Semiconductors, the p-n diode and rectification: WASSCE Physics

Every phone, radio and solar charger is built on a handful of tiny components made of silicon. Semiconductors conduct better than insulators but worse than metals — and by adding traces of other elements, engineers control exactly how.

Learning objectives

By the end of this lesson you should be able to:

distinguish conductors, insulators and semiconductors using the idea of free charge carriers
explain intrinsic and extrinsic (n-type and p-type) semiconductors
describe the p-n junction diode and its forward and reverse bias
explain half-wave and full-wave rectification

Conductors, insulators and semiconductors

Conductors	Semiconductors	Insulators
many free electrons	few free charge carriers	almost no free charge carriers
e.g. copper, aluminium	e.g. silicon, germanium	e.g. glass, rubber, plastic
resistance rises with temperature	resistance falls with temperature	very high resistance

In terms of energy bands: in a conductor the valence and conduction bands overlap; in a semiconductor there is a small energy gap that some electrons can cross when heated; in an insulator the gap is large.

Intrinsic semiconductors

A pure semiconductor (silicon or germanium) is intrinsic. At room temperature, a few electrons gain enough energy to break free of their bonds, leaving behind holes (gaps that behave like positive charges). Both electrons and holes carry current, and they are equal in number. Heating frees more of them, so the resistance falls (the principle of the thermistor).

Extrinsic semiconductors (doping)

Definition

Doping is the addition of a tiny, controlled amount of impurity to a pure semiconductor to increase its conductivity.

n-type	p-type
doped with a pentavalent element (5 outer electrons): phosphorus, arsenic, antimony	doped with a trivalent element (3 outer electrons): boron, aluminium, gallium, indium
the extra electron is free: electrons are the majority carriers	a missing electron creates a hole: holes are the majority carriers
the impurity is a donor	the impurity is an acceptor

Both types are electrically neutral overall.

The p-n junction diode

When p-type and n-type material are joined, electrons near the junction diffuse into the p-side and fill holes, and holes diffuse the other way. This creates a thin depletion layer with no free carriers, across which a small junction (barrier) potential forms (about 0.6–0.7 V for silicon).

Diode symbol with an arrow showing the direction of conduction; the diode conducts when forward-biased and blocks current when reverse-biased; below, an a.c. input wave and the half-wave rectified output with only positive half-cycles

Figure 1. The diode and half-wave rectification. WASSCE Tutor diagram.

Forward bias: p-side connected to +, n-side to −. The depletion layer narrows; once the p.d. exceeds about 0.6 V, a large current flows.
Reverse bias: p-side to −, n-side to +. The depletion layer widens; almost no current flows (only a tiny leakage current).

A diode therefore lets current flow in one direction only — the direction of the arrow in its symbol.

Rectification

Definition

Rectification is the conversion of alternating current into direct current.

Half-wave rectification: a single diode in series with the load lets through only the positive half-cycles. The output is a series of humps with gaps.
Full-wave rectification: a bridge of four diodes (or two diodes with a centre-tapped transformer) turns the negative half-cycles positive too, giving a continuous series of humps.
Smoothing: a large capacitor across the output charges at the peaks and discharges between them, giving an almost steady d.c.

In Sierra Leone

A phone charger plugged into a 230 V socket contains a small transformer (or switching circuit) to lower the voltage, a bridge rectifier of four diodes, and a smoothing capacitor, giving the steady 5 V d.c. that a phone needs.

Other diodes

Light-emitting diode (LED): emits light when forward-biased; very efficient; used in indicator lights, torches, LED bulbs and screens.
Photodiode and solar cell: produce a current when light falls on the junction.
Zener diode: designed to conduct in reverse at a fixed voltage; used to regulate voltages.

Key points

Semiconductors (Si, Ge) have resistance that falls with temperature.
n-type: pentavalent dopant, electrons are majority carriers. p-type: trivalent dopant, holes are majority carriers.
A p-n diode conducts when forward-biased and blocks when reverse-biased.
Rectification: half-wave (one diode), full-wave (bridge), smoothed by a capacitor.

WASSCE examination focus

WAEC tests this

Distinguish n-type and p-type semiconductors (dopants and majority carriers), explain forward and reverse bias of a p-n junction, sketch the I–V characteristic of a diode, and draw circuits and output waveforms for half-wave and full-wave rectification. Explain the role of the smoothing capacitor.

Quick check

What are the majority charge carriers in an n-type semiconductor?
In which bias does a diode conduct?
What is the purpose of a capacitor connected across the output of a rectifier?

Answers: 1. Electrons. 2. Forward bias (p-side to positive). 3. To smooth the output, giving an almost steady d.c.

Semiconductors, the p-n diode and rectification