Semiconductor Electronics — Class 12 MCQs with Answers

Semiconductor Electronics — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Pure silicon is an example of which type of material?

   AConductorBIntrinsic semiconductorCInsulatorDSuperconductor
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   Answer: Intrinsic semiconductor

   Explanation: Band theory: Si has a ~1.1 eV gap, wide enough to resist free conduction like a metal but narrow enough for thermal excitation — the hallmark of an intrinsic semiconductor, not an insulator.

2. Q2 of 6Easy

   The barrier potential of a silicon pn-junction diode is approximately:

   A0.7 VB0.3 VC1.5 VD5 V
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   Answer: 0.7 V

   Explanation: The built-in barrier (knee voltage) of a silicon pn-junction is ≈0.7 V — germanium's is lower at ≈0.3 V, so swapping the two values is the classic slip here.

3. Q3 of 6Medium

   In an extrinsic semiconductor at thermal equilibrium, the product of electron and hole concentrations equals:

   ANᴀ × NᴅBnᵢCnᵢ²DZero
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   Answer: nᵢ²

   Explanation: The mass-action law states that in thermal equilibrium the product of electron and hole concentrations is fixed: n·p = nᵢ², independent of doping — not Nᴀ×Nᴅ, which sets net carrier difference instead.

4. Q4 of 6Medium

   Assertion (A): A p-n junction cannot be made by simply pressing a slab of p-type semiconductor against a slab of n-type semiconductor. Reason (R): Any slab surface, however flat, has roughness much larger than the inter-atomic spacing, so continuous contact at the atomic level is not possible.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: Both are true. Surface roughness is far larger than the 2–3 Å spacing between atoms, so the two slabs touch only at a few points. The contact then acts as a discontinuity for charge carriers instead of a junction, which is exactly why A holds.

5. Q5 of 6Hard

   In a metallic conductor, the valence and conduction bands:

   AAre perfectly insulated from each otherBAre widely separatedCOverlap completelyDAre both empty
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   Answer: Overlap completely

   Explanation: In a metallic conductor the valence and conduction bands overlap completely, so electrons need essentially zero extra energy to enter the conduction band, unlike the finite gap in a semiconductor.

6. Q6 of 6Hard

   Assertion (A): A junction diode can be damaged by overheating even when it is forward biased. Reason (R): The maximum current rating specified by the manufacturer of a diode applies only to its reverse current.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is true but R is false

   Explanation: A is true: if the forward current goes above the rated value, the diode overheats and is destroyed. R is false: the rated limit applies to the forward current as well, which is exactly why a large forward current can damage the diode.

Semiconductor Electronics notes: key concepts (Class 12 Physics)

From energy bands to working devices: how band gaps separate conductors, insulators and semiconductors, intrinsic conduction by electrons and holes, doping to make n-type and p-type material, the pn junction with its depletion region and barrier potential, forward and reverse bias, rectification, and the special-purpose diodes.

Energy bands in solids

Atomic energy levels broaden into bands when atoms come together, with a forbidden energy gap separating the filled valence band from the empty conduction band.

Conductors, insulators, semiconductors

In a conductor the bands overlap; an insulator has a gap above about 3 eV; a semiconductor's gap is small enough for thermal energy to bridge it.

Band gap

Silicon's gap is about 1.1 eV and germanium's about 0.7 eV at room temperature. The smaller gap makes germanium the more thermally sensitive of the two.

Intrinsic semiconductor

Pure semiconductor in which electrons and holes are created only in pairs by thermal excitation, so their concentrations are exactly equal.

Electrons and holes

A hole is the vacancy left by an excited electron and behaves as a mobile positive charge. Electrons have the higher mobility of the two.

Doping

Adding a controlled trace of impurity to raise the carrier concentration by orders of magnitude, which is what makes practical devices possible.

n-type semiconductor

Doped with a PENTAVALENT impurity such as phosphorus or arsenic, giving a surplus of electrons as majority carriers and holes as minority carriers.

p-type semiconductor

Doped with a TRIVALENT impurity such as boron or indium, giving holes as majority carriers. The crystal stays electrically neutral overall.

Effect of temperature

Raising the temperature frees more carriers, so a semiconductor's resistance FALLS with heating — the opposite of a metal's behaviour.

pn junction

The boundary formed within a single crystal doped p-type on one side and n-type on the other. It is the building block of every diode and transistor.

Depletion region

A thin layer at the junction swept clear of mobile carriers, containing only fixed ionised dopant atoms and therefore acting as an insulator.

Barrier potential

The built-in potential across the depletion region opposing further diffusion, about 0.7 V for silicon and 0.3 V for germanium.

Forward bias

Connecting p to the positive terminal narrows the depletion region and lowers the barrier, so appreciable current flows once the barrier voltage is exceeded.

Reverse bias

Connecting p to the negative terminal widens the depletion region and raises the barrier, so only a tiny minority-carrier leakage current flows.

Rectification

Using a diode's one-way conduction to convert AC into DC. A half-wave rectifier passes one half of each cycle; a full-wave rectifier inverts and uses both.

Special purpose diodes

The Zener diode conducts in controlled reverse breakdown and regulates voltage; the LED emits light on recombination; the photodiode and solar cell convert light into current.