From: Winfield Hill
Subject: Re: Low leakage parts
Date: 3 Jan 2003 17:26:41 -0800
Organization: Rowland Institute
References: <email@example.com> <8npR9.16063$jM5.firstname.lastname@example.org>
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Phil Allison wrote...
> Tom Bruhns wrote
>> Some of you may recall some postings I've made ... [ snip ]
>> Sooo...I'd say that modern power mosfets also have pretty low gate
>> leakage current. To hold the voltage above the nom. 3V required to
>> turn the mosfet on, the average leakage must have been less than 50fA,
>> assuming 21 days, 0.01uF and 9V delta (and no arithmetic errors).
>> (It was cool, about 18C, and likely wouldn't do quite so well inside
>> a car with the windows rolled up in Phoenix in the summer...)
> ** 21 days is 1.8 exp 6 seconds, divided by 0.01 exp -6 gives
> 1.8 exp 14 ohms as the resistance in the cap, fet and wiring etc.
> I don't believe it - do you ??
> I suspect there was a DC leakage path HOLDING the 0.01 uF cap voltage
> up that was the real story here.
Phil, having made similar measurements and calculations myself, I'd
say that you could well be wrong. As Tom and I have both observed in
the past, leakage resistances in excess of 100T-ohm are not all that
unusual. Moreover, we know that sub-5fA leakage currents have been
routinely measured for NSC mosfet opamps, 10x below Tom's value.
On the other hand, FET gate leakages often tend to turn the part on.
Tom can easily expand his experiment to test such a hypothesis simply
by completely discharging the capacitor and waiting to see how long
the FET stays off. If the off state exceeds 21 days, I'd say he's
made a good point.