Gilbert Held Introduction to Light Emitting Diode Technology and Applications

[ Pobierz całość w formacie PDF ]
.7 V,a 20 mA current, an optical output of 20 mW, a luminous efficiencyof 8 lm/W, minimum lifetime of 8000 hr until the optical outputdecreases to half the initial value, and a color temperature of whitelight in the range of 3000 K and above.Currently, this LED is beingused in a range of applications such as lighting, indicators, and back-lights for liquid crystal displays.As its life is expected to improvedue to further development efforts, this LED will become suitable foradditional applications.3LEDS EXAMINEDIn the previous chapters of this book, we learned what we canconsider as basic information concerning light-emitting diodes(LEDs), which provides us with a solid foundation for under-standing how LEDs work.This information included the basicsof p-n junction and emission of both infrared and visible light.In this chapter, we will probe deeper into the manner in whichLEDs operate, focusing our attention directly on the operationof a p-n junction.In addition, we will cover two additional LEDtopics in this chapter that will serve to expand our knowledgeof LEDs.The first topic concerns a relatively new type of LEDreferred to as an organic LED.The organic LED has the potentialto make science fiction a reality, with the possibility of having alarge-screen super-thin TV that can be rolled up and put awaywhen not in use.The second topic that will be discussed in thischapter concerns the use of an LED driver.Thus, sit back andrelax as we continue our exploration of the light-emitting diode.3.1 P-N Junction OperationThe LED is a special type of diode, which is a very simple semi-conductor device.The diode is a two-terminal electronic device thatenables electric current to pass primarily in one direction, with thecurrent dependent on the voltage between the leads.When we consider the evolution of the LED from a diode, we seethat the key to an LED s ability to emit light is its p-n junction andthe doping of a substrate material with different materials to form thejunction.As we will shortly note in detail, the dopant in the n-regionprovides mobile negative charge carriers known as electrons, whereasthe dopant in the p-region provides mobile positive charge carriersreferred to as holes.Thus, when a forward voltage is applied to the p-n5354 INTRODUCTION TO LED TECHNOLOGY/APPLICATIONSjunction from the p- to the n-region, the charge carriers are injectedacross the junction into a zone where they recombine and converttheir excess energy into light.3.1.1 Semiconductor MaterialThere are two main types of semiconductor materials: intrinsic andextrinsic.In intrinsic semiconductor materials, semiconducting prop-erties occur naturally.On the other hand, in extrinsic semiconductormaterials, semiconductor properties are the result of external pro-cesses.Today, almost all semiconductors are extrinsic, as this allowsthe properties of the material to be explicitly defined.To do so, thematerial is doped by the addition of foreign atoms.LEDs are solid-state devices.As such, one of the critical elementsthat govern the operation of the device is its p-n junction.As wenoted in Chapter 1 of this book, when p-type and n-type materialsare placed in contact, the resulting junction behaves very differentlyfrom either type of material by itself.That is, current will flow in onedirection, referred to as forward bias, but not in the reverse direction,referred to as reverse bias.The inability to reverse bias results from thecharge transport process in the p-type and n-type silicon used to formthe p-n junction.3.1.2 Basic Concepts of AtomsRecall that the core of an atom is its nucleus.The nucleus contains one ormore protons and may contain one or more neutrons.Protons are posi-tively charged, whereas neutrons have no charge.Orbiting around theatom are one or more electrons.Electrons are relatively small in com-parison to protons and neutrons; however, they have a negative charge.As an example of the relationship between electrons, protons, andneutrons, let s consider the simplest of all atoms, the hydrogen atom.As illustrated in Figure 3.1, the hydrogen atom has one proton andone electron; however, the proton has approximately 1850 times themass of its electron.Elements are classified by the number of protonsthey have, which becomes their atomic number.Thus, hydrogen hasan atomic number of 1, whereas helium that has two protons has anatomic number of 2, and so on. LEDS EXAMINED 551Pe hydrogen atom has one proton and one electronFigure 3.1 The hydrogen atom.3.1.2.1 Electrical Charge An atom in its initial unaltered state has thesame number of electrons as it has protons.At this point of time,the atom s total electrical charge is said to be balanced.If the atomloses an electron, it will have more protons than electrons and itstotal charge will become positive.Conversely, if the atom gains anextra electron, it will have more electrons than protons and it willhave a negative electrical charge.In physics, an atom with a positivecharge is referred to as a positive ion, whereas an atom with a negativecharge is referred to as a negative ion.A positive ion will try to attractan electron (positives attract negatives), whereas a negative ion (repelnegatives) will attempt to lose its extra electron.3.1.2.2 Band Theory Electrons orbit around the nucleus in energylevels or bands.As the number of electrons increases, they fill thebands in a predefined order.That is, the innermost band is filled first,followed by the population of band two, and so on.Band one canhold two electrons, whereas band two can hold eight, and the lastband that holds electrons in an atom is referred to as the valence band.The first unfilled level above the valence band is referred to as theconductor band.As the bands fall further out from the nucleus, theirenergy level increases.3.1.3 Energy BandsFigure 3.2 illustrates the energy bands in the p-type and n-type sili-con at equilibrium.The open circles in p-type silicon on the left sideof the junction shown in Figure 3.2 represent holes or deficiencies56 INTRODUCTION TO LED TECHNOLOGY/APPLICATIONSP-typeN-typeLegend:Free holesFree electronsFigure 3.2 Energy bands in the p-type and n-type silicon at a point of equilibrium.of electrons in the semiconductor material, which can act as posi-tive charge carriers.In comparison, the n-type material contains freeelectrons [ Pobierz całość w formacie PDF ]

Linki