University of Wisconsin - Madison Materials Research Science and Engineering Center Education and Outreach

LEDs - Light Emitting Diodes

Band Theory

As described below, simple chemical bonding ideas can be used to provide a qualitative understanding of how the color of light emitted by an LED is tuned by changes in the semiconductor's chemical composition.

The following is a brief review of the band theory under pinning these devices.

Bonds to Bands, as more bonds occur between atoms, the energy difference becomes less, creating a band of energies.

When two atoms bond together to form M2, two molecular orbitals are formed. When three atoms bond together to form M3, three molecular orbitals are formed.

Bands in extended solids range from lower energy, most bonding, to high most antibonding orbitals

In the extended solid, many atoms interact with one another, and there will be the same number of molecular orbitals, delocalized over the entire solid, as the number of atomic orbitals being combined. The energy separation of the orbitals is so small from one such delocalized orbital to the next that they comprise a so-called "band." The band represents a kind of electronic highway allowing electrons to move throughout the solid, thereby conducting electricity. For this to occur, however, the band cannot be empty or filled with electrons. Only if the band is partially filled can a net flow of electrons occur, corresponding to an electrical current.

The band containing the valence electrons is known as the valence band. The band of unoccupied orbitals is known as the conduction band. Conduction occurs when electrons are promoted from the valence band to the conduction band, where they can move throughout the solid. The energy separation between the valence and conduction bands is known as the bandgap energy.

Schmatic band diagrams for an insulator with large band gap, a semiconductor with small band gap, and a metal with little or no band gap.
Schematic band diagrams for an insulator, a semiconductor, and a metal.

The band gap energy, Eg, shown as the double-headed arrow, is the separation between the top of the valence band and the bottom of the conduction band. The size of the band gap decreases in passing from aninsulator to a semiconductor to a metal, where it is effectively zero. Electron-hole pairs are shown for a semiconductor as filled circles (electrons) in the conduction band and open circles (holes) in the valence band.

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