[time-nuts] Re: +1/f of transistors

[ghf at hoffmann-hochfrequenz.de]

Am 2022-04-09 20:35, schrieb Lux, Jim:
> On 4/9/22 10:03 AM, usenet at teply.info wrote:
>> On 09.04.22 15:31, Richard (Rick) Karlquist wrote:
>>> 
>>> I am seeing a lot of unsupported "theories" about what should be done 
>>> to make devices with low 1/f noise.  It might be instructive for 
>>> everyone
>>> to read Marv Keshner's PhD dissertation (Stanford) discussing 1/f 
>>> noise.
>>> He looks at all kinds of theories and shows that there is no valid 
>>> cookbook for how to make low 1/f noise devices.  It's the classic
>>> non reproducible process.  I remember an FCS
>>> talk many years ago that NIST guru Fred Walls gave with some theory
>>> on how to get low 1/f noise.  Unlike his other papers which were
>>> well received (and rightly so), this one was rapidly debunked.
>>> I felt bad for Fred, getting out too far over his skills.

>> Thanks for the hint towards the thesis, I'll ask our library to fetch 
>> a copy.

>> Recently I was discussing some measurement results with my colleagues 
>> as we're trying to come up with a low noise JFET which can 
>> successfully be integrated into a SiGe BiCMOS process, and quite often 
>> we're also struggling to identify why exactly variant A has 
>> significantly lower noise than variant B, or why a new approach does 
>> not improve noise the way it was expected.
>> So from a manufacturing process design point of view, achieving low 
>> 1/f noise indeed is closer to sheer dumb luck than the proverbial 
>> "more art than science" suggest.


> This is very, very true. Some manufacturers get very low noise or very
> low leakage (or both), essentially by being "lucky".  From what I've
> been told, there's no good models, nor predictions - so people share
> "lore" of "if you get these 2Nxxxx FETs from the mfr in England,
> they're really good" until they aren't.   There isn't enough market
> for these, so I suspect research money to "solve the problem" isn't
> available.

Buy a life time supply while they are available. One reel will probably 
do.

> Like all those microwave MMICs with low noise, they worry about 100
> MHz and up (if not 1GHz), they certainly don't worry (or control) for
> noise at 5 MHz, or where the 1/f knee is. So just because you got good
> results with a batch of them, the next batch might not.  It's not even
> clear you could come up with a standardized test method, because the
> noise depends on a lot of other factors (drain current, for instance).

When it changes from lot to lot, then you have lost. You cannot catch
that on the wafer tester. No one can pay for the tester time.
A simple BJT or FET circuit is allotted a ms or so in total, maybe.
You cannot measure 1/f in the 100 Hz range in that time. The picture
of the FET amplifier I had 3 days ago took 35 minutes, per trace.
By far, most of the wall time is aquisition time for the lowest octaves.

Setting the drain current is cheap, it takes maybe a dozen of test 
vectors.
But tester time is paid in millions of vectors per second.

Gerhard