[Amps] LDMOS and DMOS RF MOSFETs

[Roger (K8RI)]

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
> ----------
> 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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