Intrinsic, n-type, and p-type Semiconductors
An intrinsic semiconductor is a pure semiconductor such as silicon or germanium. An extrinsic semiconductor is formed by adding a small controlled amount of impurity, called a dopant, to increase charge carriers; pentavalent dopants produce n-type material and trivalent dopants produce p-type material.
In one line: Five gives one extra electron; three leaves one hole.
Concept Practice Coming SoonUnderstand it
- In pure silicon or germanium, each atom forms four covalent bonds, and electron-hole pairs are generated thermally.
- The number of electrons equals the number of holes in an intrinsic semiconductor.
- In n-type semiconductors, a group V atom such as phosphorus contributes an extra electron, so electrons become majority carriers.
- In p-type semiconductors, a group III atom such as boron creates an electron deficiency in bonding, so holes become majority carriers.
- The doped crystal remains electrically neutral overall; it only has more mobile carriers of one type.
Example
Formula and rule
Carrier rule: in intrinsic semiconductors, electron concentration equals hole concentration.
In n-type material, electrons are majority carriers and holes are minority carriers.
In p-type material, holes are majority carriers and electrons are minority carriers.
Doping must be small and controlled so the semiconductor lattice is modified without becoming a metal.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
A germanium sample is doped with arsenic.
- 1Arsenic is a group V element, so it has one more valence electron than germanium needs for four covalent bonds.
- 2The sample becomes n-type, electrons are majority carriers, holes are minority carriers, and arsenic acts as a donor impurity.
Method you can reuse
- 1Check whether the semiconductor is pure or doped.
- 2If doped, identify dopant valency.
- 3Group V means donor impurity and n-type.
- 4Group III means acceptor impurity and p-type.
- 5State majority and minority carriers clearly.
Sketch it in your answer
Show silicon covalent bonds with one pentavalent dopant for n-type and one trivalent dopant for p-type. Label donor atom, acceptor atom, free electron, hole, majority carrier, and minority carrier. The important visual point is that the dopant changes carrier availability, not the basic crystal framework.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why is phosphorus called a donor impurity in silicon?
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Common wrong answer
Writing that boron-doped silicon has free electrons as majority carriers.
Why it loses marks: Boron is trivalent and produces holes as majority carriers.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
An intrinsic semiconductor is a pure semiconductor such as silicon or germanium. An extrinsic semiconductor is formed by adding a small controlled amount of impurity, called a dopant, to increase charge carriers; pentavalent dopants produce n-type material and trivalent dopants produce p-type material.
- 2
Add for 2 marks
Example: If a silicon crystal is doped with phosphorus, it becomes n-type because phosphorus has five valence electrons and one electron is loosely available for conduction. If silicon is doped with boron, it becomes p-type because boron has three valence electrons and creates a hole in the bonding structure.
- 3
Add for 3 marks
In pure silicon or germanium, each atom forms four covalent bonds, and electron-hole pairs are generated thermally.
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