C1 · BJT

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

A transistor is a device that governs a much larger collector current with a tiny base current. In this unit you raise the base current yourself and see how many times larger the collector current grows.

Grow the collector flow with the base current

The picture is a cross-section of a transistor: from left to right, the emitter, the thin base in the middle, and the collector. Because the base is so thin, almost all the gold carriers the emitter pours in are swept on into the collector, and only a tiny share leaves through the base terminal at the bottom. Raise the base current and the bundle heading for the collector grows many times thicker.

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 almost 0. The emitter sends almost no carriers, so no current flows in the collector either.

Barely on

Two junctions around a thin base

A BJT is made by placing two pn junctions back to back. A thin base sits in the middle, with a heavily doped emitter on one side and the collector on the other. In operation the emitter-side junction is forward-biased and the collector-side junction reverse-biased, and what sets it apart from simply joining two diodes is that the base is very thin.

The thin base is the secret

The forward-biased emitter junction pours enormous numbers of carriers into the base. Because the base is thin and lightly doped, the carriers barely recombine while crossing it, and the strong field of the reverse-biased 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 the base current.

Current control: a small knob, a large flow

Because the recombination ratio is constant, the collector current is always the same multiple of the base current. That multiple is the current gain β, commonly around a hundred. Raise the base current just a little and the collector current follows, β times as much. Holding a large collector flow by the handle of a small base current, this current control is the foundation of the amplification built in the next units. β, however, varies widely even between transistors of the same part number, so real circuits are designed not to depend on its exact value.

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 current is (β+1) times the base current.

On your ownα = Ic / Ie = β / ?

α is the collector current over the emitter current, close to 1.

Back to the first screen

Each time you raised the base current, the gold bundle heading for the collector grew many times thicker. What set that thickness was the thin base: the carriers the emitter poured in crossed it without recombining and were almost all swept into the collector. The handful that leaves through the base is the base current, the bundle going to the collector is the collector current, and the ratio between them is β. Holding a large flow with a small handle, this current control is the essence of the transistor.

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

What comes next

Now you hold current gain in hand. But to use it, the transistor must first be set in a switched-on state, at a point where suitable base and collector currents flow. The next unit shows DC biasing, which uses resistors to set that operating point. Before enlarging a signal, you first prepare a place to enlarge it.