Classification of Materials (Metals, Conductors, Semiconductors)

Introduction

The electrical properties of materials vary over an enormous range. Some materials readily allow electric current to flow through them, while others strongly resist it. This difference in electrical conductivity allows us to classify materials into broad categories: conductors, semiconductors, and insulators. This classification is not merely descriptive but has deep roots in the atomic structure and bonding of the materials, which determines the availability and mobility of charge carriers (usually electrons).

Understanding these classifications is fundamental to the study of electronics and electrical engineering, as different materials are used for specific purposes in electrical circuits and devices based on their conductive properties. Conductors are used for wires and components requiring low resistance, insulators for preventing unwanted current flow, and semiconductors form the basis of modern electronic components like diodes and transistors, due to their controllable conductivity.

The behaviour of electrons in materials, especially in solids, can be best understood using the concept of energy bands, which arise from the interactions between the electron orbitals of a large number of closely packed atoms. This energy band theory provides a theoretical framework to explain why some materials are excellent conductors, others are insulators, and some have intermediate, tunable conductivity (semiconductors).

Classification Of Metals, Conductors And Semiconductors (Energy Band Diagram)

The electrical conductivity of solid materials spans a wide range, from very high conductivity in metals to extremely low conductivity in insulators. Semiconductors fall in between these two extremes, possessing conductivity that is intermediate and, crucially, highly sensitive to factors like temperature and the presence of impurities. The difference in conductivity arises from the way electrons are arranged in energy levels within the material, which can be described using the Energy Band Theory.

Energy Band Theory

In an isolated atom, electrons occupy discrete energy levels. When a large number of atoms come together to form a solid, the energy levels of the individual atoms interact and split into a large number of closely spaced levels. These closely spaced levels form continuous bands of allowed energy for electrons within the solid. Between these allowed energy bands are regions of forbidden energy, called energy gaps.

The most important bands for electrical conductivity are the outermost electron bands:

• Valence Band: This is the highest occupied energy band at absolute zero temperature (0 K). It contains the valence electrons, which are involved in bonding. Electrons in the valence band are generally tightly bound to the atoms and are not free to move throughout the crystal.
• Conduction Band: This is the lowest unoccupied energy band above the valence band. Electrons in the conduction band are not bound to any particular atom and are free to move throughout the material, acting as charge carriers.

The energy separation between the top of the valence band and the bottom of the conduction band is called the band gap energy ($E_g$).

Energy band diagram showing valence band, conduction band, and band gap.

Electrical conductivity depends on the availability of free electrons in the conduction band and holes (vacancies of electrons) in the valence band. Electrons can move freely in the conduction band. When an electron leaves the valence band and moves to the conduction band, it leaves behind a vacancy (a "hole") in the valence band. This hole can also move through the valence band, effectively acting as a positive charge carrier.

Electrons need to gain energy to move from the valence band to the conduction band across the band gap. This energy can be supplied by heat (thermal energy) or light.

Classification based on Energy Band Diagrams

Materials are classified into conductors, semiconductors, and insulators based on the nature of their energy bands and the band gap between them:

1. Conductors (Metals):
   • In conductors, the valence band and the conduction band overlap, or the conduction band is partially filled.
   • There is no energy gap between the valence band and the conduction band ($E_g = 0$).
   • Electrons in the valence band can easily move into the conduction band even at room temperature, as they require negligible energy.
   • A large number of free electrons are available in the conduction band, which can readily move under the influence of an electric field, resulting in high electrical conductivity.
   • As temperature increases, the thermal motion of lattice ions increases, scattering the free electrons more frequently, which hinders their movement. This causes the conductivity of metals to generally decrease with increasing temperature, and resistivity to increase.
   • Examples: Copper, Aluminium, Silver, Gold, Iron.

   Energy band diagram for a conductor (valence and conduction bands overlap).

2. Semiconductors:
   • In semiconductors, there is a small energy gap ($E_g$) between the valence band and the conduction band.
   • The band gap energy is typically around 1 eV (e.g., $E_g \approx 1.1 \, eV$ for Silicon, $E_g \approx 0.7 \, eV$ for Germanium).
   • At absolute zero temperature (0 K), the valence band is completely filled, and the conduction band is empty. The semiconductor behaves like an insulator.
   • At room temperature, some valence electrons gain enough thermal energy to jump across the small band gap into the conduction band. This creates free electrons in the conduction band and holes in the valence band. Both electrons and holes contribute to conductivity.
   • The number of free charge carriers (electrons and holes) increases significantly with increasing temperature. Although increased thermal motion also increases scattering (decreasing mobility), the increase in carrier density is the dominant factor. Thus, the conductivity of semiconductors generally increases with increasing temperature (resistivity decreases).
   • The conductivity of semiconductors can be dramatically altered by adding small amounts of impurities (doping), making their conductivity controllable.
   • Examples: Silicon (Si), Germanium (Ge).

   Energy band diagram for a semiconductor (small band gap).

3. Insulators:
   • In insulators, there is a large energy gap ($E_g$) between the valence band and the conduction band.
   • The band gap energy is typically much larger than thermal energy at room temperature (e.g., $E_g > 3 \, eV$).
   • At room temperature, very few, if any, electrons have enough energy to jump across the large band gap into the conduction band. The valence band is essentially filled, and the conduction band is empty.
   • The number of free charge carriers is extremely small, resulting in very low electrical conductivity. They effectively block the flow of electric current under normal voltage conditions.
   • Increasing the temperature does slightly increase conductivity, but it remains very low due to the large band gap.
   • Examples: Wood, Plastic, Rubber, Glass, Diamond.

   Energy band diagram for an insulator (large band gap).

Comparison based on Conductivity/Resistivity

The classification is also evident in the ranges of resistivity ($\rho$) or conductivity ($\sigma = 1/\rho$) values at room temperature:

This band theory model provides a successful explanation for the observed differences in the electrical properties of materials and is the foundation for understanding semiconductor devices.