[time-nuts] TAPR TICC boxed (input protection)

Charles Steinmetz csteinmetz at yandex.com
Sat Apr 8 16:30:38 EDT 2017

David wrote:

> I mentioned this in connection with some manufacturers using gold
> doping in transistors which would not normally be expected to have
> gold doping.  So you end up with a bunch of lessor named 2N3904s which
> meet the 2N3904 specifications but are useless if you were looking for
> low leakage diodes.

I believe all 2N3904s and 2N3906s are gold doped.  National's certainly 
were (Processes 23 and 66), and TI's and Fairchild's are. Not heavily 
doped, like 2N2369s (with storage times of ~20nS), but just enough to 
bring the storage time down to ~100nS.   2N2219s, 2N2222s, and 2N4401s 
are also lightly gold doped.

> If [4117 leakage is] not being tested, then where is the maximum specified
> leakage number coming from?  For a small signal bipolar transistor it
> will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
> shows 10pA maximum and 1pA maximum for the A versions.
>        *   *   *
> When this discussion of low leakage input protection started, I did a
> quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
> and came up with nothing; all of the inexpensive JFETs are much worse

Same as any "guaranteed by design" spec -- by the device design. The 
4117 series is unlike any other JFET -- the geometry is TINY, and the 
4117 Idss is only 30-90uA (hundreds of times lower than other low-Idss 
JFETs). [BTW, lowest Idss is why I recommend the 4117 over the 4118 and 
4119 for use as a low-leakage diode.  The 4118 and 4119 have higher Idss 
-- up to 240uA for the 4118 and 600uA for the 4119 -- and tend to have 
higher gate leakage, as well.]

Best regards,


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