C1 · BJT

How a BJT Works: A Small Knob Holds a Large Flow

A transistor governs a far larger collector current with a tiny base current. Raise the base current yourself and watch the collector current grow by a fixed multiple.

Raise the base current to grow the collector flow

The emitter on the left pours carriers into the base. The base is thin, so almost all are swept right into the collector and only a tiny part leaks out the base terminal. Raise the base current and the collector flow thickens by a large multiple.

Base current IbIb = 2 μA

A small Ib controls a large Ic

Ib = 2 μA  →  Ic = 0.2 mA

β = Ic / Ib = 100

The base current is nearly zero. The emitter sends almost no carriers, so the collector does not flow either.

Barely on

Two junctions around a thin base

A BJT is built from two pn junctions placed back to back. A thin base sits in the middle, with a heavily doped emitter on one side and a collector on the other. In operation the emitter junction is forward-biased and the collector junction reverse-biased. What makes it different from merely wiring two diodes together is exactly that the base is very thin.

The thin base is the secret

The forward emitter junction pours an enormous number of carriers into the base. Because the base is thin and lightly doped, those carriers barely recombine while crossing it. Then the strong field of the reverse collector junction sweeps almost all of them into the collector. Only about one in a hundred recombines in the base and leaves through the base terminal, and that small share is precisely the base current.

Current control: a small knob, a large flow

Because the recombination fraction is constant, the collector current is always the same multiple of the base current. That multiple is the current gain β, often around a hundred. Raise the base current a little and the collector current rises β times as much. Holding a large collector flow with the small knob of the base current, this current control is the basis of the amplification built in the next units.

ObserveIc = β Ib

The collector current is β times the base current.

ChooseIe = Ic + ?

The emitter current is the sum of the collector and base currents.

Fill inIe = (β + ?) Ib

Combined, the emitter is (β+1) times the base current.

On your ownα = Ic / Ie = β / ?

α is collector over emitter, close to one.

Back to the first screen

Each time you raised the base current, the collector flow thickened by a large multiple. What set that thickness was the thin base. The carriers the emitter poured in crossed the thin base without recombining and were nearly all swept into the collector. The handful that leaks to the base is the base current, the bundle that goes to the collector is the collector current, and their ratio is β. Holding a large flow with a small knob, this current control is the essence of the transistor.

A BJT is two pn junctions around a very thin base. The forward emitter pours carriers into the base, and because the base is thin most are swept without recombining into the reverse collector. So a small base current controls a β times larger collector current (Ic = β Ib). This current gain is the basis of amplification, with Ie = (β+1) Ib and α = β/(β+1) ≈ 1.

What comes next

You now hold the current gain. But to use it, the transistor must be set up already on, parked at a point where suitable base and collector currents flow. The next unit sets that operating point with resistors: DC bias. Before amplifying a signal, you first build the place where it will be amplified.