What is a Semiconductor?
A semiconductor is a material with electrical conductivity between that of a conductor (copper) and an insulator (glass). The most commonly used semiconductor materials are silicon (Si) and germanium (Ge). Semiconductor conductivity changes dramatically with temperature and with the addition of controlled impurities (doping), making them ideal for electronic circuits.
1. Intrinsic Semiconductor
An intrinsic semiconductor is the pure (undoped) form of a semiconductor. In intrinsic silicon or germanium the number of free electrons equals the number of holes — and both are produced only by thermal excitation.
Energy band structure (intrinsic)
At absolute zero (0 K) the valence band is completely filled and the conduction band is empty. As temperature rises, some valence electrons gain thermal energy to jump into the conduction band, leaving behind positively charged holes. The generated electron–hole pairs (EHPs) increase conductivity. For intrinsic materials, conductivity depends largely on temperature.
- No deliberate impurities — pure lattice (Si, Ge).
- Equal number of electrons and holes:
n = p. - Low conductivity at room temperature; increases with temperature.
2. Extrinsic Semiconductor & Doping
An extrinsic semiconductor is created by adding a small, controlled amount of impurity to a pure semiconductor — a process called doping. The dopant introduces extra charge carriers (either electrons or holes), dramatically increasing conductivity.
Typical doping concentration: extremely small — around one impurity atom per 108 host atoms. The dopant type determines whether the semiconductor becomes n-type (more electrons) or p-type (more holes).
A. N-Type Semiconductor (donor)
When a pentavalent impurity (Group V — phosphorus, arsenic, antimony) is added, the impurity donates a free electron to the crystal. The extra electron is loosely bound and easily excited into the conduction band, so electrons become the majority carriers.
- Donor impurity → provides free electrons.
- Majority carriers: electrons; minority carriers: holes.
- High conductivity due to abundant electrons.
B. P-Type Semiconductor (acceptor)
When a trivalent impurity (Group III — boron, gallium, aluminium, indium) is added, it creates a missing-electron site (a hole) in the lattice. Holes act as positive charge carriers and dominate conduction in p-type materials.
- Acceptor impurity → creates holes.
- Majority carriers: holes; minority carriers: electrons.
- Conductivity typically lower than comparable n-type at identical doping because hole mobility is lower.
Energy Band Diagram (How doping changes bands)
Doping alters energy states within the band gap:
- N-type: donor level (ED) appears just below the conduction band (EC). The Fermi level shifts toward the conduction band.
- P-type: acceptor level (EA) appears just above the valence band (EV). The Fermi level shifts toward the valence band.
Key Differences — Intrinsic vs Extrinsic
| Basis of Difference | Intrinsic Semiconductor | Extrinsic Semiconductor |
|---|---|---|
| Purity | 100% pure crystal (no deliberate impurity) | Deliberately doped with impurities (trivalent or pentavalent) |
| Carrier concentration | Electrons = Holes (n = p) | Electrons ≠ Holes (n ≠ p); one type dominates |
| Conductivity | Low (temperature dependent) | Higher (depends on dopant and temperature) |
| Dependence | Only temperature | Temperature & dopant concentration/type |
| Examples | Pure Si or Ge | Si doped with P/As (n-type) or B/Al (p-type) |
Applications
Intrinsic: Primarily used for study, reference substrates, and as the starting wafer for device fabrication. Extrinsic: Form the active regions of almost every semiconductor device: diodes, transistors, integrated circuits (ICs), LEDs, solar cells and sensors.
Conclusion
Intrinsic semiconductors are pure materials where temperature alone determines carrier generation; extrinsic semiconductors are intentionally doped to create a controlled surplus of electrons or holes. This controlled conduction — by choosing donor or acceptor impurities — enables p–n junctions and the vast ecosystem of modern electronic components. Understanding intrinsic vs extrinsic behavior is foundational for electronics, device design and semiconductor fabrication.