This was on line, dated 2014. An interesting perspective on LDMOS and
plain DMOS transistors.
73
John
K5PRO
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Reports of the Death of DMOS RF MOSFETs are Greatly Exaggerated
by Mark Vitellaro, Director of Strategic Marketing, Richardson RFPD
The landscape of the RF power transistor market has recently changed due
primarily to three developments: the emergence of GaN, the maturation of
the LDMOS process, and the slowdown in the growth of the wireless
infrastructure market. The performance attributes of GaN are well known.
The technology offers a highly-valued combination of improved power
density, high gain, high frequency, wide bandwidth, and high efficiency.
Another interesting aspect of the emergence of GaN has been the
appearance of multiple independent GaN fabs, which has opened up
opportunities for several RF transistor companies that were previously
unable to develop their own competitive LDMOS strategy. While there is
certainly time for the GaN market to consolidate, there seems to be
enough diversification in product development to support several players
in the medium term. It just so happens that the timing of the GaN
awakening has occurred as LDMOS is facing challenges from a maturing
process, as well as the shrinking of the wireless infrastructure market.
Now dominated by Freescale, NXP and Infineon, LDMOS is on its 9th
generation, providing diminishing marginal returns from future process
enhancements. Furthermore, the push for higher efficiency, digital
baseband with digital predistortion and small cells in the wireless
market have reduced the LDMOS dollars per amplifier.
The combination of these forces has driven LDMOS suppliers to look
elsewhere for growth, and the largest area of interest has been the
industrial, scientific, and medical (ISM) market. This market is
fragmented and includes many high power HF applications such as RF
exciters for CO2 laser, RF generators for plasma ignition, and RF
amplifiers for MRI. Historically these applications have been served by
RF MOSFETs.
But the current generation of LDMOS was not suitable for ISM, because
the evolution of LDMOS has been driven primarily by the needs of the
wireless market—which has a completely different set of technical
requirements. So Freescale and NXP went back to the drawing board.
Freescale was the first to plant their flag with their “E” series of
enhanced rugged LDMOS. The new feature set included a very high VSWR
rating of 65:1 at all phase angles, coupled with a 3 dB overdrive
feature. Combining those features with their 50V process and wide gate
withstand voltage introduced a new class of transistor that was suited
for the high mismatch applications of the ISM market. What’s more,
Freescale introduced a 1.25kW device that could be operated single-ended
or push-pull. This new class of transistors set the ISM market on its
head. Up until this point, most systems were based on combined 300W DMOS
transistors which required extensive passive matching and combining
networks. The 1.25kW LDMOS device simplified the architecture and
eliminated significant supportive circuitry. Furthermore, as the
efficiency of these circuits were approaching 80%, it was possible to
utilize surface mount devices. It didn’t take long for NXP to notice,
and shortly thereafter, it introduced its family of “XR” Extra Rugged,
RF transistors. The family looked similar on paper, as they included a
65:1 VSWR rating and 1.2kW and 1.4kW device. But even more telling was
the 2013 announcement from NXP to close a fab that produced many RF
transistors, including their DMOS products that were optimized for
HF/ISM applications. Freescale had already exited the DMOS market more
than a decade ago when they divested their gold process fab and sold
that product line to M/A-COM Technology Solutions, which continues to
produce the line of MRF transistors in their Lowell fab today.
So now you may be asking yourself, “Wait, this blog post began on the
premise the DMOS was not dead, didn’t it? It sounds like DMOS is
declining as LDMOS encroaches into their market only to be accelerated
further now that both Freescale and NXP are focused on transitioning
customers to LDMOS technology...” Clearly LDMOS has taken share and the
suppliers are allocating more resources to support the ISM market as
wireless slows, but the remaining DMOS suppliers are not throwing in the
towel just yet. As a process technology it would seem that DMOS has
reached its process limits, but there are incremental improvements that
can be made specifically to address the ISM market, namely packaging
innovations, higher voltage, and the pulse characteristics of DMOS.
ST Microelectronics was the first to develop a plastic air cavity
package as an alternative to costly ceramic packages. Introduced as
“STAC” packaging, which stands for ST Air Cavity, they successfully
eliminated the ceramic, which reduced cost and improved thermal
dissipation and gain. Presently available in bolt-down and direct-solder
options, several established DMOS transistors are now available in the
new packaging. Microsemi also developed their own packaging enhancement.
The flangeless VRF157FL is a near equivalent to the 600W MRF157, but
Microsemi removed the CuW flange and replaced the ceramic lid with
plastic. These mechanical changes result in similar benefits as the
STAC, and hopefully that family grows as much as it has with ST Micro.
Besides packaging, increasing the supply voltage from 50V can simplify
and shrink the power supply and thus the overall system. In addition to
a smaller power supply, RF amplifiers for MRI benefit from better SNR,
which can improve the quality of the imaging. ST and Microsemi both have
100V conventional DMOS in their portfolio, the SD3933 and VRF3933. ST
also has a STAC version, the STAC3933. But Microsemi has gone even
further with their ARF family of DMOS transistors which operate at 150V
and 250V. These plastic packaged devices operate in highly efficient
Class D mode and are offered in pairs or with integrated drivers
(DRF1200). ST is developing their own 150V to 250V “Super Junction” DMOS
devices, which are in evaluation now.
Finally, DMOS exhibits a much higher peak power rating compared to an
equivalently sized LDMOS device. Combining that peak power advantage
with a plastic/flangeless package narrows the cost advantage of LDMOS.
For instance the STAC3932B is characterized for 3T MRI systems. Under
1mS, 10% conditions, the part will produce 900W peak power.
In conclusion, the combination of inherent characteristics of DMOS and
the hard work of innovating suppliers show that the incumbent DMOS
manufacturers refuse to concede victory to LDMOS. And the ultimate end
users of ISM transistors are very resistant to change and are not always
impressed with the next new thing. “Copy Exact” requirements and fear of
the unknown will ensure a portion of the ISM market continues to use
DMOS. I, for one, welcome the debate and am curious to see how this
battle plays out. While everyone else is enthralled with GaN, the LDMOS
– DMOS battle will be a very interesting one to follow.
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