Light-emitting diode physics

Light-emitting diodes (LEDs) produce light (or infrared radiation) by the recombination of electrons and electron holes in a semiconductor, a process called "electroluminescence". The wavelength of the light produced depends on the energy band gap of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light. A LED is a long-lived light source, but certain mechanisms can cause slow loss of efficiency of the device or sudden failure. The wavelength of the light emitted is a function of the band gap of the semiconductor material used; materials such as gallium arsenide, and others, with various trace doping elements, are used to produce different colors of light. Another type of LED uses a quantum dot which can have its properties and wavelength adjusted by its size. Light-emitting diodes are widely used in indicator and display functions, and white LEDs are displacing other technologies for general illumination purposes.

Electroluminescence

The p–n junction in any direct band gap material emits light when electric current flows through it, that is, electroluminescence. Electrons cross from the n-region and recombine with the holes existing in the p-region. Free electrons are in the conduction band of energy levels, while holes are in the valence energy band. Thus, the energy level of the holes is lower than the energy levels of the electrons. Some portion of the energy must be dissipated to recombine the electrons and the holes. This energy is emitted in the form of heat and light. As indirect band gap materials the electrons dissipate energy in the form of heat within the crystalline silicon and germanium diodes, but in gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP) semiconductors, the electrons dissipate energy by emitting photons. If the semiconductor is translucent, the junction becomes the source of light, thus becoming a light-emitting diode.

The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes usually recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in 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 substrates.

Refractive index

Bare uncoated semiconductors such as silicon exhibit a very high refractive index relative to air. Photons that approach the surface at too great an angle to the perpendicular experience total internal reflection. This property affects both the light-emission efficiency of LEDs as well as the light-absorption efficiency of photovoltaic cells. The refractive index of silicon is 3.96 (at 590 nm), while air's refractive index is 1.0002926. In general, a flat-surface uncoated LED semiconductor chip emits only light that arrives nearly perpendicular to the semiconductor's surface, in a cone shape referred to as the light cone, cone of light, or the escape cone. Photons arriving at the surface more obliquely, with incidence angle exceeding the critical angle, undergo total internal reflection, and return inside the semiconductor crystal as if its surface were a mirror. Internal reflections can escape through other crystalline faces if the incidence angle is low enough and the crystal is sufficiently transparent to not re-absorb the photon emission. But for a simple square LED with 90-degree angled surfaces on all sides, the faces all act as equal angle mirrors. In this case, most of the light can not escape and is lost as waste heat in the crystal. A convoluted chip surface with angled facets similar to a jewel or fresnel lens can increase light output by distributing light perpendicular to the chip surface and far to the sides of the photon emission point. The ideal shape of a semiconductor with maximum light output would be a microsphere with the photon emission occurring at the exact center, with electrodes penetrating to the center to contact at the emission point. All light rays emanating from the center would be perpendicular to the entire surface of the sphere, resulting in no internal reflections. A hemispherical semiconductor would also work, with the flat back-surface serving as a mirror to back-scattered photons.

Transition coatings After the doping of the wafer, it is usually cut apart into individual dies. Each die is commonly called a chip. Many LED semiconductor chips are encapsulated or potted in clear or colored molded solid plastic. The plastic encapsulation has three purposes:

Mounting the semiconductor chip in devices is easier to accomplish. The tiny fragile electrical wiring is physically supported and protected from damage. The plastic acts as a refractive intermediary between the relatively high-index semiconductor and low-index open air. The third feature helps to boost the light emission from the semiconductor by reducing Fresnel reflections of photons within the light cone. A flat coating does not directly increase the size of the light cone in the semiconductor; it provides an intermediate wider cone angle in the coating, but the critical angle between rays in the semiconductor and in the air beyond the coating does not change. With a curved coating or encapsulation, however, efficiency can be further increased.

Efficiency and operational parameters

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