 Hello, this is Dr. Rupali Shalke working as an associate professor at Vulture Institute of Technology at Sholapur. Now, in today's lecture in this video, we are going to discuss with the one of the micro-solid state device that is a varactor diode. Learning outcomes, after completion of this session, students are able to describe the working and construction of the varactor diode. They can also know the application areas of the varactor diode. These are the contents which we are going to discuss. Now, before discussing with the varactor diode details, we should know what is a resistance, a negative resistance device, because the varactor diodes are working on the principle of negative resistance. Now, what is exactly what devices, why these are called as a negative resistance device? Because in the negative resistance device, it allows the current in a one direction at which its resistance is very low. This is obtained when it is in a reverse bias, when the diode is connected into the reverse bias. And similarly, it does not allow the current to flow in a other direction when its resistance is very high. And this is during the reverse bias condition. These devices work on the principle of resistance, negative resistance, means what? These diodes are being connected in a reverse bias. The symbol of the varactor diode, it is like a, it has a two terminals anode and cathode. It is been a capacitor is represented with the symbol of the diode here. In series, it is a parallel plate. So, parallel plates of the capacitors. Why it is being shown as a capacitor? Because the, between the two layers, there is a depletion regions form and that depletion regions acts like a capacitance over here. Now, you can see here, this is a symbol, your p-type is connected to the negative terminal of the battery and n-region is connected to the positive terminal of the battery. So, the depletion regions acts like a capacitance between these two regions. These diodes are also known as a very cap diode because there is a variable capacitance. So, they are also known as a very cap diodes and also they are used as a variable tuner tuning the circuit. So, they can act as a tuning diodes also. This was, this diodes was first developed in 1961 by the specific semiconductors. And the importance of this diode is that they are working in a reverse bias condition. And as the capacitance is changing based on the applied voltage. So, based on the voltage, the capacitance is changing and this condition is observed in a reverse bias condition. Now, as the, the depletion regions, we are, the region is known as a capacitance over there, the junction capacitance we can call. And this, as the equation of the capacitance is given by epsilon 0, epsilon r, A by D. As this D goes on increasing, this see junction capacitance goes on changing. Now, let us see the instruction, internal structure of the varactor diode. Here, there are the two regions it is being shown of the N region, N type and N plus. No, not necessary that it should be a N plus. The, it can be a N also. The two regions are being shown because the electrons are excited to move. There are more numbers of electrons are excited to move towards the P type of material. This, for that purpose, this is the internal structure it is shown. Across, above this N type of material, P type of material is shown which is represented by the epitaxial layer. Now, what is the meaning of this epitaxial layer? The meaning of the epitaxial layer is that the width is not constant, but it goes on changing depending upon the applied voltage. So, this is not constant, it is epitaxially changing. So, even if it is exponentially changing, the width is being changed exponentially and there are two terminals P type and N type. Two terminals are being over here, metal contacts are done and this varactor diode are developed on the heatsink material. As we are telling that the junction is being created in the, at the P type of N, N type of layer, that junction acts like a capacitor and that capacitance is directly depends upon the applied voltage. The applied voltage is a reverse bias. It is exponentially depends upon the applied reverse voltage. Here N can, for example, if you consider capacitance is directly proportional to the minus 1. This is because the, as the D increases, its value goes on decreasing. Now, when, let us see how the equivalent circuit is being represented here. Yeah, at this junction, there is a resistance you can see. Resistance is created. No, it is not visible, sorry. You can see here is a resistance in the equivalent circuit and junction resistance. This we are calling it as a rj and here we are representing the capacitance which is called as a junction capacitance. Now, to this they are parallel, parallely and here will be the resistance, plate resistance which is in the series and there will be a plate capacitance for the leads which we are going to connect. See, the, similarly this circuit is here we are representing the equivalent circuit. These are the junction capacitance and junction resistance and these are the two plate we are connecting series resistance and plate capacitance. This is the equivalent circuit of the varactor diode. Now, the working of the varactor diode, as we are telling that the diode is being connected in the reverse bias, the numbers of holes from the p regions are made to move across the negative terminal of the battery. When it is connected in the reverse bias, the holes are moved towards the negative terminal of the battery and the electrons move towards the positive terminal of the battery, due to which a depletion region goes on increasing. As the depletion region goes on increasing, the D of the value goes on, the distance between these two regions goes on increasing. As the distance increases its capacitance, the junction capacitance goes on decreasing. So, when we see the characteristics gap of this, the as the reverse bias, as the reverse bias increases its junction capacitance goes on. This capacitance value goes on decreasing. The applications of the varactor diode, the varactor diode is used as a control oscillator. They are used as a parametric amplifier. They are used as a frequency multiplexer. They are used as a tuning circuit. Also, they are used as a phase lock loop and they can be also used as a frequency synthesizer. These are the few references for referring the contents. Thank you.