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

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On 07.04.22 22:58, Joseph Gwinn wrote:

>>>> There is a list in The Art of Electronics, but some of them won't be
>>>> available.   Some datasheets do have the curve - the JFE150 from TI
>>>> has
>>>> its voltage noise curve right on the front page.
>>>
>>> What those folk are currently using for capacitance multipliers and
>>> the like (where low 1/f noise is also essential) are SiGe transistors
>>> like the following:  BFP640H (Infineon), BFP780, SAV541
>>> (MiniCircuits), and 2S2114K (NPN, beta 1200) for high current
>>
>> and 2SD2704 from ROHM, even more beta
> 
> Interesting.  I would not have thought of this one.  It's big and
> slow, and made from modern very clean silicon, so it could have low
> 1/f noise.  Its transition frequency (*= gain-BW product?) is 35 MHz,
> so it ought to work on switcher noise.
> 
> 
>> Those SiGe transistors have wonderful low Rbb of just a few Ohms,
>> which results in nice low voltage noise, but some have 1/f corners
>> of 50 MHz or more; that kills my application completely.
> 
> While these chips are small, they are made from very clean material,
> so one wonders why so high. The circuit should be physically designed
> as if it were to be handling GHz signals, because it could be
> oscillating far above the capability of available instruments to
> detect.
> 
IF the base and emitter doping would be done through ion implantation, 
this can create a lot of defects, which act as recombination centres. 
In-situ doping during epitaxial growth of the SiGe base is more 
difficult to get right (especially achieving the doping profile 
necessary for proper operation of the transistor) than ion implantation, 
but can give significantly better noise performance.
For JFETs, the same problems crop up.
But even when the manufacturer in general masters the process, it's not 
given that a transfer of products to another plant of the same company 
can replicate the results. This is very much dependent not only on 
equipment and materials, but also on experience and knowledge of the 
engineers actually running the process. Swap just a single one, and the 
results may be different by several orders of magnitude. There are quite 
a few anecdotes in the industry. That's also a reason space guys often 
require sourcing parts manufactured in one named fabrication plant.


Bests,
Florian