Band Gap and Semiconductor Current Carriers

Published

If we recall some basic topics in chemistry, we’ll remember that all atoms consist of neutrons, protons, and electrons, except for a normal hydrogen atom which doesn’t have a neutron. Using the Bohr model, we can visualize that an atom has a central nucleus consisting of protons and neutrons that is surrounded by orbiting electrons.

Band Gap And Semiconductor Current Carriers Image1 522X261
Structure of an atom consisting of neutrons, protons, and electrons

The orbits surrounding the nucleus are grouped into energy levels known as shells and the outermost shell is called the valence shell. The valence shell of an atom represents a band of energy levels, which is why it’s also called a “valence band” and valence electrons are confined to that band. When a valence electron gains enough energy from an external source, it can escape from the valence band and goes to the conduction band.

Band Gap And Semiconductor Current Carriers Gif1
Valence shell - the outermost shell orbiting the nucleus

Band Gap

The difference in energy between the valence band and the conduction band is called “band gap”. It is the amount of energy a valence electron must possess so that it can jump from the valence band to the conduction band, wherein the electron is free to move throughout the material. If the band gap is really big, electrons will have a hard time jumping to the conduction band, which is the reason of material’s poor conductivity.

Energy Diagrams

Let’s try to examine the energy diagram of the three types of materials used in electronics and discuss the conductivity of each material based on their band gap.

Band Gap And Semiconductor Current Carriers Image2 522X261
Insulator energy diagram and band gap

As we can see, the band gap between the valence band and conduction band in an insulator is really big. That is why it doesn’t conduct current.

Band Gap And Semiconductor Current Carriers Image3 522X261
Semiconductor energy diagram and band gap

The band gap in a semiconductor is smaller compared to an insulator and allows valence electrons in the valence band to jump into the conduction band if it receives external energy.

Band Gap And Semiconductor Current Carriers Image4 522X261
Conductor energy diagram and band gap

In a conductor, like copper, there’s no band gap. Actually, the conduction band and valence band overlaps, which means that electrons can freely move into the conduction band. This is why you may hear the electrons in metal referred to as a “sea of electrons” - they’re just floating around.

Current Carriers

Now that we know more about band gap, let’s discuss the two types of current carriers in a semiconductor, free electrons and holes, and see how they produce current in a semiconductor.

Band Gap And Semiconductor Current Carriers Image5 522X261
Covalent bonds in a silicon crystal

Atoms may combine to form a solid crystalline material through covalent bonding. For example, a silicon atom covalently bonds with four adjacent silicon atoms to form an intrinsic silicon crystal. Intrinsic because it doesn’t contain impurities and a crystal because there is a pattern in how the atoms are connected.

Electron Current

Band Gap And Semiconductor Current Carriers Gif2
Electrons jumping into the conduction band, becoming free electrons

At room temperature, intrinsic silicon crystal gains enough heat energy that enables some of the valence electrons to jump into the conduction band, becoming free electrons. When this happens, vacancies are left in the valence band within the crystal. These vacancies are known as holes.

Band Gap And Semiconductor Current Carriers Gif3
Electron current - movement of free electrons in the conduction band when voltage is applied

Now, if we put a voltage source across an intrinsic silicon material, the thermally-generated free electrons in the conduction band will be attracted to the positive end of the voltage source. They will move toward the positive end and this movement produces current in the material. This type of current is called electron current.

Hole Current

While electron current happens in the conduction band, the other type of current, hole current, happens in the valence band. Remember that as valence electrons jumped into the conduction band, vacancies or holes are left in the valence band. Electrons that remain in the valence band can move into a nearby hole when it receives a small amount of energy. This movement produces a current in the valence band called hole current.

Band Gap And Semiconductor Current Carriers Gif4
Hole current - movement of valence electrons into a nearby hole when it receives a small amount of energy

Though the current is produced by valence electrons moving into a nearby hole, it is called hole current so that it won’t be confused with the electron current produced in the conduction band.

In Summary:

  • Band gap and the current carriers in a semiconductor.
  • The energy diagram of the three types of materials used in electronics, insulators, semiconductors, and conductors and briefly discussed their conductivities.
  • The two types of semiconductor current carriers, free electrons and holes, and mentioned how they produce current in a semiconductor material.

Check Yourself

9 Questions


Check Yourself

9 Questions


Related Tutorials

Terms Used

Make Bread with our CircuitBread Toaster!

Get the latest tools and tutorials, fresh from the toaster.

What are you looking for?