Q.A pure semiconductor is ____.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Intrinsic and Extrinsic Semiconductors
A pure semiconductor -- pure silicon or pure germanium, with no impurity added -- is called INTRINSIC. Each silicon or germanium atom has a valence of four and forms four covalent bonds with its neighbours in a tetrahedral arrangement; at absolute zero, every bond is complete and no free carriers exist. At room temperature, thermal agitation breaks a small number of these bonds, promoting a valence electron into the conduction band and leaving behind a vacancy -- a HOLE -- in the valence band, which behaves as an effective positive charge. Because every broken bond produces one electron and one hole together, an intrinsic semiconductor always has equal carrier densities, ne=nh=ni (the intrinsic carrier density), and conduction happens through BOTH carrier types drifting in opposite directions when a field is applied -- electrons toward the positive terminal, holes (via a relay of neighbouring electron jumps) toward the negative terminal. Because so few of a crystal's roughly 5×1022 atoms per cm3 actually contribute a carrier this way, intrinsic conductivity is comparatively low and not easily controlled. …
Semiconductors are classified by whether they have been deliberately doped with impurities, and a chemically pure, undoped semiconductor has its own specific name. …
"Pure" (undoped) semiconductor is, by definition, called intrinsic; adding impurities to boost conductivity produces an extrinsic (p-type or n-type) semiconductor.
A semiconductor in its chemically pure form, with no impurity atoms added, is called an intrinsic semiconductor (e.g. pure silicon or germanium crystals). Its charge carriers — electrons and holes — are generated in equal numbers purely by thermal excitation across the energy gap. When controlled amounts of impurity ('doping') are added to alter conductivity, the materia …
Showing the 12 most recent of 15 on this concept.
- CBSE 2026Set V11 markMCQQ.Which of the following pairs are elemental semiconductors?(a) Silicon and aluminium(b) Silicon and germanium(c) Germanium and cadmium(d) Aluminium and cadmium
›Reveal solutionSolution
Option (b) Silicon and germanium. …
- CBSE 2026Set A1 markMCQQ.Semiconductor is damaged by the strong current due to (A) lack of free electrons (B) excess of protons (C) excess of electrons (D) excess of neutrons
›Reveal solutionSolution
A strong current means an excess of moving electrons, whose collisions dump heat that damages the semiconductor's crystal structure.
Unlike a metal, a semiconductor has relatively few free carriers, and its conductivity rises sharply with temperature. When a strong current flows, there is an excess of electrons moving through the material; their frequent collisions with the lattice generate intense Joule heating. This heating liberates still more carriers (thermal runaway), and the temper …
- CBSE 2026Set ANNUAL1 markMCQQ.When a semiconductor is heated, its resistance(a) decreases(b) increases(c) remains unchanged(d) is uncertain
›Reveal solutionSolution
A semiconductor's resistance falls with rising temperature, because the number of charge carriers rises sharply (negative temperature coefficient of resistance).
In a semiconductor, conduction occurs through electrons and holes generated when covalent bonds break due to thermal energy. As temperature increases:
- More covalent bonds break, generating more electron–hole pairs.
- The number of free charge carriers (and hence conductivity σ) increases roughly exponentially with temperature. …
- CBSE 2025Set ANNUAL1 markMCQQ.Which one in the following is a semiconductor-(a) Phosphorus(b) Aluminium(c) Polyaniline(d) Copper
›Reveal solutionSolution
Among the four choices, polyaniline is a conducting/semiconducting organic polymer; the others are an insulating-side element, a metal, and a metal.
Phosphorus (a non-metal element, used as a dopant but not itself classed as a semiconductor here) and copper (a good metallic conductor) and aluminium (a metal, a conductor) are not semiconductors. Polyaniline is a conjugated conducting polymer whose electrical conductivity can be tuned over a wide range (from in …
- CBSE 2025Set ANNUAL1 markQ.The resistivity of semiconductors is between ____________ ohm-m to ____________ ohm-m.
›Reveal solutionSolution
Semiconductors occupy the middle ground between conductors and insulators, with resistivity roughly between 10−5 and 106 ohm-metre.
Comparing typical resistivity ranges:
- Conductors (metals): ρ∼10−8 to 10−2 Ωm
- Semiconductors: ρ∼10−5 to 106 Ωm
- Insulators: ρ∼1011 to 1019 Ωm …
- CBSE 2025Set ANNUAL1 markQ.Semi-conductor is a device that obeys Ohm's law. (T/F)
›Reveal solutionSolution
This statement is False — a semiconductor (and semiconductor devices such as diodes) generally do NOT obey Ohm's law; their V-I relationship is non-linear.
Ohm's law (V=IR with constant R) holds for an ohmic conductor whose resistance stays constant regardless of the applied voltage or current direction. A semiconductor's conductivity strongly depends on temperature and carrier concentration, and semiconductor junction devices (like a p-n junction diode) show a markedly non-linear V-I characteristic (near-zero current in reve …
- CBSE 2024Set ANNUAL1 markQ.What happens to the electrical resistance of a semiconductor when its temperature is raised?
›Reveal solutionSolution
Unlike metals, a semiconductor's resistance falls when it is heated, because more charge carriers become available for conduction.
In a pure (intrinsic) semiconductor at low temperature, very few electrons have enough thermal energy to jump across the energy gap from the valence band to the conduction band, so the number of free charge carriers (electrons and holes) is small and the resistance is relatively high.
As the temperature is raised, more valence electrons acquire enough thermal energy to break their covalent bonds and jump into the conduction band, creating additional electron–hole pairs. The number of free charge carriers therefore increases rapidly (roughly exponentially) with temperature.
…
- CBSE 2024Set ANNUAL1 markMCQQ.Example of inorganic semiconductor is -(a) Ge(b) CdS(c) anthracene(d) polyaniline
›Reveal solutionSolution
Semiconductors are classed as inorganic (elemental/compound, e.g. Ge, Si, CdS, GaAs) or organic (e.g. anthracene, polyaniline); CdS is the inorganic compound example here.
Semiconducting materials are broadly grouped as:
- Elemental inorganic semiconductors: Silicon (Si), Germanium (Ge).
- Compound inorganic semiconductors: CdS, GaAs, CdSe, InP, etc.
- Organic semiconductors: anthracene, doped phthalocyanines, polyaniline, etc. …
- CBSE 2023Set 55/1/11 markMCQQ.When an intrinsic semiconductor is doped with a small amount of trivalent impurity, then :(a) its resistance increases.(b) it becomes a p-type semiconductor.(c) there will be more free electrons than holes in the semiconductor.(d) dopant atoms become donor atoms.
›Reveal solutionSolution
Doping an intrinsic semiconductor with a trivalent impurity (like boron) creates p-type material, where holes are the majority carriers and resistance decreases — the correct option is (b).
The Core Idea: What Doping Actually Does
An intrinsic (pure) semiconductor has equal numbers of electrons and holes — both generated by thermal energy. When you add a trivalent impurity (3 valence electrons, e.g., boron, aluminium, gallium), each impurity atom replaces a silicon atom in the crystal lattice. Silicon has 4 valence electrons; the trivalent atom has only 3. That missing bond creates a hole — a vacancy that can accept an electron.
This is the key: trivalent impurities are acceptor atoms, not donors. They introduce extra holes into the crystal, making holes the majority carriers. The material becomes p-type (p for positive charge carriers).
Let’s walk through each option carefully.
Step-by-Step Analysis
1. Option (a): “its resistance increases”
Doping always adds charge carriers to the semiconductor. In an intrinsic semiconductor, the number of carriers is very small (on the order of 1010 cm−3 for silicon at room temperature). Doping with even 1 part per million of a trivalent impurity increases the hole concentration dramatically — to about 1016 cm−3 or more.
Conductivity σ is given by:
σ=q(nμn+pμp)
where q is the electronic charge, n and p are electron and hole concentrations, and μn, μp are their mobilities. Adding more carriers (holes) increases conductivity, so resistance decreases. Option (a) is false.
Watch outA common mistake is to think that “impurities block current” — but in semiconductors, controlled impurities (dopants) are what make them conduct. Pure silicon is almost an insulator; doped silicon is a useful conductor.
2. Option (b): “it becomes a p-type semiconductor”
Exactly. With trivalent doping, holes outnumber electrons. The material is p-type. This is the definition. Option (b) is true.
3. Option (c): “there will be more free electrons than holes” …
- CBSE 2023Set F1 markMCQQ.The temperature coefficient of a semi-conductor is (A) positive (B) Negative (C) Zero (D) Infinity
›Reveal solutionSolution
For semiconductors, resistance decreases with rising temperature → negative temperature coefficient of resistance.
In a semiconductor, raising the temperature releases more charge carriers (electrons and holes) by breaking covalent bonds. This large increase in carrier concentration outweighs the increased scattering, so the resistivity (and resistance) decreases as temperature increases. S …
- CBSE 2023Set ANNUAL1 markMCQQ.As temperature increases the resistance of a semiconductor(1) increases(2) decreases(3) remains constant(4) none of these
›Reveal solutionSolution
A semiconductor has a NEGATIVE temperature coefficient of resistance — its resistance falls as temperature rises.
In a semiconductor, raising the temperature breaks more covalent bonds, generating more free electrons and holes (charge carriers). Since conductivity depends on the number of available carriers, more ca …
- CBSE 2023Set ANNUAL1 markQ.What is the effect of rise in temperature on the conductivity of a semiconductor?
›Reveal solutionSolution
A rise in temperature breaks more covalent bonds in the semiconductor, generating additional free electrons and holes, which raises conductivity.
As temperature increases, thermal energy breaks more covalent bonds, creating more electron-hole pairs. Since conductivity is proportional to the number of free charge carriers, this INCREASES the conductivity of the semiconductor (equivalently, its resistance decreases) — the opposite of the …
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