CBSE Class 12 Physics Semiconductor Electronics PYQs Analysis (2021-2025)

Based on detailed analysis of last 5 years' papers. Perfect for 2026 Boards prep!

Key Trends

Weightage Breakdown: 1-3 MCQs/assertion-reason (1 mark), 1-2 short answers (2 marks), 1-2 numericals/derivations on diode/transistor/logic gates (4-6 marks).
Repeated Topics: Energy bands in solids (conductor, semiconductor, insulator), p-type & n-type doping, p-n junction (depletion region, barrier potential, forward/reverse bias), diode characteristics & applications (rectifier: half/full wave, Zener diode as voltage regulator), transistor (CE configuration, input/output characteristics, current gain β), transistor as amplifier/switch, logic gates (AND, OR, NOT, NAND, NOR, XOR), truth tables, Boolean expressions, universal gates (NAND/NOR).
Difficulty: Medium; transistor characteristics & logic gate truth tables are most repeated & scoring.
Changes: More questions on Zener diode as regulator & transistor as switch/amplifier in 2023–2025.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Explain energy bands in solids. Distinguish between conductor, semiconductor & insulator on basis of band gap.	Short Answer/Theory (3 marks)	2021, 2022, 2023, 2024, 2025	Repeated 5x; Valence band, conduction band, forbidden gap; Conductor: Eg ≈ 0, Semiconductor: Eg ~ 0.1–3 eV, Insulator: Eg > 3 eV.
Draw I-V characteristic of p-n junction diode in forward & reverse bias. Explain depletion region & barrier potential.	Diagram + Explanation (3-4 marks)	2021 Term 2, 2022, 2023, 2024	Repeated 4x; Forward: exponential rise after ~0.7V (Si), reverse: very small leakage current till breakdown.
Draw circuit diagram & explain working of full-wave rectifier (centre-tapped & bridge). Find ripple factor or efficiency.	Diagram + Theory (4 marks)	2022, 2023, 2024, 2025	Repeated 4x; Bridge rectifier: higher efficiency (~81%), ripple factor 0.48; Centre-tapped: 40.6% efficiency.
Assertion: Zener diode is used in reverse bias for voltage regulation. Reason: It operates in breakdown region with almost constant voltage.	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains.
Draw input & output characteristics of transistor in CE configuration. Define current gain β = IC/IB.	Diagram + Definition (3 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; Input: IB vs VBE (almost vertical), Output: IC vs VCE (active, saturation, cutoff regions).
Construct truth table & logic symbol for NAND & NOR gates. Show they are universal gates.	Short Answer (3 marks)	2022, 2023, 2024	Repeated 3x; NAND/NOR can make AND, OR, NOT → universal.
MCQ: In forward bias, resistance of p-n junction is: (a) high (b) low (c) infinite (d) zero	MCQ (1 mark)	2021 Term 1, 2023, 2024	Repeated 3x; Answer (b) low (conducts easily).
Explain transistor as amplifier in CE configuration. Find voltage gain Av = -β (RC/RE).	Short Answer (3 marks)	2023, 2025	Repeated 2x; Small change in IB → large change in IC → voltage amplification across RC.
Case-based: Given diode/transistor circuit or logic gate combination, find output or equivalent gate.	Case-Based (4 marks)	2023, 2025	Repeated 2x; Use truth table or diode on/off condition.
Find current gain β if IC = 5 mA, IB = 50 μA (or similar).	Numerical (2 marks)	2021 Term 2, 2024	Repeated 2x; β = IC/IB = 100.

Best Format for Preparation and Answering

Study Format: Memorize p-n junction characteristics, transistor CE curves & logic gate truth tables. Practice numericals on current gain & rectifier efficiency daily. Draw clear circuit diagrams & graphs.
Answer Format: Diagrams: Label forward/reverse regions, saturation/cutoff. Numericals: Use β = IC/IB, rectifier efficiency = (DC power / AC power) × 100. Logic gates: Draw symbol + truth table.
Tips: p-n junction characteristics & transistor as amplifier/switch are highest repeat & marks. Practice logic gate combinations (universal gates). Time: 10–12 mins for 4–6 mark questions. Very scoring with neat diagrams & correct labels.

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