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LEDs - Light Emitting Diodes

Solid State Solutions

Solid state solutions of three and four elements are commonly used inlight emitting diodes. In cases of 1:1 stoichiometry such as GaAs, Al canbe substituted for a fraction of the Ga or P can be substituted for afraction of the As, thus producing Al1-xGaxAs orGaP1-xAsx semiconductors, respectively. Fourelements can even be combined to produce Al1-xGaxPyAs1-y.

Substitutional solid solutions A1-xZx with varyingstoichiometries (varying values of x).

Blue light is emitted by LEDs containing In, Ga, and N, while GaP LEDsemit green light and and LEDs containing Ga, P and As emit red light. Light emitting diodes can be made that are solid phase solutions of threeelements, GaP1-xAsx, where x varies from 1 to 0.For x equals 0.6, the LED is red. The LED emits orange light when xequals 0.35. For x equals 0.15, the LED emits yellow light. Green lightis emitted when x = 0, i.e., LEDs with a composition of GaP.

LED strip of red, orange, yellow, green, and blue LEDs.

Letters outlined using red, orange, yellow, green, and blue LEDs.

The band gap energy can also be described qualitatively in terms ofchemical bonds. The band gap energy can be regarded as the energyrequired to free an electron from a bond in the solid, enabling it tobecome mobile and thus contribute to electrical conductivity. Connectingthe LED to a battery in an electrical circuit provides a source of energyto liberate electrons from their bonds. When some of the electrons returnto restore bonds, roughly the band gap energy is liberated, as this is thereverse of the process used to create mobile electrons that requiredenergy. Some of the energy released is in the form of light. Thus, formany LEDs the color of light is a rough measure of the band gap energy ofthe semiconductor comprising the LED.

The band gap energy can be tuned by varying the chemical composition ofthe semiconductor. Considering first solids with the diamond structure,there is a smooth variation in electrical conductivity in descending theperiodic table. In diamond itself, the carbon atoms are relatively smalland close to one another so that the bonding electrons are held verytightly by the atoms. This corresponds to a large band gap energy andpoor electrical conductivity. Indeed, diamond is a superb electricalinsulator. In passing to silicon and germanium, the atoms are larger andfarther apart, and the bonding electrons are held much less tightly. Thiscorresponds to a smaller band gap energy and a larger pool of mobileelectrons at room temperature, corresponding to semiconductor-likeconductivity. Finally, with tin, the atoms are yet larger and fartherapart. The band gap energy for tin is very small and the largeconcentration of mobile electrons corresponds to metallic conductivity.

For many combinations of elements in compound semiconductors, the trend inband gap energy can be qualitatively predicted based on interatomicdistance, with, as noted above, shorter bonds corresponding to larger bandgap energies. As the illustration shows, pairing Ga with As, P, andfinally N leads to a progressive reduction in bond length and acorresponding increase in band gap energy and LED color. This family ofsolids, in fact, permits tuning of the band gap energy from the nearinfrared (GaAs) to the red (As-rich solid solutions of GaAsxP(1-x)) to thegreen (GaP) and most recently to the blue (GaN).

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