Semiconductor Electronics: Materials, Devices, and Simple Circuits

14.1 Introduction

• Vacuum Tubes: Before transistors, vacuum tubes like diodes, triodes were used; electrons flow from heated cathode to anode in vacuum.
• Limitations: Bulky, high power, limited life, unreliable.
• Semiconductors: Introduced controlling flow of charge carriers without vacuum; small, low power, reliable.
• Early Detectors: Galena crystals used in radio wave detection.

14.2 Classification of Metals, Conductors, and Semiconductors

• Based on Conductivity:
  • Metals: Low resistivity, high conductivity.
  • Semiconductors: Intermediate resistivity.
  • Insulators: High resistivity.
• Types of Semiconductors:
  • Elemental: Si, Ge.
  • Compound: CdS, GaAs.
• Energy Bands:
  • Valence Band: Occupied by valence electrons.
  • Conduction Band: Generally empty, electrons can move freely here.
  • Energy Gap: Determines conductivity; varies for metals, semiconductors, insulators.

14.3 Intrinsic Semiconductors

• Structure: Diamond-like; Si, Ge share electrons in covalent bonds.
• Conduction: Thermal energy breaks bonds, creating free electrons and holes.
• Current: Combination of electron and hole movements.

14.4 Extrinsic Semiconductors

• Doping:
  • n-type: Pentavalent atoms (e.g., As, Sb) donate extra electrons.
  • p-type: Trivalent atoms (e.g., B, Al) create holes.
• Charge Carriers:
  • n-type: Electrons are majority carriers.
  • p-type: Holes are majority carriers.

14.5 p-n Junction

• Formation: Combining p-type and n-type semiconductors.
• Depletion Region: Formed by diffusion of charge carriers, devoid of free charges.
• Equilibrium: No net current; potential barrier forms.

14.6 Semiconductor Diode

• Structure: p-n junction with metal contacts.
• Operation:
  • Forward Bias: Reduces barrier, allows current flow.
  • Reverse Bias: Increases barrier, restricts current.
• Characteristics: Forward bias - high current, reverse bias - low current.

14.7 Applications of Junction Diode as a Rectifier

• Rectification: Converts AC to DC using diode's unidirectional current flow.
• Half-wave Rectifier: Conducts during one half-cycle of AC.
• Full-wave Rectifier: Uses both half-cycles, more efficient.
• Filters: Smooth out pulsating DC using capacitors or inductors.

Summary

• Semiconductors: Basis for modern electronics.
• Types: Intrinsic (pure) and extrinsic (doped).
• p-n Junctions: Key to device functionality.
• Diodes: Used for rectifying AC voltages.

Points to Ponder

• Energy Bands: Not localized, represent energy averages.
• Defects: Influence semiconductor properties.

Exercises

• Includes questions on semiconductor properties, junction behavior, and applications of diodes.