PN junction Diode hey, friends welcome to Kohikhi.com in the previous article we have seen that what is p-type and n-type semiconductors. PN junction Diode and we have seen that in a case of a p-type semiconductor the holes are the majority carriers and electrons are minority carriers. while in the case of the n-type semiconductor the electrons are majority carriers and holes are minority carriers.
- Topics That We Covered
- PN junction
- Unbiased p-n Junction
- Depletion region
- Forward biased P-N Junction
- Reverse biased P-N Junction
- Reverse Saturation Current
- V-I Characteristics Of PN junction Diode
PN junction now by itself, these p-type and n-type semiconductors act like a resistor, but by doing one side of silicon crystal with a p-type impurity and the other side by the n-type impurity, we can convert the silicon crystal into the PN Junction.
here this PN junction is the border where the p-type and n-type region meets. And this silicon crystal with p and n-type impurity is also known as the diode because here the p-type and n-type region acts like two electrodes.
so understanding this PN Junction we can understand all kinds of semiconductor devices. so in this p-type region, each circled negative sign represents the trivalent atom and the positive sign represents the hole. similarly, in the n-type region, each circled positive sign represents the pentavalent atom and each minus sign represents the electron.
Unbiased p-n Junction
Unbiased p-n Junction here for simplicity in both regions, the minority carriers are not shown but in very small quantity they are also present in both regions. now due to the doping, there is a large number of electrons on the N side but very few electrons on the P side, because on the P side the electrons are minority carrier sand whenever these P and N regions are joined then the electrons from the N side starts diffusing towards the P side.
So whenever the electron enters the p-type region then it becomes the minority carrier and being surrounded by so many holes its lifetime will be very short and due to that, it will get recombined with these holes. So whenever the electron diffuses across the junction then it creates the two ions. so first whenever. Unbiased p-n Junction leaves this n-type region then it leaves behind this pentavalent atom which is just one short of an electron.
so due to that, it will become a positive ion and whenever the electron enters this p-type region then it will get recommend with the hole of this trivalent atom. so after capturing the electron this trivalent atom will become a negative ion. so every time the electron diffuses to the P side from theN side then it creates one positive and the negative ion near the junction.
PN junction Now, these ions are the immobile ions and unlike the free charge carriers like electrons and the holes they cannot move so due to the recombination of these holes and electrons near this Junction, there is hardly any free charge carriers near this injunction.
or we can say that due to the recombination this Junction is depleted from the free charge carriers and due to that this region is also known as the depletion region. so this depletion region contains both positive and negative ions, which are immobile in nature, and apart from that it also contains a few charge carriers.
Depletion region PN junction which is thermally generated in this depletion region. now in this depletion region, these ions set up the electric field. And this electric field points from the positively charged ions towards the negatively charged ions and due to this electric field only a few electrons from the N side are able to cross this depletion region and the same is true for the holes in this p-side region or we can say that these immobile ions create the barrier potential for this majority carriers, so that they are not able to diffuse from one side to the other side.
so this barrier potential is also known as the built-in potential and for silicon, this built-in potential is equal to 0.7 volts while for germanium it is roughly around 0.3 volt. so the free charge carriers hard to be precise the majority of carriers in both regions have to overcome this barrier potential to cross this depletion region. And only a few majority carriers are able to cross this barrier whenever there is no external biasing.
but PN junction due to this electric field, the minority carriers in both regions will be able to cross this depletion region. For example, the holes which are minority carriers in this n-type region will get swept from the n-type region towards the pre type region and they will become majority carriers in this p-type region similarly the electrons which are minority carriers in this p-type region will get swept due to this electric field in this n-type region and once they enter this n-type region they will become the majority carriers.
so due to this built-in electric field, the majority of carriers are not able to cross this Junction but still, the minority carriers will be able to go from one region to another region. PN junction But when there is no external biasing is applied then the flow of electron due to the electric field and due to the diffusion will get canceled out each other or in other words, PN junction Diode we can say that the flow of minority charge carriers and the majority charge carrier will cancel out each other so that the overall current in the circuit will be zero.
PN junction And the same thing is also applicable to the holes. so whenever this PN Junction is not biased then the overall current in the circuit will be zero, so if this majority charge carriers on both N and P side wants to cross this depletion region then they require the external biasing voltage. PN junction Diode so if we apply the external field in the same direction as the built-in electric field, that case, this depletion region will provide more resistance to the majority of carriers.
on the other end of the applied external field is in the opposite direction to the built-in electric field in that case the resistance which is offered by the depletion region will reduce. so based on that let us see the two types of biasing for this PN Junction. And first, let ustalk about the forward bias PN Junction.
Forward biased P-N Junction
Forward biased P-N Junction so in case of a forward bias, the positive terminal of the source is connected to the P side and the negative terminal is connected to the N side. And in this forward bias condition, the external electric field is in the opposite direction to the built-in electric field, and due to that the effective electric field at the junction will reduce. so in the forward bias condition,
PN junction the electrons in the n-type regions and the holes in the p-type region will get pushed towards the junction. so due to that, the width of the depletion region will reduce. so the effective resistance which is offered by the depletion region will also reduce.
if PN junction you further increase the externally applied voltage then the width of the depletion region will further reduce and when the applied voltage is more than the barrier potential of this PN Junction then the resistance that is offered by the depletion region is negligible. so for example for the silicon crystal, if the applied external voltage is more than the 0.7 volts, PN junction in that case, the resistance that is offered by the depletion region will be negligible and in this condition, the electrons from the N side can cross this depletion region.
And PN junction Diode once they cross this depletion region then they will get attracted to the positive terminal of the battery. so once they come into the p-side region then just by passing through the holes in the p-type region they can reach the positive terminal of the battery. PN junction And similarly, the holes will be pushed towards the depletion region and once they enter the n-side region then they will get attracted to the negative terminal of the battery.
PN junction in this way, we get a flow of current due to the movement of holes as well as electrons. And as we increase the externally applied biasing voltage the more and more electrons and the holes will be able to cross this depletion region. And due to that, they will contribute to the flow of current.
so as we increase the externally applied voltage then the flow of current in the circuit will increase. Now before we go ahead let me just clear one thing. The hole is nothing but the absence of an electron at a particular location.PN junction so whenever the electrons are moving from one place to the other place or here from right to the left then in a way, we can say that the hole is also moving from left to the right so due to the movement of electrons we also get the moment of holes.
And in this way, we get a flow of current due to the electrons as well as holes. So now similarly let us see when the PN junction is reversed biased.
Reverse biased P-N Junction
Reverse biased PN Junction so in the reverse bias condition, the negative terminal of the battery is connected to the P side and the positive terminal of the battery is connected to the N side so in this condition, PN junction the electrons which are majority carriers in this n-type region will get attracted towards the positive terminal of the battery and similarly the holes on the P side region will get attracted towards the negative terminal of this battery.
And due to that, more ions will get created near the junction so we can say that the width of the depletion region will increase. so in the reverse bias condition, as we increase the reverse bias voltage the width PN junction of the depletion region will further increase and due to that now this depletion region offers more resistance to these majority carriers.
and due to that virtually there is a flow of current due to the majority carriers but in this case, due to the built-in electric field, minority carriers in both regions will be able to cross this depletion region.
so PN junction in this reverse bias condition the holes which are minority carriers in the n-type region will cross this barrier and we’ll reach the p-side region and once they reach this p-side region then they will get attracted towards the negative terminal of the battery similarly the electrons in the p-type region which are minority carriers will be able to cross this depletion region and after crossing this depletion region they will reach to the n-side region.
And once they reach this inside region then they will get attracted to the positive terminal of the battery so in this way in this reverse-bias condition, we will get a flow of current due to the movement of the minority carriers.PN junction But as the minority carriers are very less in comparison to the majority carriers, the magnitude of the current in this reverse bias condition will be very low compared to the forward bias condition.
So this flow of current which exists in this reverse bias condition is known as the reverse saturation current and that the term saturation comes from the fact that it reaches the maximum level very quickly and it does not change significantly whenever we increase this reverse bias voltage. PN junction now typically these reverse saturation currents used to be in the range of microamperes but nowadays due to the advancement in the technology for the silicon devices this reverse saturation current used to be in the range of nano amperes and here let’s denote these reverse saturation current as Is.
Reverse Saturation Current
Reverse Saturation Current So this reverse saturation current is a function of temperature. so as the temperature rises, the thermally generated electron-hole pair in the silicon crystal will increase or we can say that the minority carrier charges in the silicon crystal will increase PN junction, and due to that this reverse saturation current will also increase. PN junction So for silicon, this reverse saturation current almost gets doubled for every 10-degree rise in the temperature or we can say that for the 1-degree rise in the temperature this reverse saturation current increases by 7 percent.
so let’s say for a one PN-Junction if this reverse saturation current is 20 nano ampere at 25 degrees centigrade then at 35-degree centigrade, it will be roughly around 40 nano ampere. so after every 10-degree rise in the temperature, these reverse saturation current almost gets double. now as I said earlier this reverse saturation current does not change much even if PN junction we increase the reverse bias voltage. but there is a limit on the maximum reverse voltage which can be applied to this PN Junction
V-I Characteristics Of PN junction Diode
so if we continuously increase this reverse voltage then we will reach a point which is known as the breakdown voltage. So once the breakdown voltage is reached for the PN junction then suddenly a lot of minority carriers will appear at the depletion region and suddenly the diode conducts very heavily and we will see in detail in the next article that what happens to this PN Junction diode whenever it is used in this breakdown region.PN junction so whenever the diode is operated in the reverse bias condition then the applied voltage should be less than this breakdown voltage and in detail, we will talk about it in the next article.
so I hope in this article you understood what is PN Junction and how it can be operated in the forward and reverse bias condition. so if you have any questions or suggestions do let me know here in the comment section below.