Learnosha CBSE 12
Physics Semiconductor Electronics: Materials, Devices and Simple Circuits 1 Mark
Appeared in: 2025 (3 sets)
According to the band theory of solids, a semiconductor has a valence band and a conduction band separated by a gap, known as energy band gap. Pure semiconductors are called intrinsic semiconductors. At room temperature, some electrons from the valence band can acquire enough energy to cross the band gap and enter the conduction band. The number of conduction electrons is equal to the number of holes is an intrinsic semiconductor. The number of charge carriers can be changed by doping of a suitable impurity in a pure semiconductor. Such semiconductors are known as extrinsic semiconductors. These are of two types (n-type and p-type). A p-n junction is the basic building block of semiconductor devices. Two important processes occur during formation of a p-n junction : diffusion and drift. A 'depletion layer' is formed in a p-n junction. This is responsible for a junction potential barrier. A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage. A diode can be forward-biased or reverse-biased. The barrier height and the depletion layer width in a p-n junction changes depending on the nature of the biasing. (i) (a) Which of the following statements is not true ? (A) The resistance of an intrinsic semiconductor decreases with the increase of temperature. (B) Doping pure $ ext{Si}$ with trivalent impurities gives p-type semiconductors. (C) The majority charge carriers in n-type semiconductors are holes. (D) A p-n junction can act as a semiconductor diode. OR (b) In an unbiased p-n junction : (A) the diffusion current is zero everywhere. (B) the drift current is zero everywhere. (C) the electric potential is zero everywhere. (D) the drift current and the diffusion current cancel each other.
✓ Verified CBSE Marking Scheme Official Evaluation Steps & EL10 Insight

(i) (a) Correct Option: (C)

  • Step 1 (Conceptual Analysis): [0.5 Mark]

    According to the band theory and semiconductor physics, in an n-type semiconductor, the pentavalent impurity atoms donate extra electrons. Therefore, electrons are the majority charge carriers, and holes are the minority charge carriers.

  • Step 2 (Verification of Statements): [0.25 Mark]
    • (A) True: In intrinsic semiconductors, $R \propto e^{E_g/2kT}$, so resistance decreases as temperature increases.
    • (B) True: Trivalent impurities (e.g., Boron) create holes, resulting in p-type semiconductors.
    • (C) False: In n-type semiconductors, electrons are the majority carriers, not holes.
    • (D) True: A p-n junction diode allows current flow in one direction, acting as a rectifier.
  • Step 3 (Final Concluded Answer): [0.25 Mark]

    The statement that is not true is (C).

OR

(i) (b) Correct Option: (D)

  • Step 1 (Governing Principle): [0.5 Mark]

    In an unbiased p-n junction, the concentration gradient causes diffusion of charge carriers, creating an electric field. This field causes drift of carriers. At equilibrium, the net current $I_{net} = I_{diffusion} + I_{drift} = 0$.

  • Step 2 (Mathematical Working): [0.25 Mark]

    Since the junction is in equilibrium (unbiased), the rate of charge transfer due to diffusion is exactly balanced by the rate of charge transfer due to the internal electric field (drift):

    $$I_{diffusion} = -I_{drift}$$ $$\implies I_{diffusion} + I_{drift} = 0$$
  • Step 3 (Final Concluded Answer): [0.25 Mark]

    The correct statement is (D): the drift current and the diffusion current cancel each other.

💡 EL10 Concept Insight: Think of an unbiased p-n junction like a room with two doors: diffusion is like people walking from a crowded side to an empty side, while drift is like a security guard pushing them back; when the number of people crossing equals the number being pushed back, the room stays perfectly balanced!

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