From: Winfield Hill
Subject: Re: Low leakage parts
Date: 4 Jan 2003 07:24:43 -0800
Organization: Rowland Institute
References: <firstname.lastname@example.org> <email@example.com>
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John Larkin wrote...
> I did a post a while back: a 2N7000 with a 9-volt battery and an
> LED in its drain, gate hanging open. This can be set on or off
> and stay in that state for many days. You can also perch the
> gate voltage just at the turnon point and get the LED to stay
> on dimly, with no perceptable brightness change over many hours.
> ... I figured that the gate leakage was in the 100 electron/second
> range, maybe much less... attoamps.
At low currents, such as 1 to 5mA, the transconductance of a
2n7000 approaches that of a transistor, where an 18mV change
makes a 2x current difference. If we assume your FET stayed
within 2x current over 5 hours, that's dV/dt < 1uV/s. Given
a 40pF gate capacitance, we get I = C dV/dt = 4E-17A, or 40aA,
which is 250 e-/sec.
While impressive that's not unreasonable, and it demonstrates
that under good conditions both air and semiconductor plastics
have much better insulation capabilities than some would think.
You don't mention the FET's gate-drain voltage, but assuming
2V, we get R > 5E16 or 50000000G, or 50M Gig-ohms, another
interesting value. The high-quality plastics used by most
semiconductor manufacturers must be impressive, especially
considering that the gate leakage is likely in the gate oxide
rather than the plastic.