Figure 1The light Emitting Diode electrical symbol and practical structure diagram
The light emitting diode is P-N junction diode, which consists of two leads and semiconductor light source. When it is activated by applying the suitable voltages across its leads then it emits the light energy in the form of photons and the color of this light was determined by the band gap of semiconductor material. This light energy is produced by the recombination of electrons and electron holes with in this device and this process is called electroluminescence process. This process was stated in twentieth century from solid state material when it is heated at room temperature then it emits the light energy. At the beginning the infrared light emitting diode was developed and its light intensity was low but it is still frequently used in variety of consumer electronics circuits such as remote controls. Different sizes of light emitting diodes are available in market form 1mm2 to onward. One of the light emitting diode with electrical symbol and practical structure diagram is shown in figure 1. It is also used to make LED matrix.
Construction of Light Emitting Diode
The construction of light emitting diode is so much simple, it is made by depositing the three layers of semiconductor material on a substrate. These three semiconductor material layers are made three regions which are called a P-type region which is top one, active region which is middle one and N-type region which is bottom one. The figure 2 shows all of three semiconductor material regions.
Figure 2 The Construction Diagram of Light Emitting Diode
As Per the figure 2 the P-type region have the holes, N-type region have the elections and active region have both electron and holes. In normal condition when there are no any voltages are applied at anode and cathode then all the holes and electrons are present at their places but when the voltages are applied at this light LED then it is forward biased. Then the holes from p-type region and electrons from n-type region are pushed up towards the active region, which is also called the depletion region. Because the holes have positive charge and electron have negative charge then the light is produced by the recombination of these opposite polarity charges.
The light LED works on the same principle of simple PN junction diode means when the anode is connected to positive terminal of dc supply and cathode is connected to the negative terminal of dc supply then the PN junction is forward biased. When the PN junction is forward biased then the holes’ form P-type region and electrons from N-type region are recombined then the conduction band is formed for emitting the light energy in the form of light energy photons. This whole phenomenon is called electroluminescence phenomena and this light energy depends upon the amount of current absorbed by the LED. In other words, this light energy is directly proportional to the absorbing current means when it absorbs more current then the light would be high similarly when it absorbs low current then the light would be low. All the absorbing current not converted into light energy some of the portion of this current is converted into heat which is dissipated by the light emitting diode during light emitting into external environment. This heat is known as electron dissipate energy and this dissipate energy also depends upon the semiconductor material of light emitting diode. The inner working diagram of this light emitting diode is also shown in figure 3.
Figure 3 The Inner Working Diagram of Light Emitting Diode
When the light emitting diode is forward biased by applying 2 or 3 volts across its terminals then it emits the visible coloured, invisible infrared or laser light of narrow band width. In this condition, it shows that the forward current is almost constant at early stage but when it is fully on then the forward current is increased up constantly shown is figure 3. The light intensity depends upon this current when this current is high then the intensity of output light would be high. The voltage at which the current is increased constantly is called forward voltage and is denoted by Vd in figure 3. In reverse bias condition the reverse current or leakage current is almost linearly constant but a point is come when its PN junction is breakdown and reverse current is increased constantly in negative current region. The voltage at which the breakdown is occurred is called breakdown voltage and is denoted by Vbr shown is figure 3
Figure 3 The V-I Characteristic of Light Emitting Diode
The LEDs are divided into three major types, accordingly to its material.
Traditional Inorganic LEDs: The traditional type LEDs are having the same shape as the traditional type diode and these are available in market since 1960.These are made with inorganic material therefore these are called inorganic LEDs. Some of widely used LEDs which are made from semiconductor material compound are Aluminum gallium arsenide, Gallium arsenide. These are available in market with different shapes and colures such as surface mounted LED, Single colure LED, Multi-colure LED and Flashing LED.
Organic LEDs:This type of LED is made with organic material therefore these are called organic LEDs. These types of LEDs show the basic idea of LED. The traditional inorganic LEDs are used the inorganic semiconductor material therefore these are having a varying dopant level. These are produced a light form defined PN junction, therefore their light is shows in the form of point of light. The organic LEDs are manufactured in sheets therefore these are providing a diffuse area of light means these can be used for wide area of surface.
High Brightness LEDs: The high brightness LEDs (HBLEDs) are the type of organic LEDs and these are widely used for lightening purpose. Essentially, this type of LEDs is same as organic LEDs but only different is that these are having high brightness in output light. For generating high brightness, these LEDs are required to be able a high current and high power dissipation therefore these LEDs are mounted on heat sink for removing the unwanted heat.
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