ABOUT LED
How Does LED WorkA Light Emitting Diodes (LEDs) are semiconductor devices that emit light and have begun to see widespread use with the increased need for clean and efficient lighting.
The device’s remarkable ability to directly convert electrical energy to light comes from quantum mechanics and uses materials known as semiconductors – materials whose electrical properties lie somewhere in between that of a conductor and an insulator.
Scientists and engineers can manipulate the properties of these materials by introducing trace impurities to design LEDs to emit almost any wavelength of light they desire.
What are semiconductors?
Conductors are metals, such as gold, copper and aluminum, and contain atoms that are surrounded by a “sea” of electrons. These electrons are easy to move under the influence of an electric field and thus, will flow when a voltage is applied across its terminals.
In an insulator, such as rubber or plastic, the electrons are more tightly bound and not as free to move as they are in a conductor; they don’t respond as well to electric fields.
Semiconductors differ from conductors and insulators in that the electrons responsible for the formation of chemical bonds aren’t as strongly bound as that as insulators. While an applied electric field can easily set electrons moving in a conductor, it still takes some effort to get electrons moving in a semiconductor. The electric field has to go above a certain value for a current to flow.
Semiconductors are materials that lie somewhere between conductors and insulators.
Unlike conductors that conduct electricity through the movement of negative charge carriers or electrons, pure or intrinsic semiconductors conduct through both positive and negative charge carriers.
When an electric field is applied across a semiconductor crystal, the atom’s weakly bound electrons gain enough energy to break free and move through the crystal lattice.
During this process, these liberated electrons leave behind an “empty” space or positively charged hole. Electrons move through a semiconductor’s crystal lattice by “hopping” between lattice positions to fill the vacancies left by previous electrons.
Changing Crystal Properties
An atom in a semiconducting crystal, such as germanium, has four valence or outermost electrons. These electrons are important in chemistry and physics as they are the ones that participate in the formation of different types of chemical bonds.
In a covalent bond, a type of strong chemical bond, atoms share pairs of valence electrons. Some atoms contain less valence electrons and some contain more. Boron, for example, contains three valence electrons and is called trivalent while arsenic have five outermost electrons and is known as pentavalent.
Boron has three outermost electrons (trivalent), Arsenic five (pentavalent) and the semiconductor germanium has four.
By doping an intrinsic (pure) semiconductor or adding trace amounts of trivalent and pentavalent impurities to the crystal structure, we can change its electrical properties. Theseatoms will either add more electrons or negative charge carriers to the crystal lattice, i.e act as a donor, or add more holes or positive charge carriers, i.e. act as an acceptor.
By doping a semiconductor crystal, we can create regions where there are excess electrons or negative charge carriers and holes or positive charge carriers.
This process of adding impurities to a pure semiconducting crystal dramatically changes its electrical properties and allows us to build much of the integrated circuitry we find in the devices in use today.
By controlling how electrons flow, semiconductors have allowed us to shrink much of the circuitry in the devices, such as computers, laptops and smart phones, that we use today and it is difficult to imagine our modern world without the conveniences semiconductors bring. The material that is found in most of these devices is made up of silicon, the main element found in sand but LEDs use a different kind of semiconductor called germanium whose properties are similar to that of silicon but different, in that it allows physicists and engineers to manipulate the crystal to emit light.
Just as with silicon, an engineer can dope different regions of a germanium crystal with trivalent and pentavalent elements like boron and phosphorous to create regions in the crystal that either have an excess of negative charge carriers or electrons or an excess of positive charge carriers or holes.
A germanium semiconductor crystal shares electrons and forms covalent bonds. Under the influence of an electric field, an electron is freed and leaves behind a hole. As the electrons move to combine with holes, it releases the energy it gained from the electric field in the form of visible light. By choosing different combination of dopants, we can manipulate the color of light that is emitted.
This method of producing light has quite a few advantages over incandescent light bulbs. Incandescent lamps produce light by sending a current through a thin metallic filament wire. As the wire heats up, its atoms vibrate and release energy. As there are many ways this atom can vibrate, there are many ways in which energy can be released. Thus only a small portion of atoms vibrate in the right way to produce visible light with over 90% of an incandescent light’s energy being wasted as heat. In the case of LEDs, the energy that is released as the electron and hole recombine is quantized – the energy that is released goes into producing photons or packets of light of a particular frequency and makes for a very efficient light source. As we will see in future articles, we can use this and the way we perceive color to combine LEDs of different colors to produce white ligh