Reply split into two posts in order to better follow the flow of
discussion...
> On 12.04.22 00:02, Joseph Gwinn wrote:
>> On 09.04.22 13:38, use...@teply.info wrote:
On 07.04.22 22:58, Joseph Gwinn wrote:
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.
>
> I always wondered about such details. I gather from this and some
> following posts that it is known how to greatly reduce 1/f noise in
> transistors, but it's a nuisance, and so isn't generally done. But
> what saves us is if the intended purpose of the transistor type
> requires the cleanest of material and the best processes to yield low
> defects such as trapping centers. Such as the above-mentioned
> difficulties in getting the in-situ doping correct in SiGe
> transistors.
>
I wouldn't go as far as saying that we KNOW EXACTLY how to reduce 1/f
noise or all the other unwanted side effects (RTN, leakage currents, you
name it). We do observe however, that in many cases these effect are
somewhat correlated with each other and with a variety of defects.
In many cases, these defects can be annealed (mostly through high
temperatures), but there's you get into conflict with other physical
properties you want to achive. One example: for modern three-digit GHz
fT SiGe HBTs (and also for double-digit GHz devices...), you'll need a
very thin base and a sharp doping profile. Application of high
temperatures for extended periods not only anneals the defects, but also
enhances diffusion of the dopant atoms so the can move along their
concentration gradient. Such spreading of the doping profiles and
germanium contents however is detrimental to their RF performance, as a
wider base and/or less steep doping profiles correlate to lower fT and
fMAX.
Bests,
Florian
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