Operating Principles of LED and LCD - Report Example

Light Emitting Diode or LED is a semiconductor emitting narrow spectrum light upon introduction of electric currents in a forward direction. LED helps to save energy consumption by 75 to 85% and has a life span of 7 to 10 years. Haitz law states that the luminance performance of LED doubles every two years. 1. Operating Principle The LED functions on the principle of Electroluminescence. In general, LED’s have a semiconductor material impregnated with impurities to create a P-N junction. N type materials carry extra electrons and are hence negatively charged whereas P type material has holes where electrons jump from holes towards a positive charged area. The P type and N type materials are joined together to create a P-N junction. In case of no electricity flow the electrons occupy the holes on the P type material creating a depletion zone. The depletion zone acts as an insulating layer in the diode. In case the electric current is passed through the diode, the electrons in the N type material get attracted towards the P type material leading to removal of depletion zone. When the electrons meet the protons energy is released in the form of photons, which emit light. The energy of the band-gap is represented by the energy of the photons. The colour of the light depends on the wavelength and the type of material used in the semiconductor. The change in supply of current affects the flux of luminosity proportionately. LED in general are operated using direct currents to avoid the variation in the intensity of luminosity. The mechanism used is the “Injection Electroluminescence” where Luminescence signifies photon production; Electro suggests photon production using electric current and the Injection part deals with photon production using current carriers. 2. Materials used in LED In general, the conducting material used in LED are Aluminium-Gallium-Arsenide (AlGaAs), the colour of the light depends upon the type of material used as a semiconductor. Some of the other materials used in production of semiconductors a part of LED are Aluminum- indium-gallium-phosphide (AlInGaP) Gallium-arsenide-phosphide (GaAsP) Indium-gallium-nitride (InGaN) Fig.1 –List of Semiconductor materials (LED, 2010) Organic Light Emitting diodes are also used where a thin film of organic material is coated over the semiconductors. 3. Working process of LED LED is made of semiconductor materials which are responsible for the production of light of various colours. In case of a static diode, when no electric current is passed the P type and N type materials attract respective protons and electrons at the respective ends. A depletion zone is created which acts as an insulating platform as prevents the flow of electrons. Once the electric current is passed through the ends by connecting, anode, P type to the positive end of circuit and N type to the negative end of the circuit, the depletion zone is reduced and the movement of electrons take place. The electrons interact with the protons to produce energy packets called photons. The movement of electrons is governed by the energy levels of each electron. The movement of electron from higher level to lower levels helps in emission of energy packets called photons. The atoms are arranged in a particular format that the electrons movement stops at a shortened distance which makes it difficult for human eyes to view the process. When the circuit is connected in opposite direction with N type connected to the positive end and P type to the negative end, the flow of electrons takes place in the reverse direction and the zone of depletion gets bigger. This creates an insulating platform within the diode and stops the flow of electric current. 4. Semiconductor materials and their implications The typical semiconductor material are made up of pure Aluminium-Gallium-Arsenide (AlGaAs) where the atoms group together to disallow the movement of electrons. The case of doping where the impurities are impregnated to the semiconductor material, the conductivity of the electrons become easy and fast. LED emanates monochromatic light and the colour change as per the wavelength of the light. LED’s do not produce ultraviolet or infra-red light and can be easily used for various purposes. The flux luminosity of LED lies between 1 lumen (lm) or 50 to 100 mW for low intensity LED to 120 lumen (lm) or 5 W for high intensity ones. LED’s have been efficiently used for interior decorations, picture tubes, and traffic lights etc. to improve the efficiency of the diode emitting lights. Liquid Crystal display or LCD is the unique technology which has made display slimmer and portable. Liquid crystals more appropriately behave in midway between solid and liquid. The crystals are viscous and so the name. The crystals are sensitive to heat and extremes of temperature, as in thermometer where heat is required as compared to an LCD TV where extreme temperature destroy the liquid crystals. 1. Operating Principles and Working process The twisted nematic crystals called Twisted Nematics (TN) are affected by the electric current which causes them to unwind depending upon the degree of voltage applied. The liquid crystals react to the electric current to control light phases. The molecules that form liquid crystals are rod shaped and Thermotropic or Lyotropic. Thermotropic crystals often react to temperature and pressure changes whereas Lyotropic crystals depend on the solvency power of the mixture used. Thermotropic liquid crystals are further divided into Isotropic and Nematic crystal. Isotropic crystals are randomly arranged whereas Nematic crystals are arranged in orderly fashion. The movement of the Nematic molecules depend upon the presence of the magnetic field which helps in the orientation of the molecules. The Nematic crystals are further divided into various forms such as Smectic (molecules are arranged in layers) and Cholesteric (molecules are twisted over layers). The main principles which help in the manufacturing of LCD are driven by the following points such as :- Polarization of light can take place, Polarized light can be changed using liquid crystals, Electric current posses the power to change liquid crystal and electricity is conducted by the transparent substances. Two pieces of polarized glass with groves created using a transparent material is used and a film of nematic crystals is applied on the same. The second piece of glass with polarized film at ninety degrees to the first glass is used. When lighting falls on the filter the molecules gets polarized and the light traverses to the next layer. The light upon reaching the last glass filter vibrates at the same angle as liquid crystals. On application of electric current to the polarized film, the molecules straighten and cause difference in the angle of the light passing through the polarizing filter and this leads to darkening of the LCD screen. 2. Materials used and types of LCD There are various liquid crystals which are used for polarization in an LCD such as Nematic Crystal- These crystals are sensitive to temperature and pressure and react to the light to excite the crystal molecules. Ferroelectric liquid crystals- These crystals have twisted configuration of molecules arranged in layers which increases the efficiency of the LCD making switching response the maximum. Surface stabilized Ferro-electric liquid crystals- In this configuration a glass plate is used to apply pressure over the molecules arranged in layers. This makes the switching process fast and is used in highly advanced LCD. Common plane based LCD- These are normal LCD which are used for the purpose of information display and most commonly used in watches and mobile devices. Passive matrix LCD – These are simple grid where horizontal rows and verticals columns are joined using indium tin –oxide material. When the current passes through the glasses, the rows and columns cross at a particular pixel which unwinds the crystals and created designated pixels. Active matrix- These are the Thin Film Transistors (TFT) which are arrangement of transistor and capacitors. A particular pixel is produced using a charge at a designated row and column which excites the charge. Other areas across the glass layer are not affected by the charge and application of designated voltage helps to unwind the crystals at the prescribed places. 3. Display market and future The flat panel display (FPD) technology commands more than 90% of the LCD market and was popularized in the North America and Europe. With the advancement on the technological front new display panels have been introduced such as Field Emission Displays (FED), Organic Liquid Emission Display (OLED). OLED is the recent technological breakthrough and highly advanced over the earlier LCD versions. The colour LCD in general has red, blue and green filters with sub-pixels in order to produce colour combinations required. At present LCD technology has evolved to a greater extent with the Super Twisted Nematics (STN), Dual Scan Twisted Nematics (DSTN), Ferroelectric Liquid Crystal (FLC), and Surface Stabilized Ferro-electric Liquid Crystals (SSFLC). As the market has matured, size of LCD has increased proportionately but the advancement needs to evolve in sync with technology to provide the best to the market. References Bloomfield Louis. How Things Work. New Jersey: John Wiley & Sons, 1992. Print Dave Mentley, OLED Displays - Out of the Lab and into the Market, Commercializing OLEDs Conference, San Diego, CA, USA, October, 2000 Leo William and Zuppiroli Libero “Development of OLED’s, a next generation flat panel display technology: Experiences from the On-going Collaboration between industry and Academia.” Lausanne, Switzerland. 2001 Nakamura S. “The roles of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes” sciencemag.org 1998. Schubert Fred E. “Light Emitting Diodes and Solid-State Lighting” Department of Electrical, System and Computer Engineering 2010. Schubert Fred E. “Light Emitting Diodes” 2nd Edition. Cambridge University Press: Cambridge 2006 Summit Associates, “Light Emitting Diodes (LED) : Lighting overview” Grass Front Niches Tyson Jeff. “How LCD Works” LED Technical Information. 30 October 2008. 15 Aug 2010 Vishay “Vishay General Semiconductor.” Physical Explanation February 2010 Wilson Bill. “Light Emitting Diode” The Connexions Project. Connexion Module, May 2008. Read More