Transistors A small signal controlling a large current. Used at the extremes — fully off or fully on — it is a switch, and that is the version you need first: the thing that lets a 20mA microcontroller pin drive a motor, a relay or a strip of LEDs.

Both come in two polarities. NPN / N-channel switch the ground side of a load (low-side switching) and are the easy, common case. PNP / P-channel switch the positive side (high-side), which is more awkward to drive from a microcontroller and is best left until you need it.

Everything in that diagram earns its place:

• The base resistor limits current into the base. Without it, the base-emitter junction is effectively a forward-biased diode across your output pin, and one of the two will die.
• The flyback diode absorbs the reverse spike when an inductive load switches off. Non-negotiable for relays, motors and solenoids.
• The common ground between the microcontroller and the load supply. The transistor switches the ground path; without a shared reference nothing works.

// switching 60mA with hFE (gain) around 100
I_base minimum = 60mA / 100 = 0.6mA
// drive 5-10x harder to guarantee full saturation
I_base chosen  = 5mA
R_base = (V_pin − 0.7) / 0.005 = (3.3 − 0.7) / 0.005 = 520Ω -> 470Ω

The 0.7V is the base-emitter drop, essentially fixed for silicon. Overdriving ensures the transistor is fully saturated — behaving like a closed switch with a small voltage drop — rather than partly on and dissipating the difference as heat. Datasheet gain figures are quoted at a particular current and vary enormously between individual parts, so designing at exactly the minimum is asking for a transistor that runs warm and switches unreliably.

MOSFETs are voltage-controlled, so the gate draws essentially no steady current and a good one has very low on-resistance — far more efficient for real loads. Three specifications decide the choice:

• V_GS(th) and the R_DS(on) test condition. This is the trap. A classic IRF540 needs about 10V on the gate to turn fully on; driven from a 3.3V pin it is barely conducting, runs hot, and behaves confusingly. You want a part explicitly specified with R_DS(on) measured at V_GS = 2.5V or 4.5V — that is what "logic-level" means. The IRLZ44N and AO3400 are common logic-level choices.
• R_DS(on) — the on-resistance. Multiply by your load current squared for the heat: 0.02Ω at 2A is 0.08W, negligible. The same 2A through 0.5Ω is 2W and needs a heatsink.
• Current and voltage ratings — with the usual generous margin, and remembering motors draw several times their running current at start-up.

Two practical additions: a gate resistor of 100–220Ω limits the current spike as the gate capacitance charges, and a 10kΩ pull-down from gate to ground keeps the MOSFET off while the microcontroller boots and its pins are still floating. Without the pull-down, motors twitch on power-up.

Failure modes: transistors usually fail shorted, so the load stays permanently on — an alarming but informative symptom. If your motor runs the instant power is applied and ignores the control signal, suspect a destroyed transistor and look for the missing flyback diode that killed it.