Doping Semiconductors
Explain how doping increases conductivity by changing carrier number density, and distinguish n-type vs p-type semiconductors (A Level Physics).
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The core idea
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Learning objectives
- Explain n-type and p-type doping, donor and acceptor levels and majority carriers.
1. Definitions (Must Know)
A. Intrinsic vs extrinsic semiconductor
- An intrinsic semiconductor is a pure semiconductor crystal. Conduction is due to thermally generated electron–hole pairs.
- An extrinsic semiconductor is a doped semiconductor. Doping increases conductivity by increasing the number density of charge carriers.
B. Doping (dopant)
Doping is adding a small amount of impurity atoms (dopants) to a semiconductor to change its conductivity.
C. n-type semiconductor (donor dopant)
An n-type semiconductor is doped with donor atoms (e.g. Group V such as phosphorus).
The majority carriers are electrons.
D. p-type semiconductor (acceptor dopant)
A p-type semiconductor is doped with acceptor atoms (e.g. Group III such as boron).
The majority carriers are holes.
Doping is useful background for understanding diodes, but detailed band-structure / Fermi-level arguments are beyond what most 9478 exam questions require.
2. Key Ideas (What Earns Marks)
- Doping increases conductivity mainly by increasing the number density of charge carriers.
- n-type: electrons are majority carriers; holes are minority carriers.
- p-type: holes are majority carriers; electrons are minority carriers.
- The semiconductor remains overall electrically neutral (dopants do not “charge up” the bulk material).
3. Detailed Explanations
A. n-type doping (donor)
In a silicon lattice, each silicon atom forms 4 covalent bonds.
If a Group V dopant (e.g. phosphorus) replaces a silicon atom:
- 4 of its valence electrons form bonds
- the extra electron is weakly bound and can become a conduction electron
B. p-type doping (acceptor)
If a Group III dopant (e.g. boron) replaces a silicon atom:
- it has only 3 valence electrons for bonding
- one bond is short of an electron, creating a hole
In an electric field, holes behave like positive charge carriers.
The right-hand panel of the figure shows the corresponding acceptor model and its mobile hole.
C. Why doping increases conductivity
Intrinsic semiconductors have relatively few carriers at room temperature.
Doping supplies many additional majority carriers (electrons for n-type, holes for p-type), so the material can carry larger current for the same applied potential difference.
4. Common Mistakes
- Thinking n-type means “negative overall charge” (bulk is neutral).
- Thinking holes are actual particles like protons (holes are vacancies).
- Mixing up which dopant type creates which majority carrier (Group V → n-type; Group III → p-type).
5. Exam Tips
- If a question mentions a diode, remember it is built from a p–n junction (doping matters for the device, even if the question only tests I–V behaviour).
- If a question mentions NTC thermistors, the temperature trend is explained mainly by carrier number density increasing with temperature (not by doping).
6. Worked Examples
Modelled example 1
Identify n-type vs p-type
Problem
Study the worked solution
Classify the dopant
Method
Phosphorus is a Group V donor dopant in silicon.Reason
It has one more valence electron than the four needed for silicon’s covalent bonds.Working
5-4 = 1 extra valence electronIdentify the carrier supplied
Method
The extra electron can become a mobile conduction electron.Reason
It is weakly bound compared with the bonding electrons.Working
donor ⇒ e⁻_mobileName the material
Method
The semiconductor is n-type and electrons are its majority carriers.Reason
Donor doping makes mobile electrons far more numerous than thermally generated holes.Working
n-type: nₑ≫ nₕ
Common misconception 2
Majority and minority carriers
Learner claim
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View solution step by step
Apply acceptor doping
Method
Group III acceptors leave many vacancies in valence-band bonding states.Reason
Each acceptor is short of one electron for four silicon bonds.Working
acceptor ⇒ holeIdentify the effective carrier
Method
Those vacancies behave as mobile positive holes as neighbouring electrons fill them.Reason
The hole’s apparent motion is opposite to the individual electron hops.Working
electron hops ⇒ hole driftClassify populations
Method
Holes are majority carriers; electrons are minority carriers.Reason
Acceptor doping raises hole concentration well above the thermally generated electron concentration.Working
p-type: nₕ≫ nₑ
7. Mind Stretchers
Mind stretcher 1: Question 1Extension
Explain why increasing temperature can still increase conductivity in a doped semiconductor.
Show Answer
Doping provides majority carriers, but heating can still generate additional electron–hole pairs, increasing total carrier number density and hence conductivity.
Mind stretcher 2: Question 2Extension
Why does a doped semiconductor not become “charged” overall even though donors provide extra electrons?
Show Answer
The dopant atom that donates an electron becomes a positively charged ion fixed in the lattice. The free electron is mobile, but the total charge remains balanced: the bulk remains electrically neutral.
8. Optional (Enrichment)
A. Simulations and deeper models
- Use the Semiconductor Devices Lab to connect carrier models to device behaviour.
- For more advanced models such as Fermi levels, continue to the H3 Physics portal.