Understanding Lasers

Chapter 9.3: Light Emission at Junction

9.3 LIGHT EMISSION AT JUNCTIONS

Henry J. Round was puzzled in 1907 when he saw light emission from an impure form of silicon carbide called carborundum. He knew the light arose from the junction between a metal conductor and the material, which we now know is a semiconductor, but he did not understand what produced it. Figuring that out took other researchers decades.

Today, we understand that phenomenon, called recombination radiation, which occurs when electrons in the conduction band of a semiconductor drop into holes in the valence band. Round saw light emission from a junction between a semiconductor and a conductor like the point contact junction of the first transistor. Modern LEDs and diode lasers emit light at a p–n junction between regions of p- and n-type semiconductor, which makes them more versatile, like the junction transistors that followed the point-contact type.

Recombination releases an amount of energy equal to the gap between the conduction and valence bands. That energy is released at all p–n junctions, but what happens to it depends on the nature of the band gap. In indirect-bandgap semiconductors, the energy is dissipated within the crystal. Direct-bandgap semiconductors instead emit a photon with energy equal to the band gap. Because the band-gap energy depends on the semiconductor composition, the emission wavelength also depends on composition.

Recombination is the basis of light emission from both LEDs and diode lasers. Let us look carefully at the differences, starting with the LED.

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