As we earlier on said, a diode comprises a section of N-type material bonded to a section of P-type material, with electrodes on each end.

This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state, all of the holes are filled.

So there are no free electrons or empty spaces for electrons, and charge can’t flow, in order to get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction.

To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge, moves across the diode.

If you try to run current the other way, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, current will not flow. The negative electrons in the N-type material are attracted to the positive electrode. The positive holes in the P-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases.   

The interaction between electrons and holes in this setup has an interesting side effect in that it generates light! Now, how Can a Diode Produce Light? As a matter of fact, light is a form of energy that can be released by an atom. It is made up of many small particles like packets that have energy and momentum but no mass.

These particles, called photons (from photo for light), are the most basic units of light.  Photons are released as a result of moving electrons. In an atom, electrons move in orbitals around the nucleus. Electrons in different orbitals have different amounts of energy.

Normally, electrons with greater energy move in orbitals farther away from the nucleus.  For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy level. Conversely, an electron releases energy when it drops from a higher orbital to a lower one. This energy is released in the form of a photon. A greater energy drop releases a higher-energy photon, which is characterized by a higher frequency.  

In reality, free electrons moving across a diode can fall into empty holes from the P-type layer. This involves a drop from the conduction band to a lower orbital, so the electrons release energy in the form of photons.

This happens in any diode, but you can only see the photons when the diode is composed of certain material. The atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively short distance. As a result, the photon’s frequency is so low that it is invisible to the human eye; it is in the infrared portion of the light spectrum.

Visible light-emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock, are made of materials characterized by a wider gap between the conduction band and the lower orbitals. The size of the gap determines the frequency of the photon, in other words, it determines the colour of the light. 
 
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