Light-emitting diodes, namely LED lights, have a very important place in the electronics world. They do different jobs on different types of devices. For example, they generate numbers on digital clocks, transmit information on remote controls, illuminate clocks and let you know when your devices are turned on, they can create images on a large television screen or illuminate traffic lights.
LEDs are small light bulbs that fit easily into an electrical circuit. Unlike incandescent bulbs, they have no burning filaments, use less electricity and do not heat up. They light up only by the movement of electrons in a semiconductor material and last as long as a standard transistor. An LED's lifespan surpasses the short life of an incandescent bulb by thousands of hours. Because of these advantages, small LEDs are one of the most popular technologies used to illuminate LCD TVs.
LEDs have many advantages over traditional incandescent lamps, but their main advantage is efficiency. In incandescent bulbs, the light generation process involves generating a lot of heat (the filament must be heated to illuminate). Unless you use the lamp as a heater, this energy is completely wasted, because most of the available electricity does not go to producing visible light. LEDs generate relatively less heat. A much higher percentage of electrical energy goes directly to producing light, which significantly reduces electrical demands. LEDs per watt give off more lumens (amount of light) than regular incandescent bulbs. Light emitting diodes have a higher luminous efficiency than incandescents.
Until recently, LEDs were too expensive to be used in most lighting applications because they were built on advanced semiconductor material. The price of semiconductor devices dropped after 2000, making LEDs a more cost-effective lighting option for a wide variety of situations.
In the case of LEDs, the conductive material is typically aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all atoms are perfectly bonded with their neighbors, leaving no free electrons (negatively charged particles) to conduct electric current. In the doped material, additional atoms change the balance, either by adding free electrons or creating holes through which electrons can go. Any of these changes makes the material more conductive.
A semiconductor with extra electrons is called N-type material because it has extra negatively charged particles. In N-type material, free electrons move from a negatively charged area to a positively charged area. A semiconductor with extra holes is called P-type material because it effectively has extra positively charged particles. Electrons can jump from hole to hole, moving from a negatively charged area to a positively charged area. As a result, the holes themselves seem to move from a positively charged area to a negatively charged one.
A diode consists of a section of N-type material connected to a section of P-type material with electrodes at both ends. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill the holes in the P-type material along the junction between the layers, creating a depletion region. In a depletion region, the semiconductor material is returned to its original insulating state and all holes are filled, so there are no free electrons or vacancies for electrons and no electricity can flow.
To get rid of the depletion region, you must take electrons moving from the N-type field to the P-type field and holes moving in the opposite direction. To do this you connect the N-type side of the diode to the negative terminal of a circuit and the P-type side to the positive terminal. The free electrons in the N-type material are repelled by the negative electrode and attracted to the positive electrode. The holes in the P-type material move in the other direction.
When the voltage difference between the electrodes is high enough, the electrons in the depletion region exit their holes and begin to move freely again. The depletion region disappears and the charge moves across the diode.
If you try to run the current the other way while the P-side is connected to the negative terminal of the circuit and the N-type side is connected to the positive terminal, the current will not flow. 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. Since the holes and electrons each move in the wrong direction, no current flows through the junction and the depletion region increases.
How Can a Diode Produce Light?
Light is a form of energy that can be released by an atom. It consists of many tiny particle-like packets that have energy and momentum but no mass. These particles, called photons, 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. Generally speaking, electrons with greater energy move in orbitals farther from the nucleus.
For an electron to jump from a lower orbital to a higher orbital, something has to raise its energy level. Conversely, when an electron falls from a higher orbital to a lower orbital, it releases energy. This energy is released in the form of a photon. A larger energy drop emits a higher energy photon characterized by a higher frequency.
As we saw earlier, free electrons moving through a diode can fall into empty holes in the P-type layer. This involves a drop from the conduction band to a lower orbital so electrons release energy in the form of photons. This happens with any diode, but you can only see photons when the diode is made of a certain material. For example, the atoms in a standard silicon diode are arranged in such a way that the electron falls a relatively short distance. As a result, the frequency of the photon is so low that it cannot be seen by the human eye. It is in the infrared part of the light spectrum. In addition, infrared LEDs are ideal for remote controls.
Visible light-emitting diodes (LEDs), such as those that illuminate 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, that is, the color of the light. While LEDs are used in everything from remote controls to digital displays in electronics, visible LEDs are popular thanks to their long lifetime and miniature size. Depending on the materials used in the LEDs, they can be manufactured to shine in all colors of the infrared, ultraviolet and visible spectrum in between.
While all diodes release light, most do not do so very effectively. In an ordinary diode, the semiconductor material itself absorbs most of the light energy. LEDs are specially made to emit a large number of photons. Additionally, it is housed inside a plastic bulb that concentrates the light in a specific direction. Most of the light from the diode bounces off the sides of the bulb and travels along the round end.