# 2N7000 NMOS only switches properly when gate and source is connected with 1MΩ

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.

• Is your transistor mounted upside down? Feb 1, 2019 at 10:20

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.