Inside the transistor
A valve for current controlled by a small current (a BJT) or a voltage (a MOSFET). It amplifies, it switches, and billions of them make every chip. Every picture below is computed while you watch.
What it is
Two p-n junctions back to back (a BJT) or a channel induced by a field (a MOSFET). In a BJT, a small base current lets a much larger collector current flow: Ic = β · Ib. In a MOSFET, the gate voltage draws a conducting channel into existence between source and drain, and the gate itself takes no steady current: Id = ½ k (Vgs − Vth)² in saturation.
How it’s made: the planar process
The BJT, live
An NPN cross-section: electrons injected from the heavily doped emitter cross the very thin base and are swept into the collector. A few recombine in the base, and the base current replaces them: that small current controls the large one. Below, the output curves for several base currents and where you are.
The MOSFET, live
An n-channel MOSFET: the gate sits on a thin oxide above p-type silicon. Raise Vgs past the threshold and its field pulls electrons up to the surface: an n-type channel appears between source and drain. Raise Vds and the channel thins toward the drain until it pinches off: the current then stops growing (saturation).
A textbook square-law MOSFET: Vth 2 V, k 0.2 A/V².
As a switch: fully on, or a heater
A switch is either fully on (almost no voltage across it) or fully off (no current). Halfway, it carries the current with volts across it, and V × I becomes heat. The classic maker mistake: a standard MOSFET driven from a 3.3 V microcontroller pin.
A 12 V, 2 A lamp. Typical figures, simplified; the heating is shown sped up.
What goes wrong
- Half on: too little gate (or base) drive, or switching too slowly: it heats. See above.
- Inductive spikes: switching off a relay or motor without a flyback diode (see the inductor).
- ESD: a MOSFET’s gate oxide is a few tens of nanometres; a static spark can punch through it.
- Heat and second breakdown: past the safe operating area, current crowds into a hot spot and it fails, usually shorted.
Use it on the bench
- Transistor as a switch — a BJT switching a load (about it)
- BJT current gain (β) — measure a BJT’s current gain (about it)
- MOSFET low-side switch — a MOSFET low-side switch (about it)
- Common-emitter amplifier — a common-emitter amplifier (about it)
- PWM actuator: DC motor through a MOSFET — PWM: a motor through a MOSFET (about it)
Check yourself
Exam-style questions (ECE, EE and NC-II reviewers ask these). Each answer links to the scene that shows it.
1. A BJT with β = 100 has 50 µA of base current in its active region. The collector current is about:
Ic = β · Ib = 100 × 50 µA = 5 mA. Below saturation the collector current follows the base current, almost independent of Vce. See the scene ▸
2. Where does a BJT’s base current go, physically?
Most electrons from the emitter cross the thin base to the collector; a few recombine there. The base current supplies those holes. Watch the orange ones in the BJT scene. See the scene ▸
3. A MOSFET’s gate current in steady state is:
The gate is a capacitor plate. It takes current only to charge and discharge, which matters when switching fast. See the scene ▸
4. What happens below a MOSFET’s threshold voltage Vth?
The gate field must draw enough electrons to the surface to invert the p-type body into an n channel. Slide Vgs across 2 V in the MOSFET scene. See the scene ▸
5. Driving an IRF540N’s gate from a 3.3 V microcontroller pin to switch a 2 A load usually:
Its Rds(on) is specified at Vgs = 10 V. At 3.3 V it barely conducts, and Vds × Id becomes watts of heat. Use a logic-level MOSFET or a gate driver. Try it in the switch scene. See the scene ▸
6. A transistor used as a switch should be:
Fully on: little voltage across it; fully off: no current. Either way V × I is small. Halfway is where it heats. See the scene ▸
7. The planar process builds transistors by:
Photolithography defines where each dopant goes; a chip needs dozens of masked layers aligned to nanometres. See “How it’s made”. See the scene ▸
8. In saturation, a BJT switch has between its collector and emitter about:
Vce(sat) is small for a single transistor; a Darlington cannot saturate its output transistor fully, so it drops more and heats more. See the scene ▸
Inside the other parts
The resistor · the capacitor · the inductor · the transformer · the diode and LED · the battery · all the parts