Semiconductor Electronics: Materials, Devices, and Simple Circuits

14.1 Introduction

• Controlled Flow of Electrons: Fundamental to electronic circuits.

  • Before transistors (1948), vacuum tubes (diodes, triodes, etc.) were used.

  • Vacuum tubes are bulky, high power-consuming, and less reliable.

• Transition to Semiconductors:

  • 1930s: Realization that solid-state semiconductors can control charge flow.

  • Semiconductors are smaller, low power, and more reliable than vacuum tubes.

• Historical Context:

  • Early use of galena as a radio wave detector.

  • Introduction to semiconductor physics and devices like junction diodes and bipolar junction transistors.

14.2 Classification of Metals, Conductors, and Semiconductors

• Conductivity Classification:

  • Metals: Low resistivity (10⁻² to 10⁻⁸ Ωm).

  • Semiconductors: Intermediate resistivity (10⁻⁵ to 10⁶ Ωm).

  • Insulators: High resistivity (10¹¹ to 10¹⁹ Ωm).

• Types of Semiconductors:

  • Elemental: Silicon (Si), Germanium (Ge).

  • Compound: Inorganic (e.g., GaAs, CdS) and Organic (e.g., polythiophene).

• Energy Bands:

  • Electrons in solids form energy bands (valence and conduction bands).

  • Band gaps determine electrical properties (insulators, semiconductors, metals).

14.3 Intrinsic Semiconductor

• Crystal Structure: Si and Ge have diamond-like structures with covalent bonds.

• Thermal Excitation: At higher temperatures, electrons can break free, creating holes.

• Carrier Concentration: In intrinsic semiconductors, the number of free electrons equals the number of holes (nₑ = nₕ = nᵢ).

14.4 Extrinsic Semiconductor

• Doping: Adding impurities to enhance conductivity.

  • n-type: Doped with pentavalent elements (e.g., As, P) which donate extra electrons.

  • p-type: Doped with trivalent elements (e.g., B, Al) which create holes.

• Carrier Concentration:

  • n-type: nₑ >> nₕ.

  • p-type: nₕ >> nₑ.

• Charge Neutrality: Overall charge neutrality is maintained in doped semiconductors.

14.5 p-n Junction

• Formation: Created by joining p-type and n-type semiconductors.

• Charge Carrier Movement:

  • Diffusion: Holes move from p to n, electrons from n to p.

  • Drift: Electric field created by charge separation opposes further diffusion.

• Equilibrium: No net current flows; a potential barrier forms.

14.6 Semiconductor Diode

• Structure: A p-n junction with metallic contacts.

• Forward Bias:

  • p-side connected to positive terminal, reducing barrier potential.

  • Current increases as more carriers cross the junction.

• Reverse Bias:

  • n-side connected to positive terminal, increasing barrier potential.

  • Current is minimal, primarily due to minority carriers.

14.7 Application of Junction Diode as a Rectifier

• Half-Wave Rectifier: Allows current flow during positive half-cycles of AC.

• Full-Wave Rectifier: Utilizes both half-cycles for current flow.

• Filtering: Capacitors are used to smooth the output voltage, providing a steady DC output.

Summary

1. Semiconductors are essential for modern electronic devices.

2. Conductivity varies among metals, semiconductors, and insulators.

3. Doping alters the conductivity of semiconductors, creating n-type and p-type materials.

4. p-n junctions are crucial for semiconductor device functionality, allowing controlled current flow.

5. Diodes rectify AC to DC, with filtering techniques enhancing output stability.

Points to Ponder

• Energy bands in semiconductors are delocalized.

• Defects and stoichiometric ratios in compound semiconductors affect properties.

Exercises

1. Identify true statements regarding n-type silicon.

2. Analyze statements for p-type semiconductors.

3. Compare energy gaps of carbon, silicon, and germanium.

4. Explain hole diffusion in unbiased p-n junctions.

5. Discuss effects of forward bias on p-n junctions.

6. Determine output frequencies for half-wave and full-wave rect