Semiconductor Materials: Properties, Carriers & Doping: quick revision, 15 bites

What Makes a Semiconductor Special

Not quite metal, not quite insulator.

A semiconductor's conductivity sits **between** that of a conductor and an insulator — and crucially, it can be **tuned** by orders of magnitude.

Takeaway. The tunable conductivity of semiconductors is why they power virtually all modern electronics.

Three Ways to Control Conductivity

Doping, heat, or light — pick your lever.

Takeaway. These three factors make semiconductors uniquely controllable for device engineering.

Key Properties of Semiconductors

Nine properties that define every semiconductor.

Takeaway. The negative temperature coefficient and non-ohmic behaviour distinguish semiconductors from metals.

Group IV Elements: The Covalent Lattice

Four valence electrons build the entire crystal.

Silicon ($1s^2 2s^2 2p^6 3s^2 3p^2$), Germanium, and Tin each have **4 valence electrons** that form covalent bonds with four neighbours, completing an octet.

Takeaway. The covalent bonding of Group IV elements is the foundation of all elemental semiconductor behaviour.

What Exactly Is a Hole?

It's not a particle — but it acts like one.

Takeaway. Holes are conceptual entities representing electron vacancies, not physical positive particles.

How Two Types of Current Flow

Electrons and holes move in opposite directions.

Takeaway. In pure semiconductors, conduction is due to both free electrons and the motion of bound electrons (described as hole movement).

Intrinsic vs Extrinsic Semiconductors

Pure vs doped — the game changes completely.

Takeaway. Doping converts low-conductivity intrinsic semiconductors into device-ready extrinsic ones.

n-Type Semiconductors: Donor Doping

Add a pentavalent atom, get a free electron.

Adding a **pentavalent** atom (As, Sb, Bi, P) to silicon: four electrons form covalent bonds, the **fifth is loosely bound** and easily promoted to the conduction band.

Takeaway. n-type = negative majority carriers (electrons) from donor impurities.

p-Type Semiconductors: Acceptor Doping

Add a trivalent atom, create a hole.

Adding a **trivalent** atom (In, B, Al, Ga) to silicon: only three covalent bonds form, leaving the **fourth bond with a hole**.

Takeaway. p-type = positive majority carriers (holes) from acceptor impurities.

Holes Are Not Positive Particles

The most common misconception in semiconductors.

Takeaway. Hole conduction = bound electron conduction described in reverse. No real positive particle exists.

Compound Semiconductors: Beyond Group IV

Mix two groups, get a semiconductor.

Takeaway. Binary/compound semiconductors expand the range of band gaps and properties beyond elemental Group IV materials.

Band Gap Comparison: Diamond vs Glass

Why diamond and glass are insulators.

Takeaway. Large band gaps (>3 eV) make materials insulators — electrons cannot easily jump to the conduction band.

Exam Favourites from This Topic

These questions appear again and again.

Takeaway. Focus on band diagrams and the donor/acceptor distinction — they carry the most marks.

Remembering n-type vs p-type

n = negative, p = positive. Simple.

Takeaway. n-type = pentavalent donor, p-type = trivalent acceptor. The first letter tells you the majority carrier.

Where Semiconductors Power Your World

From your phone to solar panels.

Takeaway. Doping and carrier control are the foundation of diodes, transistors, solar cells, and photodetectors.