LED - Basic Physical Function

LEDs are p-n junction devices constructed of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), gallium phosphide (GaP) for IR, red and green LEDs; or gallium nitride(GaN) and indium gallium nitride (InGaN) for blue LEDs. The junction in an LED is forward biased and when electrons cross the junction from n- to p-type material, the electron-hole recombination produces photons in the IR, near UV or visible in a process called electroluminescence.

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. Advances in materials science have made possible the production of devices with a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate that are transparent to the emitted wavelength, and backed by a reflective layer to increase the LED efficiency. The refractive index of most LED semiconductors is quite high. The refractive index of the package material should match the index of the semiconductor, otherwise the produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat, thus lowering the efficiency. The large index is usually one of the dominant causes of LED inefficiency. Often more than half of the emitted light is reflected back at the LED-package and package-air interfaces. The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized. An anti-reflection coating may be added as well. Other strategies for reducing the impact of the interface reflections include designing the LED to reabsorb and reemit the reflected light (called photon recycling) and manipulating the microscopic structure of the surface to reduce the reflectance, either by introducing random roughness or by creating programmed moth eye surface patterns.