You can also follow him on Twitter at www.twitter.com/RFLeonard