[time-nuts] TAPR TICC boxed (input protection)
davidwhess at gmail.com
Sat Apr 8 11:52:56 EDT 2017
On Thu, 6 Apr 2017 22:23:43 -0400, you 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
>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.
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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