Science behind LED

LED or light-emitting diode is a semiconductor device that emits light on certain wavelength. Semiconductor device is an electronic component that uses the electronic properties of semiconductor material and organic semiconductors. An electronic component is any physical entity in an electronic system used to affect electrons or their associated fields. Semiconductors are the invention of modern electronic. It has an electronic conductivity (mutual of electrical resistivity and measures the capability of a material to conduct electrical current) value between conductor (type of material that allows electric current to flow in one or more directions) and insulator (a material whose internal electric charges do not flow freely and thus creates an obstruction to conduct electric current in an electric field). LED is consisting of a chip of semiconducting material doped (with a dopant [a mark of impurity element within a substance to alter the electrical properties] with contaminations to create a p-n junction (an interface between two types of semiconductor p-type [larger hole concentration or positive charge of hole] and n-type [larger electron concentration or negative charge of electron]).

The semiconductors are of two types direct band gap and indirect band gap. The minimum energy state in the conduction band (quantifies the range of energy needed to free electron from the bond of atom) and the maximal energy state in the valence band (highest range of electron energies where electron is present in zero temperature) are characterised by certain crystal momentum or k-vector (a vector or carrier is associated with an electron in a crystal lattice [a small box or array considerably repeating in all the three space directions]). If the k-vectors are same then it is identify as ‘direct band gap’ and vice-versa. LEDs are made up of materials with the quality of direct band gap with energies.

In the early stage, to make LED a device made of gallium arsenide (a compound of the elements gallium and arsenic) was used. With the advancement of material science new devices are made with shorter wavelengths, emits light in various colours. Now a day in many commercial or residual LEDs sapphire substrate are used. Sapphires are signified as most hard stone thus it is used in many scientific instruments and in very thin electronic substrate which are used as insulating wafers of solid state electronics. Solid-state electronics are those circuits made of solid materials where electrons or other charged carriers are limited within the solid material. Solid state refers to light produced by solid state electroluminescence. Electroluminescence is an optical phenomenon (an observable event that results as the interaction of light and matter) and electrical phenomenon (a division of electromagnetic phenomena) where a material produces light in response to the electrical current passage or to a strong electrical field. 

Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Thus, light removal in LEDs is an important aspect of LED production, subject to much research and development.
Silicon has a high refractive index relative to open air. This characteristic helps in light production of LED as well as light absorption capability of photovoltaic cells. Photovoltaic cells are an electrical device that converts energy of light into electricity via photovoltaic effect (the creation of voltage or electrical current in a material upon exposure to light).

Active area of LED is encased in plastic or ceramic housing. The housing may incorporate one or many dies. A die is a small block of semiconducting material where a functional circuit is fabricated. The dies are also referred as chips. Most of the semiconductor chips of LEDs are encapsulated in coloured or clear molded plastic cells. The plastic is used as it protects the tiny and delicate electrical wires. It also acts like an intermediary between the low index open air and high index semiconductor. Plastic makes it easier to mounted semiconductor chips in a device.

At first LEDs were designed to consume in between 30 to 60 mW (milliwatts) of electrical power. Philips Lumileds first invent one watt power LEDs. Larger semiconductor die sizes were used to handle large power inputs. They were mounted in metal slugs to move heat from dies. The most important feature of LED light is luminous efficacy. 

In September 2003 a company Cree Inc. demonstrated a new type of Blue LED with 24mW at 20 aM. They then produced 65lm/W at 20mA white LED light. In 2012 they announced a white LED of 254lm/W and recently in March 2014 they announced a White LED emitting 303lm/W.
These capabilities are for the LED chips at low temperature. Lighting works in higher temperature. In 2009 a test of Department of Energy (US) showed that in average efficacy of LED is still 46 lm/W.

Efficiency droop is the decrease in luminous efficacy of LEDs as the voltage increases above tens of aM (milliamps). LEDs operating at higher electrical currents generate higher heat which minimizes its life. Thus instead of increasing current levels one bulb is comprised with multiple LEDs to increase its lumen. Hence it don’t produce much heat and do not consume more electricity. As because it produces less heat thus can be used in cold climate or in refrigerators. On the other side as it consumes minimum electricity it also makes a economical change in the electricity bill.