 Hello everyone! Welcome to this video lecture. Myself Deepali Vardkar, working as assistant professor at WIT, Solapur. In this video lecture we will study Light Emitting Diode, LED. These are the learning outcomes. At the end of this video lecture, student will be able to examine working of LED, student will be able to draw the VA characteristic of LED and applications of LED. Before going to LED, first we will see what is the light. The light is the type of energy that can be released by an atom. Light is made up of many small particles called photons. Photons having energy and momentum but no mass. The light emitting diode that is the LED, it is an optical semiconductor device that converts electrical energy into light energy. So, whenever the voltage is applied across this device, this device emits the light. This is the real appearance of LED. The symbolic representation of LED in that there are two terminal, anode and cathode. The symbolic representation is similar to the PN junction diode. Only change that the arrow pointing outside from this diode. So, this arrow indicates the light is being emitted from this device. Now LED construction, this is the construction diagram of LED. It is surrounded by transparent hard plastic epoxy resin. Its shape is hemispherical. Due to this shape, the light is emitted uniformly in all direction. There are two metal electrodes, anode and cathode which is used for external electric connection. Anode and cathode is connected by a small wire. There is one chip which is placed in reflective cavity. This chip consists P type and N type semiconductor materials. You see how this structure of LED is made. Yes, there are three basic layers P type semiconductor, N type semiconductor and in between that active region that is the depletion region. So, the P type semiconductor consist majority charged carrier poles, N type semiconductor consist majority charged carrier electrons and the depletion region consist immobile ions. The most of the photons emitted from this depletion region. So, this region is called as a active region. LED operates in a forward biasing mode. So, in forward biasing mode, the positive terminal of external supply is connected to the P type semiconductor. Negative terminal of external DC source is connected to the N type semiconductor. So, whenever the battery connected in this way, the majority charged carrier electrons from N type will repel towards the junction. So, some of these electrons will recombine with the holes which are present in this depletion region to ions that is at the positive ions. And some of the electrons will cross the junction and they will combine with the holes which are present in a P side. Similarly, due to positive terminal, holes in a P type semiconductor will repel towards the junction. Some of holes will combines with the electrons which are present at the depletion region and some of holes will cross the junction and will recombine with the electrons which are present in a N type semiconductor. So, whenever there is a whole electron combination takes place, the photons emitted that is the light emitted. So, we will see how this light is emitted whenever the recombination takes place. So, electrons, pre-electrons present in a higher energy level that is in a conduction band and holes present in a lower energy level that is in a balanced band. So, whenever the electron whole recombination takes place, these electrons jumps from higher energy level to the lower energy level. So, the extra energy which is with this electron is released in the form of photons or in the form of light. Now, here in LED the materials which are used for this manufacturing of LED is a gallium arsenide or gallium phosphide. So, these all these materials having the very large energy gap between the conduction band and valence band. And due to that the electrons jumps at a large distance and release the higher energy whose frequency is very high and which is visible to human eye. So, whenever in this type of material whole electron combination takes place, the energy released that energy is visible to the human eye. So, light emits in case of these materials that is in LED. In case of P N junction diode that is whenever the material used like a silicon or germanium in that case the energy gap between the valence band and conduction band that energy gap is very small. So, electrons jumps at a very small distance where whenever there is a electron whole recombination takes place. So, whenever there is a electron whole recombination takes place electrons jumps at a very small distance and release energy this energy is in the form of heat. So, the photons which are emitted here or the energy which is emitted here that energy having very lower frequency and that frequency is invisible to human eye. So, in these type of materials silicon or germanium wherever there is a electron whole recombination takes place no any light emits or photons emitted here. So, this is the basic difference between P N junction diode and LED. Intensity of light from LED this intensity of light is depends upon the energy gap between the conduction band and valence band. If the large energy gap present then the intensity of light emitted by that LED is also high. So, as the energy gap between conduction band and valence band decreases the intensity of light is also decreases. Now the question what is the relation between forward current and light intensity. In the previous two slides we have seen the actually working of LED in forward biasing. So, what is the relation between forward current and light intensity. Forward biasing as the forward voltage increases there are large number of whole electron combination takes place and as the whole electron combination takes place large number of photons emitted. So, as forward voltage increases light intensity also increases. So, as the forward current increases light intensity also increases. So, there is a linear relation between forward current and light intensity or output light. Colours of LED the material which is used for constructing LED determines its colour. The wavelength or colour of the emitted light depends on the forbidden gap or energy gap of the material. So, these are the some examples of compound semiconductors which are used for the manufacturing LED. If gallium arsenide is used then that LED will emit infrared. If gallium arsenide phosphide is used then that LED will emit red light. If gallium arsenide phosphide with nitrate is used it will emit yellow. If aluminium gallium phosphide is used that LED will emit green light. If silicon carbide is used that LED will emit blue light. If gallium indium nitrate is used that LED will emit white light. So, in this way for different combinations of these semiconductors different colours of light is emitted by LED. There are number of applications of LED. Out of that the one of the most important application that is the 7 segment display. In the 7 segment display the 7 LEDs arranged in a rectangular format. Here these 7 LED are connected by two way either common cathode and common anode. In common cathode cathode terminals of this all 7 LEDs are connected together and they are connected to the ground. The segment which is to be on that particular segment or that respective LED is get connected to the positive terminal. One example suppose we have to display one here. So, respective segments for to display one that is the B and C. So, the LED which are present here B and C these LED get connected to the positive. So, this LED becomes forward biasing and other remaining LEDs becomes reverse biasing. So, this LED becomes on and emit a light. And due to this here the display that is that particular segment this segment becomes emits the light. Now in common anode the all anode terminals of this all 7 LEDs are connected together and it is connected to the positive terminal. The segment which is to be display that respective LED is get connected to the ground connection. So, that it becomes forward biasing. Suppose we have to display one then the B and C the LED which are present here these are get connected to the ground and all other LEDs remains off. So, the LEDs which are present at the B and C becomes forward bias and they emit a light. So, here one number is get displayed. So, in this way the 7 segment display is used to display number from 0 to 9. Some more applications of LED it is used in alarm system, calculators, picture phones, traffic signals, digital computers. It is also used in multimeters, microprocessors, digital watches, automatic heat lamps, camera flashes and aviation lightning. These are the references. Thank you.