Conductors, semiconductors and p–n junctions
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This whole topic is qualitative — there are no calculations. The marks are for using precise band-theory wording, so learn the exact phrases in the model answers below.
Energy bands: conductors, insulators & semiconductors
Key idea
Electrons in a solid sit in energy bands separated by gaps. Whether a solid conducts depends on the band populations and the size of the gap.
- In a single atom electrons occupy discrete energy levels. When many atoms bond into a solid, these levels spread into energy bands separated by gaps.
- The two bands that matter:
- valence band — the highest band that is normally filled with the atoms' outer electrons;
- conduction band — the band above it; electrons here are free to move and carry current.
- For a solid to conduct it needs both free electrons and accessible empty states to move into.
- Conductors (metals): a band is only partly filled, or the valence and conduction bands overlap. Electrons move into empty states with almost no extra energy, so metals conduct well.
- Insulators: the valence band is full and the gap to the empty conduction band is large. At room temperature there is not enough energy to lift electrons across the gap, so no conduction.
- Semiconductors: the valence band is full but the gap is small. At room temperature some electrons gain enough energy to cross into the conduction band, allowing some conduction.
- Temperature effect: heating a semiconductor lets more electrons reach the conduction band, so its conductivity increases as it gets hotter (the opposite of a metal).
Model answer — Explain (band theory)
Explain, using band theory, why an insulator does not conduct electricity but a semiconductor conducts a little at room temperature. 3 marks
Model answer
In an insulator the valence band is full and the gap to the conduction band is large, so at room temperature electrons cannot gain enough energy to reach the conduction band — there are no free electrons, so no conduction. In a semiconductor the band gap is small, so at room temperature some electrons gain enough energy to move into the conduction band, allowing some conduction.
Model answer — Explain (temperature)
A semiconductor's resistance falls as its temperature rises. Explain this using band theory. 2 marks
Model answer
Heating gives more electrons enough energy to cross the small band gap into the conduction band; with more free charge carriers the material conducts better, so its resistance decreases.
Practice 1
Which statement(s) is/are correct?
I — In metals the highest occupied band is not completely full.
II — In insulators the highest occupied band is full.
III — The valence–conduction gap is smaller in semiconductors than in insulators. 1 mark
Answer
All three (I, II and III) are correct.
I — metals have a partly filled (or overlapping) highest band.
II — insulators have a full valence band.
III — the gap is smaller in a semiconductor than in an insulator.
Doping & the p–n junction
Key idea
Doping a semiconductor with impurities makes p-type or n-type material; joining them forms a p–n junction with an electric field across it.
- Pure semiconductors barely conduct. During manufacture they are doped — small amounts of impurity atoms are added to increase conductivity, giving two types:
- n-type — doped to provide extra free (negative) electrons as charge carriers;
- p-type — doped to provide "holes" (effectively positive carriers / missing electrons).
- When p-type and n-type are formed in adjacent layers, the boundary is a p–n junction. An electric field exists across the junction; its properties are used in diodes, LEDs and solar cells.
- Bias (the p.d. applied across the junction):
- forward bias — reduces the electric field in the junction (lets current flow);
- reverse bias — increases the electric field in the junction (blocks current).
Model answer — State / Explain
State what is meant by forward bias, and describe its effect on the electric field at a p–n junction. 2 marks
Model answer
Forward bias is connecting the supply across the junction so that it reduces the electric field in the p–n junction, allowing charge to cross and a current to flow. (Reverse bias would increase the junction field.)
Model answer — Explain
Explain how doping changes a pure semiconductor, and name the two types of doped semiconductor produced. 2 marks
Model answer
Doping adds impurity atoms that increase the conductivity of the semiconductor; the two types produced are p-type and n-type.
Practice 2
A p–n junction is reverse biased. State the effect on (a) the electric field at the junction and (b) the current that can flow. 2 marks
Answer
(a) Reverse bias increases the electric field at the junction.
(b) (Almost) no current flows — the junction blocks it.
p–n junction devices: LEDs & solar cells
Key idea
An LED turns p.d. into light; a solar cell turns light into p.d. — both are p–n junctions run in opposite directions.
- LED (light-emitting diode) — a forward-biased p–n junction diode that emits photons. The forward-bias p.d. makes electrons move from the conduction band of the n-type towards the conduction band of the p-type. Photons are emitted when electrons "fall" from the conduction band into the valence band. (Higher-energy / bluer light needs a bigger band gap.)
- Solar cell (photovoltaic cell) — a p–n junction designed so that a p.d. is produced when photons are absorbed (the photovoltaic effect). Absorbed photons raise electrons from the valence band into the conduction band; the junction's field drives the conduction-band electrons towards the n-type side, so a p.d. is produced across the cell.
- Memory hook: LED = electricity → light (electrons fall, photon out); solar cell = light → electricity (photon in, electron raised, p.d. out).
Model answer — Explain (band theory)
Using band theory, explain how a forward-biased LED emits light. 3 marks
Model answer
The forward-bias p.d. makes electrons move from the conduction band of the n-type towards the conduction band of the p-type across the junction. When these electrons fall from the conduction band into the valence band, they lose energy that is emitted as photons (light).
Model answer — Explain
Explain how a solar cell produces a potential difference when light shines on it. 3 marks
Model answer
Absorbed photons give energy to raise electrons from the valence band into the conduction band. The electric field at the p–n junction drives these conduction-band electrons towards the n-type side, so a potential difference is produced across the cell (the photovoltaic effect).
Practice 3
State the energy change that takes place in (a) an LED and (b) a solar cell. 2 marks
Answer
(a) LED: electrical energy → light energy.
(b) Solar cell: light energy → electrical energy.