Thanks, all, BTW.

On 21 Sep 2004 19:50:22 -0700, Winfield Hill
wrote:

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.

I admit I never thought of doing it this way. Incidentally, talking of
very low currents, on P.170 you've reproduced a picture of a static
damaged MOSFET. I assume from the commentary that this one was totally
trashed, but you've stated before that these devices can be partially
blown and still function, albeit to an impaired extent. Could one
feasibly measure any such 'minor damage' (through static) by checking
for picoamp range current leakage across the insulating layer?

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.

Oh bugger.

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

Yes, I think the same caveat applies for simulating FETs - and that's
just in the active region!

--

"What is now proved was once only imagin'd." - William Blake, 1793.