Module 1 · NEET Physics

Semiconductor Electronics

Energy bands, diodes, transistors, logic gates, and applications.

Energy Bands · Diodes · Transistors · Logic Gates · Applications

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Learning Objectives

Understand energy bands, intrinsic/extrinsic semiconductors, and doping
Analyse diode circuits — forward/reverse bias, rectifiers, Zener regulation
Study transistor action — CB, CE, CC configurations and amplification
Identify basic logic gates and their truth tables

Key Concepts

Energy Bands in Solids

In solids, atomic orbitals overlap to form energy bands. The valence band is filled with electrons, while the conduction band is empty or partially filled. The band gap E_g determines conductivity: metals (no gap), semiconductors (~1 eV), insulators (>3 eV).

Intrinsic & Extrinsic Semiconductors

Pure (intrinsic) Si or Ge has equal electrons and holes at a given temperature. Doping introduces impurities: n-type (pentavalent — P, As, Sb) adds extra electrons; p-type (trivalent — B, Al, In) creates excess holes. Majority and minority carriers govern device behaviour.

p-n Junction Diode

When p-type and n-type materials are joined, a depletion region forms. Forward bias reduces the barrier — current flows exponentially. Reverse bias widens the depletion region — only a tiny leakage current flows. The I-V characteristic shows a knee voltage of ~0.3 V (Ge) or ~0.7 V (Si).

Diode Applications

Half-wave rectifier: conducts during one half-cycle, efficiency 40.6%. Full-wave rectifier: uses centre-tap or bridge (4 diodes), efficiency 81.2%. Ripple factor is reduced using filter capacitors. Zener diode: operates in reverse breakdown — used for voltage regulation.

Bipolar Junction Transistor

An npn or pnp sandwich: emitter (heavily doped), base (thin, lightly doped), collector. In active region: emitter-base forward biased, collector-base reverse biased. Current gain β = I_C/I_B. Three configurations: CB (low input impedance, high voltage gain), CE (medium impedances, high current & voltage gain — most common), CC (high input impedance, voltage gain ~1 — buffer).

Logic Gates

Basic gates: NOT (inverter), AND (output 1 only if all inputs 1), OR (output 1 if any input 1). Universal gates: NAND and NOR can implement any Boolean expression. XOR gives 1 when inputs differ.

Solved Example 1

A silicon diode has a forward voltage drop of 0.7 V. If connected in series with a 470 Ω resistor and a 5 V supply, find the diode current.

a) 7.2 mA b) 9.1 mA c) 10.6 mA d) 11.8 mA

Solution: V_R = 5 - 0.7 = 4.3 V. I = V/R = 4.3/470 = 9.15 × 10^-3 A ≈ 9.1 mA. Hence option (b) is correct.

Solved Example 2

In a common-emitter amplifier, the input signal is applied between:

a) Base and collector b) Base and emitter c) Collector and emitter d) Emitter and ground

Solution: In common-emitter (CE) configuration, the input is applied between base and emitter, and output is taken between collector and emitter. The emitter is common to both. Hence option (b) is correct.

NEET Trick

Remember transistor configurations by the common terminal: CB (common base), CE (common emitter), CC (common collector). For CE: β = α/(1-α) where α = I_C/I_E.

Solved Example 3

The output of a NAND gate is LOW when:

a) All inputs are LOW b) All inputs are HIGH c) Any input is LOW d) Any input is HIGH

Solution: NAND = NOT + AND. The AND part gives HIGH only when all inputs are HIGH. The NOT inverts it — output is LOW when all inputs are HIGH. Hence option (b) is correct.

Practice Questions

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