Hi there, people,
I'm not exactly an electronics engineer, but I always thought I had the very basics covered. Now I was trying to build a little circuit with a microcontroller driving a few LEDs and thought to use a MOSFET as a little driver transistor for the LEDs. And doing that on my breadboard, I came across an interesting little effect I'm not quite sure how to explain.
So I got some 2N7000 n-channel MOSFETs(in a TO-92 package) I still had around here and started by testing if I had calculated the current-limiting resistor correctly, so I just build the circuit up, connected the source to ground, drain to the LEDs and hooked the gate directly to my 5V supply. And my LEDs lit up nicely, no overcurrent, nothing getting hot, fine so far.
However, when I disconnected the gate voltage, I noticed that the LEDs didn't go off directly, they slowly faded out over like three seconds.
With a 10kOhm pull-down resistor on the gate, the transistor shuts off immediately.
Now I think that the only thing that could cause this kind of behaviour would be the gate capacitance, keeping the gate charged for a moment and thus leaving the transistor conductive.
So here's the question: Is that true? Because the length of time the transistor stayed on kind of baffled me, I mean it must be discharging via gate-source leakage, right?
What's a typical small signal MOSFET gate capacitance going to be? I couldn't find any info on that in the datasheet of the 2n7000, but assuming a gate-source resistance of 1.5GOhms(10nA leakage @ 15V), a discharge time of 3s and a charged voltage of 5V, I get some 500pF gate capacitance. That sounds like quite a lot to me, or am I calculating this wrong here?
Greetings
Laertes