Diodes: n-type and p-type material, the p-n junction and depletion layer, forward and reverse bias, the diode I-V characteristic, and the rectifier, Zener, light-emitting and photodiode.

What this dot point is asking

The diode is the first active component on the course and the gateway to understanding all semiconductors. WJEC expects you to explain the p-n junction in terms of n-type and p-type material and the depletion layer, sketch and read the diode I-V characteristic, and know the behaviour and use of four diode types: the rectifier, the Zener, the light-emitting diode and the photodiode. Zener regulation calculations and depletion-layer explanations are reliable exam earners.

The answer

The p-n junction

This built-in barrier is what gives the diode its one-way behaviour. The barrier must be overcome before majority carriers can cross the junction.

Forward and reverse bias

The diode I-V characteristic

The characteristic curve is flat (near zero current) for reverse and small forward voltages, then rises sharply once the forward voltage passes the threshold. The sharp knee at about $0.7\,\text{V}$ is the diode's signature, and it is strongly non-ohmic: resistance is high below the threshold and low above it.

The four diode types

Examples in context

Example 1. A bridge rectifier in a power supply

Four rectifier diodes arranged as a bridge let both halves of the AC waveform drive the load in the same direction, giving full-wave rectified DC. Each diode conducts on alternate half-cycles, and the $0.7\,\text{V}$ drop across the conducting pair is why bridge outputs are slightly below the peak input.

Example 2. A Zener reference for a comparator

A Zener diode in reverse breakdown supplies a rock-steady reference voltage to a comparator, far more stable than a plain potential divider because the breakdown voltage barely changes with current. This is why precise threshold circuits use a Zener rather than two resistors.

Example 3. A photodiode light meter

Operated in reverse bias, a photodiode passes a current proportional to the light intensity. Feeding this current into an op-amp (a transimpedance amplifier) converts it to a voltage, giving a fast, linear light sensor used in cameras and optical communication receivers.

Try this

Q1. State the approximate forward voltage at which a silicon diode begins to conduct, and explain what happens to the depletion layer at this point. [2 marks]

• Cue. About $0.7\,\text{V}$; the forward voltage has narrowed the depletion layer enough to overcome the potential barrier, so current flows.

Q2. A red LED dropping $1.8\,\text{V}$ is driven from a $5.0\,\text{V}$ supply and should carry $12\,\text{mA}$. Find the series resistor needed. [2 marks]

• Cue. Resistor drops $5.0 - 1.8 = 3.2\,\text{V}$; $R = \frac{3.2}{0.012} = 267\,\Omega$, so use a standard $270\,\Omega$.