"Semiconductors and Semiconducting Devices"

About

Semiconductors are the backbone of modern electronics — from computer chips to smartphones. This chapter introduces the physics of semiconductors, how doping creates n-type and p-type materials, the formation and behaviour of the p-n junction, and the working of diodes and transistors.

Key Concepts

28.1 Energy Bands

28.2 Intrinsic Semiconductor

Pure semiconductor. At 300 K, for silicon: $n_i=1.5\times 10^{16}$ m⁻³.

• Electron concentration ($n$) = hole concentration ($p$) = $n_i$
• Charge neutrality is maintained

28.3 Extrinsic Semiconductor

Doping: Adding controlled impurity to increase conductivity.

• n-type: Electrons = majority carriers, holes = minority
• p-type: Holes = majority carriers, electrons = minority
• Extrinsic has lower resistivity than intrinsic

28.4 p-n Junction

Formation:

1. Majority carriers diffuse across the junction
2. Depletion region forms — contains uncompensated acceptor/donor ions
3. Barrier potential develops: Si ≈ 0.7 V, Ge ≈ 0.3 V

Biasing:

Diode: Allows current in one direction only — rectification.

28.5 Transistor

A three-terminal device with two p-n junctions (npn or pnp).

Configurations: Common base (CB), Common emitter (CE), Common collector (CC).

Uses: Amplification, switching, oscillation.

INTEXT QUESTIONS 28.1

Q1. At 300 K, pure silicon has $n_i=1.5\times 10^{16}$ m⁻³. What is the concentration of holes and electrons?

Ans: In intrinsic silicon: $n=p=n_i=1.5\times 10^{16}$ m⁻³.

Q2. n-type semiconductor is obtained by doping with:

Ans: (ii) Pentavalent impurity. (P, As, Sb — Group V, 5 valence electrons → extra electron).

Q3. An intrinsic semiconductor can be converted into extrinsic by addition of ______. This process is called ______.

Ans: Impurity and doping.

Q4. Electrons in n-type and holes in p-type are the ______ carriers.

Ans: Majority.

Q5. An extrinsic semiconductor has ______ resistivity compared to intrinsic.

Ans: Lower.

INTEXT QUESTIONS 28.2

Q1. Fill in the blanks:

(a) When a p-n junction forms, the majority carriers diffuse across. (b) Uncompensated ions form the depletion region. (c) Barrier potential: Si = 0.7 V, Ge = 0.3 V. (d) Electrons diffuse from n to p due to concentration difference.

Terminal Exercise

1. Distinguish between conductors, semiconductors, and insulators on the basis of energy band theory.

2. What is doping? Distinguish between n-type and p-type semiconductors.

3. Explain the formation of a p-n junction. What is a depletion region and barrier potential?

4. Draw and explain the V-I characteristics of a p-n junction diode.

5. Explain how a p-n junction diode works as a half-wave rectifier.

6. Describe the working of an npn transistor. Define $\alpha$ and $\beta$.

7. A transistor has $\beta =100$. If the base current changes by 20 μA, find the change in collector current.

8. Why is silicon preferred over germanium for making semiconductor devices?

9. In a transistor in CE configuration, $I_B=50$ μA, $I_C=5$ mA. Find $I_E$ and $\beta$.

10. Distinguish between Zener breakdown and avalanche breakdown.

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