Transistors

MOSFET

A switch controlled by voltage. Put a few volts on the gate and a channel opens between drain and source that can carry amps. The gate itself draws almost no current, so a microcontroller pin can drive it.

Think of it as: A tap: a light touch on the handle (gate) controls a big flow of water.

Pins

G (gate): : The control input. Insulated from everything else, like a tiny capacitor.
D (drain): : Connects to the load (motor, LED strip). On a TO-220 the metal tab is also the drain.
S (source): : For an N-channel low-side switch, this goes to ground.

Big power MOSFETs come in TO-220 or D-PAK packages that bolt to a heatsink; small ones are SOT-23 chips. Always check the pin order in the datasheet: it varies between parts.

What to look for inside

This is a cross-section of an N-channel MOSFET switching a 12 V, 0.5 A load. The gate voltage slowly rises from 0 V to 5 V and falls again.
Below the threshold (about 1 V here) there is no channel and the switch is off.
Above it, + charge on the gate pulls electrons up under the oxide, forming a thin conducting channel (cyan) from source to drain.
On the way up and down it passes through a half-on state where it gets hot (red glow). Switching quickly through that region is what keeps it cool.

The numbers that matter

Spec	Typical
V_GS(th) Where it just starts to turn on. Not where it is fully on: look at R_DS(on) for that.	1 V to 4 V
R_DS(on) at V_GS Resistance when fully on. Check the value at the gate voltage you actually have.	5 mΩ to 100 mΩ
V_DS max The highest voltage it can block when off.	20 V to 100 V
I_D max / power Heat is I² × R_DS(on). Big currents need a heatsink.	amps · watts
Gate charge Q_g How much charge it takes to switch. Bigger means slower switching from a weak pin.	5 nC to 70 nC

Where you’ll use it

A microcontroller switching a motor, LED strip, heater or solenoid (low-side switch).
Reverse-polarity protection with almost no voltage loss (P-channel).
The fast switches inside buck converters and motor drivers.

Common mistakes

Leaving the gate floating. It picks up noise and turns half on and overheats. Add a 10 kΩ pull-down from gate to source.
Using a standard MOSFET from a 3.3 V pin. It only half turns on and gets hot.
No flyback diode across a motor or relay coil. The spike at turn-off can destroy the MOSFET.

Lessons that use it

Go deeper

I_D = \tfrac{k}{2}\,(U_{GS} - U_{th})^2

Saturation region: the current grows with the square of the gate overdrive.

R_{DS} \approx \tfrac{1}{k\,(U_{GS} - U_{th})}

Ohmic region: fully on, it behaves like a small resistor.

P = I_D^2\,R_{DS(on)},\quad t_{sw} \approx \tfrac{Q_g}{I_{gate}}

Conduction heat, and roughly how long a switch takes.