Roger,
That's a fabulous tutorial. Thanks very much!
Jim Garland W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Roger (K8RI)
> Sent: Thursday, September 12, 2013 6:42 AM
> To: amps@contesting.com
> Subject: Re: [Amps] design approach for QRO solid state
>
> This may not be germane to the building of the amplifier and it is by
> necessity an over simplification, but it is the temperature problem
> explanation for those not familiar to transistor failure and I hope it
> is of interest to some.
>
> Transistors do age with use. Typically, unlike tubes the shelf life of
> modern transistor is almost indefinite except for contaminates introduce
> during manufacture which is rare now days.
>
> However with use, the dopant (intentional impurities induced during
> manufacture to create N and P type material.) migrates with voltage and
> current flow, but under normal conditions this migration is miniscule,
> but complicated. For simplicities sake we just say the primary carriers
> (and minority) carriers move toward junctions in bipolar transistors.
> A similar effect takes place near the gate in FETs.
>
> Of course if the temperature gets high enough we end up with a
> catastrophic failure, rather than aging.
>
> It might help to explain that during manufacture the device is placed in
> a furnace, heated and exposed to a gas to produce a layer of oxide.
> Later the structure the device is etched in the oxide to expose the
> Silicon. The device goes back in the furnace and exposed to a gas
> containing the dopant that creates the N or P type material at an
> elevated temperature by diffusing into the Silicon. This operation is
> usually repeated many times to make a device.
>
> So when a transistor is exposed to elevated temperatures it is pretty
> much in the same conditions used to manufacture it. Normally we are no
> where near this temperature, but the warmer the device the faster the
> dopant migrates.
>
> The modern materials are extremely pure. Far more so than the industry
> was capable of 30 or 40 years ago, let alone 50 years ago when the
> industry was in its infancy, BUT all material does contain some
> contaminants. Typically for N type material, it is a P type and for P
> type it is N type and these are called minority carriers.
> With age and heat the migration "slowly" creates more minority carriers
> which I guess a very loose analogy would be a tube becoming gassy or
> contaminated. I say very loose because the entire mechanism is
> different. Only the results are similar in causing a degradation of the
> device. I did say at the beginning this had to be an over simplification.
>
> Referring to the purity level I believe we are now in the parts per
> trillion whereas we were in parts per billion just a decade or two back.
> I was given the analogy for parts per billion, think of as cube of
> white bricks one city block on a side. some where in that cube is "one
> black brick". We are now talking about purities on the order of a
> thousand times greater. Poly crystal Silicon coming out of the reactors
> is now more pure than we could get from float zone refining in the early
> days and that stuff was expensive! (As much as $165 USD a gram).
>
> As a side note, in the early days reactors produced poly rods of 3/4
> to 1 inch in diameter and 16 inches long and operated at around 10" of
> water pressure. Today's rod size and the operating pressures are
> proprietary, but I can tell you that it takes a strong, chain hoist to
> remove the rods from the reactors and pressures are in PSI rather than
> inches. Growth rates are also much faster. But the industry has always
> been "Feast or Famine". I believe poly has sold for as little as $6 USD
> a kilo (long time ago) and as I believe as high as over $400 USD per
> kilo. As a SWAG I'd guess it's presently around $60 a Kilo. None of the
> manufacturers want their competition to know what they are getting but
> this price has a direct impact on device development, availability, and
> price. IIRC there are 3 main manufacturers that are capable of
> weathering the Famine and right now it is a major famine with I believe
> around 30 some manufacturers.
>
> One of the main producers just finished an expansion project of over a
> Billion dollars as well as developing a new facility. They were in
> another Billion dollar expansion when the recession set in.
>
> Back to devices:
> Keeping the temperature of the device low is a science by itself. The
> first and main problem is getting the heat from the Silicon die (the
> device itself) to the case as this device is surprisingly small. The
> temperature difference between the device and case ( called delta T) is
> the single most important limitation of the device. Far more so than
> cooling the case and its fixed within the design so we can do nothing
> about it other than to cool the case as much as possible and this is a
> real challenge. This is also why we find high power devices listed in
> pulsed service.
>
> Without going into a long explanation of the cooling I'll just say it is
> essential to keep the device as cool as is practical as there is little
> difference between a temperature that will allow normal operation, one
> that will produce accelerated aging and one that will produce
> catastrophic failure. It's much easier to cool multiple devices with the
> power divided between them. That's not to say cooling at solid state
> QRO is simple as it's a long ways from that with exotic heat sink
compounds.
>
> 73
>
> Roger (K8RI)
>
> .
> > Rather than a single big device, I like and prefer the idea of multiple
> > parallel/push-pull RF power devices to share the load and more evenly
> > distributed the massive heat into the copper spreader and heatsink
system.
> > Devices tend to prematurely fail when their thermal aspects are not
properly
> > attended to, rather than a failure attributed to load / VSWR mismatch.
>
>
> >
> > Leigh
> > VK5KLT
> >
>
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