Ian Stirling wrote...

Paul Burridge wrote:
Helmut Sennewald wrote:

Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

Is it 1nA? I thought it was 5. No matter.
Yes, I'd expected someone to point out that the "negligible current"
point was the likely problem area. I can't honestly say that I have,
because my DVM drops out at 0.01mA! However, in the context of the

Turn it to voltage. Most meters (the four I've measured) have a 1Mohm
or so input impedance on the 200mv range. This is 200nA full scale.
Probably best to do the test twice, with the meter leads reversed though.

The multimeters on my bench have infinite input impedance on the 200mV
scale, although some let you turn on an internal resistor. We can use
external 10M or 100M if we like, which gives 1pA and 0.1pA measurement
resolution with a 4.5-digit meter.

The relevant "subthreshold" formula is Id = k e^(Vgs - Vt), which is
an exponential equation that clearly shows there's no sudden threshold
for FET current = negligible. Paul can take a look at AoE page 123,
and observe the measured gate-voltage to drain current relationship for
a typical MOSFET, which shows the standard smooth 100 or 150mV/decade
(p or n-type) gate-voltage change over a wide 7-decade current range.

1 or 5nA makes no difference? Nope, that apparently small detail makes
a predictably large 70 to 100mV difference. One must pay attention to
the specifications in this matter, 1nA, 1uA, whatever - it's a big deal.

BTW, as I've pointed out several times, power MOSFET spice models are
completely wrong in this region, just forget considering their results.


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)