 Hello everyone. Welcome to this video lecture. Myself Dipali Vadkar working as assistant professor at WIT Solapur. In this video lecture, we will study the PN junction diode. At the end of this video lecture, student will be able to examine working of PN junction diode. Student will be able to draw the VI characteristic of PN junction diode and state applications of PN junction diode. PN junction diode, it is the basic building block of semiconductor devices. It is made by joining P type semiconductor with N type semiconductor by a special fabrication process. Diode means two electrodes. So, the terminal which is connected to the P side, it is called as anode and the terminal which is connected to the N side, it is called as cathode. This is the symbolic representation of PN junction diode. Here the arrow head represents the direction of conventional current flowing through the device. This bar represents cathode and this is the real appearance of PN junction diode. The PN junction diode in that current conduction takes place due to the two types of semiconductor material that is the P type and N type. The P type semiconductor material is made up by adding the trivalent impurity into pure semiconductor. By addition of this trivalent impurity provides the free holes. So, the current conduction takes place in P type saving conductor, it is due to the holes. So, holes are the majority charge carriers and the electrons here are the minority charge carriers. The N type semiconductor is made up of adding pentavalent impurity into pure semiconductor. So, addition of this impurity provides large number of free electrons. So, current conduction in N type semiconductor is due to the electrons. So, majority charge carriers here are electrons and minority charge carriers are holes. When P type semiconductor is joined to the N type semiconductor then there is a formation of PN junction diode. So, the boundary or interface between this P type and N type semiconductor it is called as a PN junction. The PN junction working modes, the PN junction works basically in three modes. First it is equilibrium, it is also called as zero biasing mode. Here no external potential energy that is no external supply is applied to the PN junction. Second forward biasing mode, here the positive terminal of external DC source is connected to the P region and negative terminal is connected to the N region. And in reverse biasing mode the negative terminal of external DC source is connected to the P region and positive terminal is connected to the N region. Now, first mode that is the equilibrium mode. So, here no any external supply is given to the device. So, in this condition at the initial condition the majority charge carrier that is the electron from N side these electrons will diffuse into the P side. The atoms from N side which donates this electron these atoms are get converted into the positively charged ion. And the atom from P side who accept these electrons these atoms get converted into the negatively charged ion. So, these positively charged and negatively charged ions accumulate near the junction. So, there is a one layer of this immobile ions towards N side positive and towards P side negative ions. So, this layer is called as a depletion region. Due to this positively charged ions there is a positive potential different ion here the negative potential. So, this the potential difference is generated here. So, this is nothing but building potential it is also called as a potential barrier. So, once this depletion region generates here the majority charge carriers cannot cross the junction. Now, next mode that is the forward biasing. In this mode the positive terminal of external DC source is connected to the P side of semiconductor. Negative terminal of external DC source is connected to the N type of semiconductor. So, the battery connected in this way the majority charged carriers that is the electron from N side will repel towards the junction due to this negative terminal. And the majority charged carriers that is the holes which are the positively charged holes will repel towards the junction due to this positive terminal. If due to this movement of electrons or repulsion of electrons and holes towards the junction the width of depletion region decreases as the external voltage increases width of depletion decreases. And at one particular voltage the depletion region completely collapse and this majority charged carriers can cross the junction easily. Due to this movement of majority carriers through the device the current flows through the device and that current is called as a forward current. So, the VI characteristic that is nothing but curve between the voltage across the junction along x axis and current through the circuit along y axis. It describes the DC behavior of the diode. So, these are the forward VI characteristic. In this characteristic on x axis there is a forward voltage on y axis there is a forward current. In between region A to B that is at the initial condition when the forward voltage is very small the current flowing through the device is also very small. After point B the current flowing through the device increases exponentially. At point B the depletion region completely get collapse and the current flowing through the circuit increases. So, this point is called as a knee voltage or it is also called as a cut-in voltage. Now, next reverse biasing of PN junction. In reverse biasing negative terminal of external DC source is connected to the P side and positive terminal is connected to the N side. So, due to this positive terminal the negatively charged electrons from N side attracts towards the positive terminal and positively charged holes from P type will attract towards the negative terminal. And due to that the width of depletion region increases that is depletion region expands here. So, due to this the barrier resistance is increases and majority charged carriers cannot flow through the junction. So, there is no zero current due to the majority charged carriers. Now, question what happens to the minority charged carriers when the PN junction diode is reverse bias? Pause the video for a while and think. So, in reverse biasing at the P side there are minority charged carrier that is the electrons and at the N side minority charged carriers that is the holes. Now, you will see how these electrons and holes minority charged carriers generated. So, whenever the heat energy is applied to the atoms the electrons from a valence band will break the covalent bond with parents atom and these electrons becomes free. Due to this electrons the whatever the current flows through the device that current is called as a reverse saturation current. I will launch a breakdown. Whenever the reverse voltage across the diode increases minority charged carriers that is the reelectrons which are present in a P side gains a higher energy and their movement takes place with the higher speed. These electrons will collides with the atoms and they knock off the more electrons. So, these free electrons again collides with the another atom and they knock off more number of electrons. So, in this way large number of electrons generated here due to this the current flows through the device. So, we will see how this avalanche breakdown takes place. So, when due to the kinetic energy these electrons get a higher energy these electrons collides with the atoms and they knock off more electrons. These electrons again collides with the another atom then knock off more electrons. So, due to this continuous collision large number of electrons are generated and due to this large amount of current flows through the device. So, this condition is called as a avalanche breakdown mechanism. What are the reverse VI characteristics of PN junction diode? So, here at the initial condition very small reverse current flows that current is called as a saturation current. This current is due to the minority charge carriers which are thermally generated means they are depend upon this current is depend upon the temperature not on a reverse voltage. So, even increasing reverse voltage here the current remains constant. So, this is the reverse saturation current. After one particular voltage the current increases rapidly. So, that voltage is called as a breakdown voltage where the breakdown of PN junction takes place. So, PN junction diode can act as an automatic switch. So, when diode is a reverse bias switch is open and when diode is a forward bias the switch is closed. These are the some more application of PN junction diode. PN junction diode act as a rectifier to convert AC into DC as a switch in computer circuit. It act as a detector in radios to detect audio signal. It is also used as a LED to emit different colors. These are the references. Thank you.