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

[Charles Steinmetz]

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.

> Out of curiosity, and I tried to look this up years ago, what doping
> is used for PNP RF transistors and saturated switches if it is not
> gold?  Does it also increase leakage?

Gold doping doesn't affect the speed of BJTs in the active region very 
much -- its purpose is to reduce minority carrier lifetime and, thereby, 
to reduce storage time when a transistor recovers from saturation.  I'm 
not sure how manufacturers deal with this in the case of PNPs.  [Note 
that the list of fast PNP small-signal switching transistors is very 
short, and the fastest of them are slower than the slowest fast NPN 
switches.]

> 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.

> 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