[sdiy] BS170, low leakage

[Czech Martin]

I just tested the D-NMOST BS170.

The objective was to find a low input leakage device.
This comes handy for several applications where
very little currents are involved.

The question was if it has any ESD protection at the gate.
Data sheets usually do not mention this.
http://www.fairchildsemi.com/ds/BS/BS170.pdf
Obviously none!

First I destroyed one. My soldering iron had the ground
not connected. So ESD can happen at home as well!
Ok, next one, with table matt, wrist strap and iron
all well conected to mains protective ground:
Viola: about 200fA!
I measured this with a 1.5nF polystyrol cap at the input,
watching the BS170 source follower droop.
Of course, one should not be fooled by cap "hysteresis",
which will lead to some droop on it's own.

This is pretty good. The leakage will not increase on a cheap
pcb (paper insulation), however with moisture/aging this could
degrade. A CA3140 has a much higher leakage current,
I measured a few nA, always going down to negative supply.
This is because of leaky zener diode ESD protection.

The BS170 drain source leakage is higher, about 1nA, but there
are circuits to cure this.

So perhaps the BS170 is a good device for switching current
mirrors etc.

 
              
m.c.