Important Objective Questions – Semiconductor Electronics: Materials, Devices and Simple Circuits (Class 12, MP Board/NCERT)

1. Energy Bands & Classification of Solids

1. On the basis of energy bands, the classification of solids is done into: Answer: Metals, Insulators, and Semiconductors.
2. The energy gap between conduction band and valence band is maximum in: Answer: Insulators (~6 eV or more, e.g., diamond ~5.4 eV).
3. Energy gap (Eg) for germanium and silicon respectively is: Answer: Ge ≈ 0.7 eV, Si ≈ 1.1 eV.
4. In conductors, the conduction band and valence band: Answer: Overlap each other (no energy gap).
5. In insulators, at room temperature, the valence band is: Answer: Completely filled, and conduction band is empty; a large energy gap separates them.

2. Intrinsic and Extrinsic Semiconductors

1. A pure semiconductor at 0 K behaves like a/an: Answer: Insulator.
2. Pentavalent impurities (like P, As, Sb) added to a pure semiconductor produce: Answer: n-type semiconductor (donor impurity).
3. Trivalent impurities (like B, Al, In, Ga) added to a pure semiconductor produce: Answer: p-type semiconductor (acceptor impurity).
4. In n-type semiconductors, majority and minority carriers are respectively: Answer: Electrons (majority), holes (minority).
5. In p-type semiconductors, majority and minority carriers are respectively: Answer: Holes (majority), electrons (minority).
6. The process of adding impurity to a pure semiconductor is called: Answer: Doping.
7. A semiconductor is electrically neutral even though it has n-type and p-type regions because: Answer: The number of positive and negative charges (donor/acceptor ions plus free carriers) balances out overall - the material has no net charge.
8. In an intrinsic semiconductor, the number of electrons (ne) and number of holes (nh) are related as: Answer: ne = nh = ni (intrinsic carrier concentration).

3. p-n Junction Diode

1. The barrier potential (depletion layer voltage) for silicon and germanium diodes is approximately: Answer: Si ≈ 0.7 V, Ge ≈ 0.3 V.
2. The region near a p-n junction depleted of free charge carriers is called: Answer: Depletion region/layer.
3. In forward bias, the p-n junction diode offers: Answer: Low resistance (majority carriers flow easily, depletion width decreases).
4. In reverse bias, the p-n junction diode offers: Answer: High resistance (depletion width increases; only a small reverse saturation current flows due to minority carriers).
5. The current in a forward-biased diode is mainly due to: Answer: Majority carriers (diffusion current).
6. The reverse saturation current in a diode is due to: Answer: Minority carriers (drift current), and it is temperature-dependent.
7. The dynamic resistance of an ideal diode in forward bias is: Answer: Zero (for an ideal diode); for a real diode, rd = ΔVd/ΔId.

4. Rectifiers

1. A p-n junction diode is used mainly as a: Answer: Rectifier (converts AC to DC).
2. In a half-wave rectifier, the output frequency is _____ the input AC frequency. Answer: Equal to (same as) the input frequency.
3. In a full-wave rectifier, the output frequency is _____ the input AC frequency. Answer: Double (twice) the input frequency.
4. Number of diodes required in a full-wave (center-tap) rectifier circuit is: Answer: 2.
5. The ripple factor is minimum for: Answer: Full-wave rectifier (compared to half-wave).
6. In a half-wave rectifier, during the negative half cycle of AC input, the diode is: Answer: Reverse biased, so no current flows (output = 0).

5. Special Purpose Diodes

1. A Zener diode is normally operated in: Answer: Reverse breakdown region.
2. The main use of a Zener diode is as a: Answer: Voltage regulator.
3. A photodiode is normally operated in _____ bias. Answer: Reverse bias (current increases with light intensity due to generation of electron-hole pairs).
4. An LED emits light when it is: Answer: Forward biased (recombination of electrons and holes releases energy as photons).
5. The energy band gap of the semiconductor used in an LED determines its: Answer: Colour/wavelength of emitted light.
6. A solar cell works on the principle of: Answer: Photovoltaic effect (junction is not biased externally; generates its own EMF when illuminated).
7. For a solar cell, the I-V characteristic is drawn in which quadrant? Answer: Fourth quadrant (it delivers power, doesn't consume it).

6. Junction Transistor

1. A transistor has how many p-n junctions? Answer: Two (emitter-base junction and collector-base junction).
2. In a transistor, which region is lightly doped and thin? Answer: Base.
3. In normal transistor operation, the emitter-base junction is _____ and collector-base junction is _____. Answer: Forward biased; reverse biased.
4. The relation between current amplification factors α and β is: Answer: β = α/(1-α), or α = β/(1+β).
5. In a transistor, IE = IB + IC. This is called: Answer: The basic transistor current relation (Kirchhoff's current law applied to transistor).
6. Which transistor configuration is most commonly used as an amplifier? Answer: Common Emitter (CE) configuration.
7. In CE configuration, the current gain β is defined as: Answer: β = ΔIC/ΔIB (at constant VCE).
8. In a transistor amplifier, a small change in base current produces a large change in: Answer: Collector current (amplification).
9. A transistor can be used as a switch when it operates in: Answer: Saturation (ON/switch closed) and cutoff (OFF/switch open) regions.
10. In a CE transistor amplifier, the output voltage is _____ with the input voltage (phase relation). Answer: 180° out of phase (inverted).
11. Voltage gain of an amplifier is given by: Answer: Av = -β(Rc/Rin) (negative sign shows phase reversal in CE mode).

7. Digital Electronics / Logic Gates

1. The basic logic gates are: Answer: AND, OR, NOT.
2. A NAND gate is a combination of: Answer: AND gate followed by NOT gate.
3. A NOR gate is a combination of: Answer: OR gate followed by NOT gate.
4. NAND and NOR gates are called "universal gates" because: Answer: Any other logic gate (AND, OR, NOT) can be built using only NAND gates or only NOR gates.
5. The output of an AND gate is HIGH only when: Answer: All inputs are HIGH (1).
6. The output of an OR gate is LOW only when: Answer: All inputs are LOW (0).
7. A NOT gate is also called a: Answer: Inverter.
8. Boolean expression for the OR gate is: Answer: Y = A + B.
9. Boolean expression for the AND gate is: Answer: Y = A·B.
10. Boolean expression for the NOT gate is: Answer: Y = A̅ (complement of A).

Quick Reference Table – Key Values