Lecture Notes on Conductivity, Semiconductors, and Electronic Devices

Introduction

• Lecture started at 5:30, will finish by 7:00.
• Focus on important topics: opto-electronic devices, photo cells, solar cells, LEDs, and semiconductor basics.
• Marks usually focus on 2 or 3 marks questions.

Conductivity and Resistivity

Basics

• Metals: High conductivity, low resistivity.
• Semiconductors: Moderate conductivity, moderate resistivity.
• Insulators: High resistivity, low conductivity.
• Relation: Resistivity is the reciprocal of conductivity.

Types of Semiconductors

Classification

• Elemental Semiconductors: Silicon and Germanium.
• Compound Semiconductors: Inorganic, organic, organic polymers.

Energy Bands

• Valence Band: Electrons bound to nucleus.
• Conduction Band: Electrons free to move.
• Energy Gap (Forbidden Gap): Determines conductivity:
  • Insulators: Gap > 3 electron volts.
  • Semiconductors: Gap < 3 electron volts; allows conduction.
  • Metals: Negligible or overlapping gap.

Intrinsic and Extrinsic Semiconductors

Intrinsic Semiconductors

• Pure semiconductors (e.g., silicon, germanium).
• At 0 Kelvin, nothing in conduction band; at room temperature, electrons get excited to conduction band.
• Charge Carriers: Number of electrons = number of holes.
• Law of Mass Action: nₑ × nₕ = nᵢ².

Extrinsic Semiconductors

• Doped Semiconductors: Add impurities to modify properties.
• Types:
  • p-type: Add trivalent impurity (more holes).
  • n-type: Add pentavalent impurity (more electrons).
• Impurity Conditions:
  • Size similar to silicon.
  • Few parts per million of the lattice.

p-n Junction and Diodes

Formation

• p-n Junction: Combination of p-type and n-type semiconductors.
• Depletion Region: Area around the junction where mobile charge carriers are absent.
• Barrier Potential: Potential difference across the depletion region.

Biasing

• Forward Bias: Reduces barrier potential, increases current.
• Reverse Bias: Increases barrier potential, minimal current.

Characteristics

• Forward Bias Current: Significant when external voltage > barrier potential.
• Reverse Bias: Defines breakdown voltage (Zener breakdown or Avalanche breakdown).

Rectifiers

Half-Wave Rectifier

• Converts AC to DC by allowing only one-half of the AC waveform to pass.

Full-Wave Rectifier

• Converts AC to DC by using both halves of the AC waveform.

Opto-Electronic Devices

Photo Diode

• Used to measure light intensity, operates in reverse bias.
• Working: Light breaks bonds, generating electron-hole pairs.
• Current: Increases with light intensity.

LED

• Operates in forward bias.
• Working: Electrons recombine with holes, releasing energy as light.

Solar Cell

• Converts light energy to electrical energy.
• No external bias: Light excites electrons, creating current flow.

Important Concepts and Formulas

• Conductors, Semiconductors, Insulators: Differ by energy band gaps.
• Photo Diode, LED, Solar Cell: Various biases and applications.
• Rectification: AC to DC conversion.

Summary

• Key concepts covered on semiconductors, electronic components, and their applications.
• Understanding conductivity, resistivity, and energy bands is crucial for electronics.
• Devices like LEDs and solar cells have specific functions and bias requirements.