Introduction
Conductors: Materials that allow electrons to flow through them. (ex. metal, human body)
Insulators: Materials that resist flow of electrons. (ex. plastic, styrofoam)
Semiconductors: Materials that have properties of both conductors and insulators.
Semiconductors
Semiconductors are made up of a crystalline material. The most commonly used element in semiconductors is silicon. Silicon has a crystalline structure at room temperature and its atoms are arranged in a way that makes it very stable at room temperature.
The valence band contains electrons that are present in the valence shell of an atom which are at a lower energy level. The conduction band contains free electrons that are responsible for carrying electric current. In a conductor, there is no gap between the valence band and the conduction band so the electrons can move freely (see figure above). A band gap–which is defined as the amount of energy that must be overcome for an electron to travel between the two bands–exists in a semiconductor and an insulator.
In a semiconductor, when all the electrons are located in the valence band, it is called an intrinsic semiconductor and does not conduct any electricity. For a semiconductor to conduct electricity, energy must be applied to the system. Energy can be generated in a variety of forms. For example, if the temperature of the system were to be raised, some electrons may have enough energy to jump up to the conduction band. The electrons that have reached the conduction band are able to move freely and thus conduct electricity. When this happens, a hole is created in the valence band where the electron that transferred to the conduction used to be. This allows the electrons to move in the valence band hence producing conductivity in the lower band as well.
In an N-type conductor, donor atoms located in the band gap will donate electrons to the conduction band (see figure on the right). In a P-type conductor, acceptor atoms located in the band gap will take away electrons from the valence band, hence forming holes in the valence band. Conduction occurs at the conduction band in the former and in the valence band in the latter.
LEDs
LEDs, which is short for light emitting diodes, are made from a combination of an N-type semiconductor and a P-type semiconductor. Notice that there is an unbalance between the energy level of the two bands of an N-type semiconductor and that of an P-type semiconductor. When no voltage is applied to the system, nothing happens since the electrons in the conduction band in the n-type region are not able to move to the p-type region. Similarly, the electrons in the p-type region are not able to travel to the n-type region. When a voltage is applied to the system, the barrier flattens and the electrons are able to flow freely throughout both the n-type and the p-type regions (see figure below). When the electrons in the conduction band and the holes in the valence band align perpendicular to each other, a phenomenon called electron-hole recombination (EHR) occurs. As the electrons fall down to the valence band, they release energy in the form of photons, producing light as a result. Note that the direction of the current matters. If the negative side was hooked up to the p-type semiconductor, the barrier between the n-type region and p-type region will be raised and no electrons will be able to flow freely across the two semiconductors.
LED and color
LED comes with a few different colors. This has to do with the amount of energy an electron releases during EHR. From [E = h*v] and [c = h*λ], where E = energy released by electron, h = frequency, c = speed of light, and λ = wavelength, we get the relationship [E = (c*v)/λ]. This means that the greater the energy, the shorter the wavelength. Therefore, LEDs that produce yellow light have a higher band gap than the ones that produce red light.
Why are LEDs efficient?
Before getting into why LEDs are said to be efficient, let’s quickly take a look at how a traditional light bulb works. Light is generated in an incandescent bulb by applying current and generating large amounts of heat from the tungsten filament (which can be treated as a resistor) to the point where white light is produced. The reason why incandescent bulbs are less efficient than LEDs is because they require a significant amount of heat in order to produce light. Moreover, the heat generated inside the bulb is released to the surrounding environment, causing an increase in room temperature. And as we all know, an increase in room temperature will prompt individuals to turn on their air conditioning (which in itself uses a great amount of energy) more frequently. LEDs on the other hand, as mentioned above, produce light directly from the electric circuit rather than relying on an external apparatus to generate light and thus release less amount of excess heat compared to incandescent bulbs.
Sources
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[2] “Top 5 Reasons for Silicon Uses in Electronics as a Semiconductor Material”. ElProCus. https://www.elprocus.com/what-are-the-reasons-behind-silicon-uses-in-electronics/ Last Accessed: 12 June 2020.
[3] McWhorter, Paul. “Arduino Tutorial 2: Understanding How Light Emitting Diodes (LEDs) Work”. https://www.youtube.com/watch?v=9uHZB7-T_XA&list=PLGs0VKk2DiYw-L-RibttcvK-WBZm8WLEP&index=2 Last Accessed: 12 June 2020.
[4] Hunter, Ashley. “How Incadescent Light Bulbs Work”. 1000Bulbs.com. https://blog.1000bulbs.com/home/how-incandescent-light-bulbs-work Last Accessed: 12 June 2020.
[5] “Semiconductor material…Silicon”. ShinDengen Electric Manufacturing. https://www.shindengen.com/products/semi/column/basic/semi/semi_basic.html Last Accessed: 12 June 2020.
[6] “Difference Between Conductor, Semiconductor and Insulator”. Electrical Technology. https://www.electricaltechnology.org/2019/10/difference-between-conductor-semiconductor-insulator.html Last Accessed: 12 June 2020.
[7] “How a PN-Junction Diode works”. University of St. Andrews. https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/pn_junc/pn_junc.htm Last Accessed: 12 June 2020.
[8] Wagner, Eugene P. II. (2016). “Investigating Bandgap Energies, Materials, and Design of Light-Emitting Diodes”. Journal of Chemical Education. https://www.fkit.unizg.hr/_download/repository/Wagner_2016.pdf Last Accessed: 12 June 2020.
[9] Silva, Jefferson R. et al. (2019). “Light, photosynthetic capacity and growth of papaya (Carica papaya L.): A short review.” https://www.researchgate.net/figure/Electromagnetic-spectrum-The-visible-portion-of-the-electromagnetic-spectrum-is-just-a_fig2_332031989 Last Accessed: 12 June 2020.
(Thumbnail Image Source) [10] PiccoloNamek. (2005). “File: RBG-LED.jpg”. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:RBG-LED.jpg Last Accessed: 12 June 2020
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