PN Junction Diode (No Applied Bias)

in this lecture we will start a new chapter semiconductor diodes semi conductor diodes this is the name of the chapter and in this chapter we will learn how a junction is formed between P type and N type semiconductor we will also learn volt ampere characteristics of the two terminal device obtained by using p and Junction and the device is called as PN Junction diode so I will write this down we will learn volt ampere characteristics or VI characteristics of PN PN Junction diode PN Junction diode is a two terminal device and there's a difference between PN Junction and PN Junction diode we obtain PN Junction diode by using the PN Junction and we can do do this easily by attaching the metal contacts at the two terminals so if I attach one metal contact like this then we have p and Junction diode by using the p and Junction we will do the same thing for this terminal also and uh because of this we have three possibilities first possibility is no bias condition no bias conditions in second possibility we have forward forward bias condition and in third possibility we have reverse bias condition in this lecture we will deal with no Bas condition and in the coming presentations we will complete forward and reverse Bas condition of the PN Junction diode now there is one question what is the meaning of bias what do we mean by biasing the two terminal device so I will write this down the meaning of bias the term bias refers to the application of external voltage across the two terminals when we apply external voltage across these two terminals we call it biasing I will write this down application application of external external voltage across the [Music] two terminals is called as bias and if we apply no external voltage it is called as no bias condition so in this lecture we are not going to apply any external voltage to this PN Junction diode and we will analyze the PN Junction diode in this situation the next next thing is the BJT the bipolar Junction transistor this chapter is very important because we will use the concepts and the the is developed in this chapter for learning BJT BJT stands for bolar bolar junction transistor and this is very important topic in analog electronics and it depends on PN Junction diode so if you have better understanding of PN Junction diode you can easily understand the bipolar Junction transistor BJT is a three terminal device this is three terminal device and you can consider this as the device having two diodes BJT you can consider as a device having two diodes in which one diode is biased by the current of other diode so you can see how much it depends on the concept that we are going to learn so without wasting any time we will start with p and Junction we already know what is p type and N type Semiconductor in case of P type semiconductor we add trivalent impurities in case of P type semiconductor we add trivalent impurities and because of this we have holes holes as the majority charge carriers on the other hand for n type semiconductor we add pentavalent impurities and because of this we have electrons as majority charge carriers and if you talk about the minority charge carriers then for p type we have electrons as the minority charge carriers and for n type we have holes as minority charge carriers and if you remember the minority charge carriers the minority charge carriers depends only on temperature because electron gains the thermal energy and breaks the calent bond so it only depends on the temperature and it will not depend on the external voltage that we are applying so this is something we already know and uh now we have to see what will happen if we combine P type and N type semiconductor together when we combine P type when we combine P type and N type semiconductors together by any means like diffusion or an implantation these are the process to combine p and N type semiconductors we dope one side of the semiconductor with the trivalent impurities and we dope other side of the semiconductor with pentavalent impurities we will have P type on one side like this and we will have n type on the other side if we consider the P type if we consider the P type I will clear this portion if we consider the P type then we have positive immobile ions or we can say acceptor ions these are the acceptor ions and we have holes we have holes as the majority charge carriers and on nide we have donor ions [Music] donor ions and electrons are the majority charge carriers so this is what we have when we join P type semiconduct conductor an N type semiconductor and this is the junction this is the junction and if we talk about holes then concentration of hole is higher on the P side as compared to the N side and because of this we have concentration gradients of holes from P side to n side and similarly we have concentration gradient of electrons from n side to P side and if we try to understand this thing by the help of pressure difference let's say we have a A and B these are the two reasons and pressure at a is PA a and pressure at B is PB and let's say PA a is greater than PB then air will flow from A to B right the same thing will happen with holes on P side concentration of hole is higher so holes will move towards the end side like this and if you talk about electrons then concentration of electron is more on N side so electrons will move to P side like this and as you can see we have movement of charge through the cross-sectional area if we consider this Junction and let's say the area of Junction is a then there is movement of charge carriers through this area and whenever this happens there is current and as this process the process of movement of charge carriers from high concentration to low is called as diffusion we call this current diffusion current this process is called as diffusion and the current is called as diffusion current this diffusion current is very important and we have to use it a lot while solving the numerical problems and diffusion current is because of majority charge carriers so we have one new thing with us diffusion current so let us try to understand it once more diffusion current is because of the movement of charge carriers the majority charge carriers from high concentration to low concentration we call it diffusion current because the process of movement is called as diffusion and this is because of majority charge carriers right the next important thing is depletion layer now we have to understand understand how depletion layer is formed in p and Junction diode because of diffusion holes from P side holes from P side combines with electron on N side because we have electrons as the majority charge carriers on nide and in the same way electrons from n side combine with the holes on P side and as the recombination takes place immobile ions will surface out I will write this point down immobile ions will surface out because of what because of diffusion now what do we mean by what do we mean by immobile ions will surface out what do we mean by surface out surface out is nothing but uncovering of immobile LS this acceptor ions and these donor ions are immobile ions because they're fixed they are not moving and when we uncover them we call immobile ions are surfaced out because initially electrical neutrality was maintained with acceptor ions we had holes so electrical neutrality was maintained in the same way with donor ions we had electrons but as these holes are combining with electrons we are left with positive and negative ions because of this depletion layer is formed in PN Junction diode the reason with no mobile charges but only uncovered immobile ions is called as depletion reason I will write this down because this is important depletion reason in this reason there is no mobile charges no mobile charges but only but only uncovered uncovered immobile ions are there and as I have already explained why there is no mobile charges because the holes and electrons are combining with each other and we are left with negative and positive ions so I will try to draw this quickly I will try to draw the depletion layer quickly these things are very important because this will help you understand the basics and after this you can understand any concepts in analog electronics and if you fail to understand this important points then it will be very tough for you to understand the coming presentations first I will make p side we have acceptor ions on the P side and on the IDE we have donor ANS so we have positive sign for them holes are the majority charge carriers in P side so we have holes as the majority charge carriers and we have electrons as the minority charge carriers in the same way for n side we have electrons as the majority charge carriers and we have holes as the minority charge carriers now the electrons and holes have recombined in this region so we are left with we are left with negative immobile an and positive immobile ANS like this because holes and electrons have already recombined with each other and you can clearly see because of this we have layer of negative charges negative immobile ions on this side and we have layer of positive immobile ions on this side so we can say that this is acting as the potential difference and because of this potential difference there is an electric field from right to left that is from positive to negative there is an electric field like this capital E is the electric field from positive to the negative this is the junction I forgot to make Junction here this is the junction and this is the depletion Reon now this depletion reason this depletion reason is also called as space charge reason space charge reason or depletion layer depletion layer these are the names for depletion reason now there is one question why the width of depletion reason why the width of depletion reason is fixed because this hole can combine with this electron in the same way all the holes on the P side can combine with the electrons all the electrons from the N side and the width of depletion reason is equal to the width of the p and Junction diode but this does not happens what is the reason behind this as I have already told you we have electric field developed because of the potential difference and due to this electric field there is no movement of charge carriers because this positive layer will repel the holes right because hole is equivalent to positive charge and this positive layer will repel the holes so there is no movement of holes and in the same way this negative layer this negative layer will repel the electrons from the inside so the width of depletion reason is fixed and as you can see this potential is acting as the barrier to the movement of holes and electrons it is acting as barrier for the further movement of free charge carriers we call it barrier potential we call it barrier potential or we call it built in potential built in potential right and we are calling it barrier potential because it is acting as the barrier for the further movement of free charge carriers let's talk about minority charge carriers in this scenario we are done with the majority charge carriers we have seen there is diffusion current because of the movement of majority charge carriers and after the formation of depletion reason the majority charge carriers will not combine with each other and hence there is no movement of majority charge carriers and as we already know in case of P type semiconductor electrons are the minority charge carriers and for n type we have holes as the minority charge carriers and because of this electric field intensity e electrons will move to the right and the holes will move to the left so there is current and because of drift of electrons towards the positive layer and drift of holes towards the negative layer we call this current drift current and this drift current this drift current having the direction opposite to the diffusion current and under steady state condition the diffusion current is equal to the drift current before that I will complete this the the drift current is is due to the minority minority charge carriers right and the diffusion current was because of majority charge carriers and under under steady state diffusion current diffusion current is equal to the drift current this is very important Point diffusion current is equal to the drift current under steady state condition and uh we can say that we can say that the net current is equal to zero in case of open circuited PN Junction diode the net current the net current is equal to Z Z for open circuited p and Junction diode so this is all for this lecture in the next lecture we will find out the expression for VB that is the barrier potential and we will also talk about width of depletion reason WD width of depletion region so this is all for this lecture see you in the next one