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

David davidwhess at gmail.com
Sat Apr 8 11:52:56 EDT 2017

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,

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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