“Understanding Materials: A Guide to Conductors, Insulators, and Semiconductors”

Author:

Prof. Kali Chandrakant

M.Sc., M.Ed., D.C.S.

50+ Years of experience in Physics teaching

 1.Introduction:

      Have you ever wondered why copper is used in electrical wires while glass is used as an insulator? The answer lies in how materials conduct electricity. Based on their electrical conductivity, materials are classified into three main categories: Conductors, Insulators, and Semiconductors.

     In this article, we will explore these materials through the lens of Band Theory. We will explore their distinct properties, atomic structures, and applications, with a focus on understanding how these materials behave under the influence of an electric field.

 2.Band Theory: 

2.1. Main points:

1. The bonding of atoms, due to the sharing of electrons, is called covalent bonding.
2. In the crystal, closely spaced energy levels form a band called as the energy band.  Each orbit has a separate energy band.
3. A band of energy levels associated with valence electrons and uppermost filled band is calledvalence band. Electrons from other bands cannot be removed but electrons from valence band can be removed by supplying a little energy.
4. The empty band above valence band lowest unfilled band is conduction band.
5. The valence band and conduction bandare separated by a gap called forbidden energy gap.

 2.2.Band structure:

   a) The electrons in an isolated atom occupy discrete energy levels. When atoms are close to each other, these electrons can use the energy levels of their neighbors.

   b) When the atoms are all regularly arranged is called the crystal lattice of a solid, the energy levels become grouped together in a band. This is a continuous range of allowed energies rather than a single level. There will also be groups of energies that are not allowed, is known as a band gap E_g.

  c)  Similar to the energy levels of an individual atom, the electrons will fill the lower bands first.

  d)  The Fermi levelgives a rough idea of which levels electrons will generally fill up to that level.  

Fig. A Band diagram

3. Conductors:

 3.1. Definition and Properties:

     Conductors are materials that allow electric current to flow easily through them. They possess a large number of free electrons, which are electrons that are not bound to individual atoms and can move freely within the material.

• High Electrical Conductivity: This is the defining characteristic of conductors. They offer very little resistance to the flow of electric current.
• Low Resistivity: Resistivity is the inverse of conductivity. Conductors have very low resistivity values.
• Metallic Bonding: Most conductors are metals, characterized by metallic bonding where electrons are delocalized and shared among many atoms.
• Temperature Dependence: The conductivity of most conductors decreases with increasing temperature. This is because the increased thermal vibration of atoms hinders the movement of free electrons.

 3.2. Atomic Structure:

      Conductors have a partially filled valence band or overlapping valence and conduction bands. This means that electrons can easily move into the conduction band with minimal energy input. In simpler terms, there is no energy gap, or a very small one, between the valence and conduction bands. (Fig. A)

 3.3. Examples:

• Copper (Cu): Widely used in electrical wiring due to its excellent conductivity and relatively low cost.
• Aluminum (Al): Lighter than copper and also a good conductor used in power transmission lines.
• Silver (Ag): The best conductor of electricity, but its high cost limits its use to specialized applications.
• Gold (Au): Highly resistant to corrosion, making it suitable for use in electronic connectors and other critical applications.

 3.4. Applications:

     Conductors are essential components in virtually all electrical and electronic devices.

  They are used in:

• Electrical Wiring: To carry electricity from power sources to appliances and other devices.
• Electronic Circuits: To connect various components and allow current to flow through the circuit.
• Power Transmission Lines: To transmit electricity over long distances.
• Heat Sinks: Some conductors, like aluminum, are also good thermal conductors and are used to dissipate heat from electronic components.

 4. Insulators:

 4.1. Definition and Properties:

    Insulators are materials that resist the flow of electric current. They have very few free electrons and a large energy gap between the valence and conduction bands.

• High Electrical Resistance: Insulators offer very high resistance to the flow of electric current.
• High Resistivity: Insulators have very high resistivity values.
• Covalent Bonding: Many insulators are characterized by covalent bonding, where electrons are shared between atoms in a way that restricts their movement.
• Temperature Dependence: The conductivity of most insulators increases slightly with increasing temperature, but this increase is usually negligible.

 4.2. Atomic Structure:

      Insulators have a large energy gap between the valence and conduction bands. This means that a significant amount of energy is required to move electrons from the valence band to the conduction band, making it difficult for current to flow. (Fig. A)

 4.3. Examples:

• Rubber: Used to insulate electrical wires and cables.
• Glass: Used in insulators for power lines and in electronic components.
• Plastic: Used in a wide variety of applications, including insulation for wires, cables, and electronic components.
• Ceramics: Used in high-voltage insulators and other applications where high temperature resistance is required.
• Wood: Used in some low-voltage applications, but its insulating properties can vary depending on moisture content.

 4.4. Applications:

    Insulators are used to prevent the flow of electric current in unwanted directions and to protect people from electric shock.

 They are used in:

• Electrical Wiring and Cables: To insulate conductors and prevent short circuits.
• Power Lines: To insulate the conductors from the supporting structures and prevent current leakage.
• Electronic Components: To isolate different parts of a circuit and prevent interference.
• Protective Gear: Used in gloves, boots, and other equipment to protect electricians and other workers from electric shock.

5. Semiconductors:

5.1. Definition and Properties:

     Semiconductors are materials that have electrical conductivity between that of conductors and insulators. Their conductivity can be controlled by factors such as temperature, light, and the presence of impurities.

• Intermediate Conductivity: Semiconductors have conductivity values between those of conductors and insulators.
• Controllable Conductivity: The conductivity of semiconductors can be significantly altered by doping (adding impurities) or by applying an electric field.
• Temperature Dependence: The conductivity of semiconductors generally increases with increasing temperature.
• Energy Gap: Semiconductors have a smaller energy gap than insulators, but a larger energy gap than conductors.

5.2. Atomic Structure:

    Semiconductors have an energy gap between the valence and conduction bands that is small enough to allow some electrons to move into the conduction band at room temperature. The number of electrons in the conduction band can be increased by doping the semiconductor with impurities.     (Fig. A)

 5.3. Examples:

• Silicon (Si): The most widely used semiconductor material in the electronics industry.
• Germanium (Ge): Used in some specialized applications, but less common than silicon.
• Gallium Arsenide (GaAs): Used in high-speed electronic devices and optoelectronic devices.

 5.4. Doping:

   Doping is the process of adding impurities to a semiconductor to increase its conductivity.

There are two types of doping:

• N-type Doping: Adding impurities with more valence electrons than the semiconductor material (e.g., adding phosphorus to silicon). This creates an excess of free electrons, increasing conductivity.
• P-type Doping: Adding impurities with fewer valence electrons than the semiconductor material (e.g., adding boron to silicon). This creates “holes” (electron vacancies) that can move through the material, effectively carrying positive charge and increasing conductivity.

 5.5. Applications:

Semiconductors are the foundation of modern electronics. They are used in:

• Transistors: Used to amplify and switch electronic signals.
• Diodes: Used to allow current to flow in only one direction.
• Integrated Circuits (ICs): Complex circuits containing millions or billions of transistors and other components on a single chip.
• Solar Cells: Used to convert sunlight into electricity.
• LEDs (Light-Emitting Diodes): Used to emit light when current flows through them.

  6. Conclusion:

    Conductors, insulators, and semiconductors are essential materials with distinct electrical properties that make them suitable for a wide range of applications. Understanding their properties and behavior is crucial for designing and developing electrical and electronic devices. The ability to control the conductivity of semiconductors through doping has revolutionized the electronics industry, leading to the development of countless innovative technologies.