Lecture on Energy Bands and Classification of Solid Materials

Introduction

• Focus on energy bands and classification of solid materials.
• Discuss valence band and conduction band.

Energy Bands

Valence Band

• Associated with valence electrons.
• Valence electrons:
  • Bounded in atomic structure (e.g., silicon atom: 2 electrons in the first orbit, 8 in the second, and 4 in the outermost orbit).
  • Connected by covalent bonds.
  • Do not participate in the flow of current.
• Valence band energy is low due to bounded electrons.

Conduction Band

• Associated with free electrons.
• Free electrons:
  • Not connected in atomic structure.
  • Participate in the flow of current.
• Conduction band energy is high due to free electrons.

Energy Band Diagram

• Conduction band energy is high.
• Valence band energy is low.
• Gap between conduction band and valence band is the forbidden energy gap.
  • Represents the energy needed for an electron to move from valence to conduction band.

Forbidden Energy Gap

• Necessary for the flow of current.
• Ways to provide energy for electron transition:
  • Apply electric field, potential difference, or heat.

Classification of Solid Materials

Conductors

• High conductivity.
• Conduction and valence bands overlap.
• Examples: Copper, Aluminum.
• Conductivity: (\sigma > 10^7 \text{ per ohm-meter}).

Semiconductors

• Moderate conductivity.
• Forbidden energy gap exists (e.g., Silicon: 1.1 eV).
• Examples: Silicon, Germanium.
• Conductivity: (10^{-6} < \sigma < 10^4 \text{ per ohm-meter}).

Insulators

• Very low conductivity.
• High forbidden energy gap ((> 6 \text{ eV})).
• Examples: Mica, Wood.
• Conductivity: (\sigma << 10^{-10} \text{ per ohm-meter}).

Conductivity and Resistivity

• Conductivity ((\sigma)) is inversely proportional to resistivity ((\rho)).
• Relation: (\sigma \propto \frac{1}{\rho}).
• Conductivity unit: per ohm-meter.
• Resistivity unit: ohm-meter.

Summary

• Valence band: low energy, no current flow.
• Conduction band: high energy, participates in current flow.
• Classification based on energy band diagram.
• Importance of understanding forbidden energy gap and materials' conductivity.

Note

• Remember the distinction between valence and conduction bands and their roles in electrical conduction.
• Understanding the forbidden energy gap is crucial for classifying materials as conductors, semiconductors, or insulators.