discharge tranny & leakage

[Martin Czech]

Thanx for your answers

> > I'm looking for a good available discharge tranny,
> > be it bipolar or jfet with low on resistance
> 

SNIP

> What about a MOSFET? You can get cheap MOSFETs with very low
> on-resistance.
> 

SNIP

>Use a MOSFET, like a 2N7000 or other types (Siliconix and IR have a wide selection).

Ok,
it's my fault, the question was too unprecise.
What I really wanted to know was a good switching
device with low JUNCTION LEAKAGE, not gate or base.
That is low current
in "off"-mode, even if it's hot (50C).
To discharge capacitors with little effect on charging time.

Does the mosfet drain-junction offer any advantage over
the normal bipolar collector-junction ?
What about threshold voltage variation during lifetime?
And consequently subtreshold currents ?
This is a big problem in submicron processes today,
but I don't know if it is also in discrete semiconductors.

And what about ESD? I mean, you can damage a junction very
quickly, (in fact, I see 100 times more defect junctions
then gate oxides, whatever the books say. This is due to the fact that
reverse biased junctions avalanche very quickly, and then may
fall into snapback and finaly into second breakdown)
and get unexpected leakage, and lifetime problems,
but you won't even notice the discharge into the semiconductor.
And if it's soldered into the pcb (sockets ? no good),
it's not so easy to find out the source of leakage.

m.c.