Junction Transistors

Sandwich a very thin slice of one type of semiconductor between two of the other type and a tiny current can control a big one. That is a transistor: the amplifier, the oscillator and, billions of times over, the computer chip.

Structure#

A junction transistor has three layers. In an n-p-n transistor a thin p-type layer sits between two n-type layers; in a p-n-p it is the other way round.

• Base (middle): very thin (about 1 μm) and lightly doped.
• Emitter: heavily doped; it supplies the charge carriers.
• Collector: moderately doped, and larger, to collect them. The collector–base junction has a larger area than the emitter–base one.

Each layer has a terminal, so the transistor is a three-terminal device. In normal use the emitter–base junction is forward-biased and the collector–base junction is reverse-biased. In the symbol, an arrow on the emitter shows the direction of current through the emitter–base junction: outward for n-p-n (current from base to emitter), inward for p-n-p.

How it works (n-p-n)#

The forward bias makes the emitter pour electrons into the base. The base is so thin, and has so few holes, that almost none of the electrons recombine there. Instead they reach the collector–base junction, where the reverse bias’s field sweeps them into the collector. They return to the emitter through the external batteries.

So the collector current I_C is nearly the whole emitter current I_E. The small difference, from electrons that leave through the base terminal, is the base current I_B, typically 1–5 % of I_E. Holes play a small part too, but the lightly doped base keeps it minor. By Kirchhoff’s junction rule:

Why a small base current controls the collector current: a few electrons from the emitter recombine with holes in the base, and some holes diffuse into the emitter. If those lost holes were not replaced, the base would charge negatively and choke off the flow from the emitter. The base current replaces them. Increase it a little and the collector current rises a lot.

Three ways to connect it

One terminal is shared between the input and output circuits and kept at zero potential: common-emitter (the most used), common-base or common-collector.

The common-emitter amplifier#

A battery biases the base–emitter junction forward; another holds the collector positive. The signal v_i is added in series in the base circuit, and the output is taken across a large load resistor R_L in the collector circuit.

• The forward-biased base–emitter junction has a small input resistance R_BE, so the signal changes the base current by ΔI_B = v_i/R_BE.
• The collector current changes by ΔI_C = βΔI_B.
• The voltage across the load changes by v_o = R_LΔI_C.

The oscillator#

An oscillator produces an alternating voltage of chosen frequency using only DC supplies. It combines a common-emitter amplifier with an LC circuit that resonates at

Part of the output is fed back to the input through the LC network. The amplifier boosts it, part is fed back again, and so on: the circuit sustains itself with no external signal. Only the frequency ν₀ that the LC network favours builds up, and changing L or C tunes it.

Characteristics#

For an n-p-n transistor in common-emitter mode, the base circuit is the input and the collector circuit the output.

• Input characteristic (I_B against V_BE): like a forward-biased diode. Almost no current until V_BE passes the barrier, then a rapid rise, though I_B stays in microamperes because most carriers go on to the collector.
• Output characteristics (I_C against V_CE, one curve per I_B): I_C rises steeply at small V_CE, then levels off at a value set mainly by I_B. Larger base currents give higher curves.