Understanding Semiconductors

What are Semiconductors?

• Semiconductors are materials that have properties of both conductors and insulators.
• They are able to conduct electricity to some extent.
• Some examples of semiconductors are carbon, silicon, and germanium.
• Semiconductors can behave as conductors at certain voltages and temperatures, and as insulators at others.

Semiconductors as Conductors vs. Insulators

• Conductors are materials that allow electricity to flow through them easily, such as metals.
• Insulators are materials that do not allow electricity to flow through them, such as wood and plastic.

Semiconductors at Zero Kelvin

• Semiconductors act as perfect insulators at zero Kelvin (absolute zero).
• At this temperature, no current can flow through them.

Key Properties of Semiconductor Materials

• The outermost shell (valence shell) of semiconductor atoms has four electrons.
• The farther an electron is from the nucleus, the easier it is to remove.
• Germanium has the outermost shell farther from the nucleus than silicon, making it easier to remove electrons.
• This makes germanium a better conductor than silicon at normal temperatures.

Why Silicon is Used in Semiconductors

• Silicon is more abundant in nature than germanium, making it an ideal material for creating semiconductor devices.

What is Energy Band Theory?

• This theory explains the behavior of semiconductors and other materials.
• It uses a graph to show how energy levels change based on interatomic distance.
• The X-axis represents the distance between atoms.
• The Y-axis represents the energy levels of the electrons.

Energy Levels in Atoms

• Electrons in atoms have kinetic energy due to their movement and potential energy due to their position within the electric field created by the nucleus.
• The total energy of an electron is the sum of its kinetic and potential energies.
• Each orbital in an atom has a specific energy level, and electrons in higher orbitals have higher energy levels.

How Atomic Spacing Affects Energy Levels in Solids

• When atoms are far apart, their energy levels are distinct and predictable.
• As atoms are brought closer together, the energy levels of their outer shell (valence) electrons begin to interact.
• This interaction causes the energy levels to split and spread out.
• The energy levels of electrons in the inner shells are less affected.

Creation of Valence and Conduction Bands

• As the atoms get closer together in a solid, the energy levels in the outer shell become a continuous band, called the valence band.
• Electrons within the valence band are tightly bound to the atoms, creating a lower energy state.
• When enough energy is provided, the electrons can jump to a higher energy state called the conduction band.
• The conduction band is a continuous band of energy levels where electrons can move freely, making the material conductive.
• The energy gap between the valence band and conduction band influences the material's conductivity.

Differences in Energy Gaps for Different Materials

• Conductors: The valence and conduction bands overlap, meaning electrons can easily move into the conduction band, giving them high conductivity.
• Insulators: There is a large energy gap between the valence and conduction bands, so electrons can only move into the conduction band when provided with a significant amount of energy. This makes the material a poor conductor.
• Semiconductors: The energy gap between the valence and conduction bands is moderate, making them able to conduct electricity under specific conditions (voltage, temperature).

Conclusion

• Energy band theory provides a framework to understand the behavior of semiconductors and other materials based on their energy band structure.
• The size of the energy gap significantly affects the conductivity of a material.
• Semiconductors play a crucial role in electronic devices due to their unique behavior and ability to control the flow of electricity.

Energy Band Theory

• The energy band theory explains the electrical conductivity of materials.
• Materials are categorized based on the energy band gap between the valence band and the conduction band.
• The valence band is the outermost band of electrons that are bound to atoms.
• The conduction band is the band where electrons can move freely, allowing for electrical conductivity.

Conductors

• Conductors have a zero energy band gap.
• The valence and conduction bands overlap.
• Electrons in the valence band can easily move to the conduction band, resulting in high electrical conductivity at all temperatures.
• This is because there is no energy barrier between the valence and conduction bands.

Semiconductors

• Semiconductors have an energy band gap of less than 3 electron volts.
• The energy band gap is small enough that electrons in the valence band can gain energy and move to the conduction band.
• The conductivity of semiconductors increases with increasing temperature.
• Silicon has an energy band gap of 1.1 eV.
• Germanium has an energy band gap of 0.7 eV.

Insulators

• Insulators have a large energy band gap, typically greater than 3 electron volts.
• The energy band gap is too large for electrons in the valence band to overcome, preventing them from moving to the conduction band.
• Insulators have very low electrical conductivity.
• Examples of insulators include wood, plastic, and glass.

Energy Band Diagram for Conductors, Semiconductors, and Insulators

• The energy band diagram helps visualize the energy levels and band gaps in materials.
• Conductors have no gap between the valence and conduction bands.
• Semiconductors have a small energy band gap.
• Insulators have a large energy band gap.