the MOSFET
/ MOSS-fet /
MOSFET stands for metal-oxide-semiconductor field-effect transistor, and the name is a tiny cross-section of the device. A thin layer of insulating oxide (glass) sits on top of the silicon, and a metal (or poly-silicon) gate sits on top of the oxide. The gate is therefore separated from the silicon by an insulator, like one plate of a capacitor hovering just above the surface. Push the gate voltage high enough and the field reaches through the oxide and attracts a thin conducting channel into the silicon underneath, joining the source and the drain.
A MOSFET has four parts: the gate (the control, like the base of a BJT), the source and the drain (the two ends of the channel, where current enters and leaves), and the body or substrate (the bulk of silicon underneath, very often tied to the source). Because the gate is an insulated plate, no steady current flows into it: the channel between source and drain is controlled purely by the gate-to-source voltage. Below a certain THRESHOLD VOLTAGE there is no channel and the device is off; above it, a channel forms and the device conducts.
The MOSFET is the workhorse of modern electronics. As a switch it can turn loads on and off with very little loss, and the same device scaled to billions per chip forms every logic gate in your phone and laptop. The most important practical warning is that the gate oxide is microscopically thin: a static-electricity zap from your fingers (ESD) can punch a hole straight through it and kill the part silently. Handle MOSFETs with care and grounding.
An N-channel logic-level MOSFET with the source at ground: leave the gate at 0 V and the drain is open; raise the gate to 5 V (above its 2 V threshold) and source-to-drain conducts like a closed switch.
The gate-to-source voltage alone decides on or off.
The gate oxide is so thin that ordinary static (ESD) can destroy it instantly. Use a grounding strap and antistatic handling for any bare MOSFET.