I was under the impression that Si based transistors would handle higher
temperatures than GaN.
Is there something that will handle more than 300 W output that isn't
pulsed and can handle CW and SSB in a linear mode?
That something needs to be cost efficient enough not to require a second
mortgage.
73, Roger (K8RI)
On 5/4/2017 3:24 AM, John Lyles wrote:
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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