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

Adrian Godwin artgodwin at gmail.com
Sat Apr 8 13:38:27 EDT 2017

The Siliconix PAD1 at 1pA and 0.8pF is still available :


On Sat, Apr 8, 2017 at 4:52 PM, David <davidwhess at gmail.com> wrote:

> On Thu, 6 Apr 2017 22:23:43 -0400, you wrote:
> >David wrote:
> >
> >> I know one  thing to watch out for if you are looking for low
> >> leakage is gold doping
> >
> >Anything that increases carrier mobility increases leakage current (all
> >else equal -- i.e., for each particular device geometry).  This accounts
> >for the much higher leakage of Schottky and germanium junctions.
> 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.
> >> And I have another question if you know.  How is rb'Cc measured?
> >
> >One way is to drive the transistor with a medium-high frequency (well
> >down the 1/f portion of its current gain curve -- typically 10-50MHz for
> >small-signal BJTs) and measure the base-collector phase shift.  It can
> >also be calculated from fT and Cc-b.  There is a JEDEC standard for
> >measuring rb'Cc, but I'm not finding my copy at the moment.  It may be
> >posted on the JEDEC web site.
> I thought there was a more sophisticated way but that sure sounds like
> something Tektronix would have done for grading parts.
> The JEDEC standard is probably what I need to find or at least start
> with.  Thank you for the tip.
> >> The advantage of the 4117/4118/4119 is that the leakage is already
> >> tested to a given specification so no qualification or testing is
> >> necessary.
> >
> >That may be true, but there is nothing in the data published by Vishay,
> >Fairchild, Calogic, or InterFET to indicate this.  Spot-checking, along
> >with the part design, should be sufficient to guarantee meeting the
> >spec.  I'll try to remember to ask the Vishay process engineer next time
> >I talk to her.
> >
> >Best regards,
> >
> >Charles
> If they are 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
> until you get to premium devices.
> (1) I only picked the InterFET datasheet because it was the most
> readily available of the ones you mentioned.  The current Fairchild
> and Linear Systems datasheets show the same thing.
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