[sdiy] Temperature compensation results

[Ian Fritz]

Hi Rene et al. --

At 04:12 PM 6/15/2003, René Schmitz wrote:

>>In an extreme case, I would imagine that one transistor might have a 
>>large defect, such as a dislocation pileup originating from a substrate 
>>imperfection, and that the other transistor would not.  Then the Is 
>>factors would be *qualitatively* different for the two devices, since one 
>>would have to include an extra term to describe the large current from 
>>the defect-induced leakage path.
>
>Right, that could happen. Although I think that there would also be a
>large mismatch between them caused by Is alone, and on top of that the
>temperature dependance of Is is qualitatively altered.

Right.  (I thought that was what you were asking. Sorry if I misunderstood.)

>>More generally, I believe a good model would include leakage from a 
>>distribution of defects.  Mismatch would then depend on what the actual 
>>distribution in each device is.  So I would guess that the boundary you 
>>are asking about isn't sharp.
>
>I suspect the same. However the difficulty in that is that you have to
>have a look at the chip to get a grip on where the defects are.

Yes, detailed electron microscopy, etc.

>>Interestingly, I think this discussion has a connection to some work I 
>>did a couple of years ago to select a transistor noise source.  I went 
>>through about 50 2N3904 devices looking for the ones with the lowest 1/f 
>>noise.  I only found one with really low 1/f noise, and that was the one 
>>with the lowest overall noise level.  This suggests to me that most of 
>>the devices have significantly more leakage than is ideal.  So I agree 
>>that there are non-ideal leakage processes in the pairs we choose as matched.
>
>I guess this is another reason why low noise dual parts have usually
>large geometry. The larger the chip, it gets more probable that the
>defects are more evenly spread across both halves of the device. (If the
>chip gets larger than the average distance between defects that is.)
>Also a localised defect doesn't contribute as much as in a small
>geometry device.

If it's a dead short it would!  But generally I think you are right

>>Perhaps the devices we pick as being matched have simply have a low level 
>>of similar defect distributions.  This suggests a new way to pick matched 
>>pairs: compare reverse leakage currents and pick pairs with matched and 
>>relatively low leakage.  Of course no one will agree to doing it this 
>>way, because it's not how Moog did it.  :-(
>
>I guess a practical way could be to use the extrapolation of the log(Ic)
>vs Vb slope as in the GP parameter extraction. (The leakage currents are
>very difficult to measure directly.) But do it for several temperatures.

Doh, I keep forgetting how small the leakage currents are.  I guess we 
wouldn't much want to do any of this at home!  What about just measuring 
the noise? Low noise and low leakage should go together, no?

   Ian