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

[usenet at teply.info]

On 10.04.22 21:03, ghf at hoffmann-hochfrequenz.de wrote:
> Am 2022-04-10 18:09, schrieb usenet at teply.info:
>> On 10.04.22 04:47, ghf at hoffmann-hochfrequenz.de wrote:
>>> Am 2022-04-09 20:35, schrieb Lux, Jim:
>>>> On 4/9/22 10:03 AM, usenet at teply.info wrote:
> 
>>>>> 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.
>>>
>> Unfortunately, that's not always an option. Sometimes you only learn
>> that Part A is exceptionally good only when it isn't anymore. One of
>> our customers got bitten by that, relying on parts which exceeded
>> specs until they got transferred to another fab. Afterwards, the part
>> still met their specs, but didn't meet the customers requirements. Of
>> course the manufacturer put in some effort trying to make the device
>> as good as it was because it was a good customer, but there's only so
>> much effort you can justify for 100k parts a year. For small-value
>> varicaps, where you can dice easily 30k-50k pieces out of a single
>> wafer, the customer would happily have bought a full manufacturing lot
>> had they known before the fact.
>>
>>>> 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.
>>>
>> It depends a bit on the needs. Whether we're talking about DC, analog,
>> RF or digital testers for example. But even for the most complex
>> tests, tester time is cheap compared to engineering time for test
>> setup.
> 
> I've worked for a tester manufacturer near Stuttgart for a project, and
> their customers had a completely different view about this. :-)
> 
> I had a mid-scale mixed signal tester just for myself to exercise my
> software; only half a day per week for someone else's regression tests.
> In our hall there were 300 engineers, almost entirely for software, only
> 15 or so for hardware design. A different customer of mine was quite
> proud on their E5052B; in the neighbor cubicle someone sent half a dozen
> of them one morning to calibration. That did not create any bottleneck.
> That all must be paid for. Then you arrive at options: Normal vector
> speed is <crawl>. For $$$ you get 10eXYZZY vectors at warp speed. Once.
> 
> My VNA cannot handle mixers because of a missing software option. As much
> as I hate it, I can understand it.
> 
> 
>> But of course you're right, as 1/f takes long, one will have to trade
>> test time versus lower frequency corner and sample count. Doing full
>> manufacturing screening (as in test every single manufactured device)
>> is prohibitively costly more or less independent of lower frequency
>> corner for all but the most demanding applications. Testing a dozen
>> devices out of a full wafer (with, say, 10 k devices per wafer) is
>> manageable. Last year I measured a few samples for 1/f down to 0.01
>> Hz. That's a matter of starting the measurement on friday afternoon
>> and then going home for the weekend. Next sample on the next weekend
>> ;-) There was no point in starting the measurement before friday
>> afternoon in any case as during the week the environment was more
>> noisy than the DUT...
> 
> Reminds me of our bit error rate measurements in fiber optics. We had
> to make sure, that NOBODY, not even the CEO took his cell phone into the 
> lab.
> 
If it was only cellphones it would have been too easy. Shielding 900 and 
1800 MHz is not really a problem. But removing the crud the MBE 
downstairs leaves on the mains lines is something that I didn't manage 
successfully yet. But on weekends there's nobody operating it 8-P

> Can you tell us what you used as bias and power supplies for these 0.01 Hz
> measurements? A tiny hint? Please, pretty please?
> 
Sure I can. It's not much black magic involved after all in the parts I 
know: For Collector Bias, I used a stack of rechargeable batteries and a 
few relays to change voltage, plus a DMM (HP 34401A) to measure the 
actual voltage and current. For the base, two of these had to do 
together with another relay-switched resistive divider. That has been 
provided by our customer along with the DUTs. I believe it was 2V 
lead-acid batteries, as they were a tad heavy. On the last sample I also 
tried one of our Agilent E5270B SMU Mainframes, and worked quite okay as 
well as the DUTs were not as low noise as has been hoped. But with that 
I only went down to 0.1 Hz due to time constraints. You have to keep in 
mind however that the test system in use (Proplus 9812DX) also does some 
filtering (unfortunately I can't give you details here as I don't know 
exactly) on the supplies, which probably helped as well.

At least I'm not aware of situations where the E5270B actually posed a 
limit here.

Bests,
Florian DH7FET