From: John Larkin
Subject: Re: Low leakage parts
Date: Sat, 04 Jan 2003 11:04:12 -0800
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On 4 Jan 2003 07:24:43 -0800, Winfield Hill
> 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.
> - Win
Try this some time, it's fun. I have a 9-volt battery wired to a
resistor, an LED, and a pair of alligator clip leads, that I keep on
my desk for checking continuity and LED polarities and stuff like
that. So I just clipped it to the 2N7000 source and drain. With the
LED dim, Vd was likely around 6 volts.
You can set the gate voltage by touching the battery terminals with
two fingers, and touching the gate briefly with a finger from the
other hand... a human potentiometer. You can also charge-pump the gate
by moving a pencil point from b+ or b- to the gate many times. This
would be a very nice classroom experiment. If you blast the s-g air
gap with alphas from a smoke detector source, the LED dims; blast the
g-d gap, and it gets brighter.
Simple things like this amuse me disproportionately.
But you're not allowed to play with stuff like this until you finish
the Third Edition.