A semiconductor is a material whose electrical conductivity lies between that of a conductor (like copper or silver) and an insulator (like glass or rubber). At room temperature, semiconductors conduct electricity partially — they have more resistivity than metals but far less than insulators. Crucially, their conductivity increases with temperature, unlike metals where conductivity decreases with temperature. Silicon (Si) and Germanium (Ge) are the most widely used semiconductors. They are the foundation of modern electronics: transistors, diodes, integrated circuits (ICs), solar cells, and LEDs all rely on semiconductor materials.
A semiconductor has electrical conductivity between a conductor and an insulator.
Resistivity range: 10⁻⁴ to 10⁴ Ω·m (conductors: ~10⁻⁸; insulators: ~10¹² Ω·m).
Bandgap: 0.1–3.0 eV (conductors: ~0 eV; insulators: >5 eV).
Semiconductor conductivity INCREASES with temperature (opposite to metals).
Most used semiconductors: Silicon (Si, bandgap 1.12 eV) and Germanium (Ge, bandgap 0.67 eV).
Intrinsic: pure semiconductor (Si, Ge). Extrinsic: doped with impurities to increase conductivity.
n-type: doped with pentavalent atoms (P, As); majority carriers = electrons.
p-type: doped with trivalent atoms (B, Al); majority carriers = holes.
Applications: transistors, diodes, ICs (computer chips), solar cells, LEDs.
Electrical conductivity is explained by band theory (energy bands):
Conductors (metals): • Valence band and conduction band overlap — electrons move freely • Resistivity: ~10⁻⁸ Ω·m • Example: copper, silver, aluminium
Insulators: • Very large energy gap (bandgap) between valence and conduction bands (>5 eV) • Electrons cannot jump to the conduction band at room temperature • Resistivity: ~10¹² Ω·m or more • Example: glass, rubber, diamond (as insulator)
Semiconductors: • Small energy gap (bandgap): typically 0.1–3.0 eV • At room temperature: a few electrons can jump to the conduction band due to thermal energy • Silicon bandgap: 1.12 eV • Germanium bandgap: 0.67 eV • Resistivity: approximately 10⁻⁴ to 10⁴ Ω·m
Key property: Semiconductor conductivity INCREASES with temperature • Higher temperature → more electrons gain energy to cross the bandgap → more current carriers → higher conductivity • This is opposite to metals (where higher temperature means more lattice vibrations, impeding electron flow → lower conductivity)
Semiconductors are classified into two main types:
Intrinsic Semiconductors (Pure): • Pure semiconductors without any added impurities • Silicon (Si): 4 valence electrons, forms covalent bonds with 4 neighbours • Germanium (Ge): also 4 valence electrons, similar structure • At room temperature: a small number of electron-hole pairs are generated by thermal energy • Electron: negatively charged current carrier in conduction band • Hole: positively charged 'vacancy' left behind when an electron jumps up — moves in the valence band • Conductivity is low; increases with temperature
Extrinsic Semiconductors (Doped): Doping means adding a tiny amount of impurity (dopant) to dramatically increase conductivity.
n-type semiconductor: • Doped with pentavalent atoms (5 valence electrons): phosphorus (P), arsenic (As), antimony (Sb) • 4 of the 5 electrons form covalent bonds with Si neighbours; the 5th electron is loosely bound and free to move • Majority carriers: electrons (negative charges) • Minority carriers: holes • n stands for 'negative' (majority carriers are negative)
p-type semiconductor: • Doped with trivalent atoms (3 valence electrons): boron (B), aluminium (Al), gallium (Ga) • Only 3 electrons to form bonds — creates a 'hole' (missing electron) that acts as a positive charge carrier • Majority carriers: holes (positive charges) • Minority carriers: electrons • p stands for 'positive' (majority carriers are positive)
Note: Both n-type and p-type are electrically neutral overall — doping doesn't add or remove charge, it adds charge carriers.
Elemental semiconductors: • Silicon (Si) — atomic number 14; most widely used; basis of computer chips and solar cells • Germanium (Ge) — atomic number 32; used in the first transistors; now used in infrared optics and some diodes
Compound semiconductors: • Gallium arsenide (GaAs) — fast switching, used in LEDs (red/infrared), solar cells, high-frequency electronics • Gallium nitride (GaN) — high-power LEDs (blue, white), power electronics • Indium phosphide (InP) — laser diodes, optical fibre communications • Cadmium sulphide (CdS) — photoconductors, solar cells • Silicon carbide (SiC) — high-temperature, high-power electronics
Semiconductors are the backbone of modern technology:
A semiconductor is a material whose electrical conductivity is between that of a conductor (like copper) and an insulator (like glass). At room temperature, semiconductors partially conduct electricity. Their conductivity increases with temperature — unlike metals. Silicon (Si) and germanium (Ge) are the most common semiconductors. They are used to make transistors, diodes, solar cells, and integrated circuits.
n-type semiconductor: doped with pentavalent atoms (like phosphorus or arsenic). The extra electron from the dopant becomes a free electron — the majority carrier is a negative electron. p-type semiconductor: doped with trivalent atoms (like boron or aluminium). The missing electron from the dopant creates a 'hole' — the majority carrier is a positive hole. Both types remain electrically neutral overall; doping simply creates more charge carriers.
In semiconductors, electrons need energy to jump across the bandgap from the valence band to the conduction band. At higher temperatures, more thermal energy is available — more electron-hole pairs are generated — so more current carriers exist and conductivity increases. This is the opposite of metals, where higher temperature increases lattice vibrations that obstruct electron flow.
The bandgap of silicon (Si) is 1.12 eV at room temperature. This means an electron needs at least 1.12 eV of energy to jump from the valence band to the conduction band. The bandgap of germanium (Ge) is 0.67 eV (smaller, so easier to conduct at room temperature).
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