Conductors, insulators and semiconductors, n-type and p-type doping, the p-n junction diode, and the operation of LEDs and photodiodes.

What this key area is asking

The SQA wants you to distinguish conductors, insulators and semiconductors by their conduction, explain how n-type and p-type doping create charge carriers, describe how a p-n junction acts as a one-way diode, and explain the operation of an LED and a photodiode in terms of photons and electron-hole pairs.

Conductors, insulators and semiconductors

Pure silicon has each atom bonded to four neighbours, so its outer electrons are tied up in bonds and few are free to move. This is why a controlled impurity is added to make it useful in electronics.

n-type and p-type doping

The majority carriers are electrons in n-type and holes in p-type. Because doping massively increases the number of mobile carriers, even a tiny doping fraction transforms an insulating crystal into a useful semiconductor.

The p-n junction diode

When p-type and n-type materials are joined, electrons from the n-side and holes from the p-side combine near the boundary, leaving a thin region with no free carriers called the depletion layer. This layer sets up an internal electric field that opposes further crossing.

LEDs and photodiodes

A light-emitting diode (LED) is a p-n junction run in forward bias. As electrons cross the junction and recombine with holes, each electron drops to a lower energy state and the released energy comes out as a photon. The photon energy equals the band gap, so $E = hf$ sets the colour of the light.

A photodiode runs the process in reverse: incoming photons with enough energy create electron-hole pairs in the depletion layer, and these carriers produce a current. In photovoltaic mode the device generates its own emf from light (a solar cell); in photoconductive mode the light changes its conductivity so it acts as a fast light sensor.

Examples in context

The LEDs in phone screens, traffic lights and torches are forward-biased p-n junctions whose colour is fixed by the band gap of the semiconductor used. Solar panels on a roof are large arrays of photodiodes in photovoltaic mode, turning sunlight directly into an emf. A rectifier in a power supply uses diodes to convert AC to DC by passing current only on the half-cycles of one polarity. Camera sensors and barcode scanners use photodiode arrays to convert light patterns into electrical signals. In every case the one-way junction behaviour, and the swapping of energy between electrons and photons, is doing the work.

Try this

Q1. State the majority charge carrier in a p-type semiconductor. [1 mark]

• Cue. Holes (positive charge carriers).

Q2. State the bias direction in which a diode conducts. [1 mark]

• Cue. Forward bias (p-side to the positive terminal).

Q3. Explain why an LED emits a photon when it conducts. [2 marks]

• Cue. An electron recombines with a hole at the junction, dropping to a lower energy level, and releases the energy difference as a photon of energy $E = hf$.