Semiconductor Electronics

1. Introduction

Semiconductor electronics forms the basis of all modern electronic devices. This chapter covers energy bands, diodes, transistors, and logic gates.

2. Band Theory

2.1 Energy Bands

Valence band: Occupied by valence electrons. Conduction band: Empty or partially filled band. Forbidden gap: Energy gap between valence and conduction bands.

2.2 Classification

3. Intrinsic and Extrinsic Semiconductors

Intrinsic: Pure semiconductor. n_i = n_e = n_h. Extrinsic: Doped with impurities.

n-type: Doped with pentavalent atoms (P, As, Sb). Majority carriers: electrons. p-type: Doped with trivalent atoms (B, Al, In). Majority carriers: holes.

4. p-n Junction Diode

4.1 Formation

When p-type and n-type semiconductors are joined, a depletion region forms at the junction due to diffusion. An internal barrier potential (0.7V for Si, 0.3V for Ge) opposes further diffusion.

4.2 Biasing

Forward bias: p to positive, n to negative. Current flows after overcoming barrier potential. Reverse bias: p to negative, n to positive. Very small current (leakage current) flows.

4.3 V-I Characteristics

Forward current increases exponentially after the knee voltage. Reverse current is very small and nearly constant until breakdown.

5. Rectifiers

Half-wave rectifier: Converts AC to DC using one diode. Efficiency: 40.6%. Ripple factor: 1.21. Full-wave rectifier: Uses two diodes. Efficiency: 81.2%. Ripple factor: 0.48. Bridge rectifier: Uses four diodes. No centre tap required.

5.5 Zener Diode as a Voltage Regulator

A Zener diode operates in reverse bias at the breakdown voltage. It maintains a constant output voltage despite variations in input voltage or load current.

Working: When input voltage increases, the Zener current increases but the voltage across the Zener remains constant at V_Z. The excess voltage drops across the series resistor R_S.

Applications: Power supply regulation, voltage reference in circuits.

6. Transistor

6.1 Types

npn: Two n-type regions with thin p-type base. pnp: Two p-type regions with thin n-type base.

6.2 Configurations

Common Base (CB): Low input resistance, high output resistance, current gain α ≈ 0.98. Common Emitter (CE): Moderate input/output resistance, current gain β = α/(1-α) ≈ 50-200.

6.3 Transistor as an Amplifier

In CE configuration, a small change in base current causes a large change in collector current. Voltage gain: A_V = β × R_C/R_B.

6.4 Transistor as a Switch

In cutoff region: transistor is OFF (no current). In saturation region: transistor is ON (maximum current).

7. Logic Gates

NOT: Y = Ā (inverter). AND: Y = A·B. Y = 1 only if both inputs are 1. OR: Y = A + B. Y = 1 if at least one input is 1. NAND: Y = A·B (universal gate). NOR: Y = A + B (universal gate). XOR: Y = A⊕B. Y = 1 when inputs differ.

8. Worked Problems

Problem 1: Find the current through a Si diode in forward bias with V = 0.7V and series resistance 100Ω. Solution: V_R = 0.7 - 0.7 = 0V (at knee voltage). I = 0A at exact knee. For V = 1V: I = (1-0.7)/100 = 3 mA.

Problem 2: In a CE amplifier, β = 100, I_B = 20 μA, find I_C. Solution: I_C = βI_B = 100 × 20×10^{-6} = 2×10^{-3} A = 2 mA.

Problem 3: Realize the AND gate using NAND gates. Solution: Connect two NAND gates: first NAND gives A·B, second NAND (as inverter) gives (A·B) = A·B.

9. Common Mistakes

'Students often forget that in forward bias, the diode does NOT conduct below the knee voltage (0.7V for Si). The current rises exponentially only after this threshold.'

10. ISC Exam Focus

11. Self-Test Questions

1. Distinguish between intrinsic and extrinsic semiconductors. Explain how n-type and p-type semiconductors are formed.
2. Draw the V-I characteristics of a p-n junction diode in forward and reverse bias.
3. Explain the working of a full-wave rectifier with a circuit diagram.
4. In a CE transistor amplifier, β = 150, I_B = 40 μA. Find I_C and I_E.
5. Write the truth table and symbol for NAND and NOR gates. Show how NAND can be used as a universal gate.