James Arthur posted the lead, here's the thread.
Newsgroups: sci.electronics.design From: firstname.lastname@example.org (Tom Bruhns) Date: 3 Jan 2003 15:46:02 -0800 Local: Fri, Jan 3 2003 6:46 pm Subject: Low leakage parts
Some of you may recall some postings I've made over the past couple of years about the self-discharge rate of polyester and polypropylene caps. The time constants I saw were on the order of a few years for the polyester and over 50 years for the polyprops.
A month or so ago, someone asked about making a simple toggle circuit for turning 12V lights on and off, and I posted a couple solutions. One of them was a "this is really simple, but it probably won't work very well" circuit using just a capacitor to hold the voltage on the gate of a power mosfet. Well, I built that ckt, using an 0.01uF cap across the gate-source, and toggled it "on", so the cap was charged to about 12V. Then I disconnected the power and went on a holiday trip. Just came back, and it's still in the "on" state, three weeks later. 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...)