I have been trying to drive a 12 V LED array with an IRF3205 MOSFET. I wanted to do this:
However, when I tested it, the LED was only being given 6 V and the MOSFET was dropping 10 V across the drain and source. Going through the things that could be wrong, I found that giving the gate 5 V causes the problem to be solved, and the LED gets the full voltage of the 12 V power supply. The LEDs draw about 100 mA when they are being driven correctly in this way.
What is going on, and is there a way I could drive this chip with a 3.3 V Raspberry Pi GPIO pin?