I am building a circuit on a breadboard to see if I can get a 2N7000 NMOS to switch using a 3.3V V_gs signal. As far as I know, I've set up the circuit properly:
simulate this circuit – Schematic created using CircuitLab
However,
- This circuit causes the LED to light up regardless if V_gs is 3.3V or 0V. The transistor conducts no matter what.
- It works properly when I connect a 1MΩ resistor between Gate and Source. Now, when V_gs is 0V, the LED turns off and when V_gs is 3.3V, the LED turns on. I discovered this when I was probing Gate and Source with a multimeter, and I wondered if an 1MΩ resistor would achieve the same effect.
Can someone explain this phenomenon for me? Thank you very much.
Best Answer
With your multimeter or the resistor, you are providing a path to ground at the gate. The gate is essentially a small capacitor, and is very high impedance. This means that charge may accumulate, and raise \$V_\mathrm{gs}\$ sufficiently to allow the MOSFET to conduct. Hence, your LED was lighting up. It is always good practice to include this "pull down" resistor.