forward bias and reverse bias of pn junction ppt to pdf

Forward Bias And Reverse Bias Of Pn Junction Ppt To Pdf

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Diode is a two terminal PN junction that can be used in various applications. One of such applications is an electrical switch.

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In chapter 1 — Understanding the PN junction , we have seen how a PN junction is formed from a p-type and n-type semiconductor. We have also learned about diffusion current, depletion region, drift current and barrier potential. Lets just make some questions. What is the use of a PN junction? Why have scientists created a pn junction device? What kind of problem it solves? Learning anything is really fun when we question it. So these are our questions. Why there exists a pn junction in this world!

To get an answer to all these questions, lets first try to understand the characteristics of a PN junction. To apply an external voltage, we usually connect 2 metallic contacts at the two ends of the pn junction known as terminals ; one on the p-side and other on the n-side. A PN junction with two metallic contacts is known as a pn junction diode or a semiconductor diode. PN junction diode is symbolically represented as shown in picture. The direction of arrow is the direction of conventional current flow under forward bias.

Now lets try applying an external voltage to the pn junction diode. There are two ways in which we can bias a pn junction diode. The basic difference between a forward bias and reverse bias is in the direction of applying external voltage. Image Source. Forward biasing a pn junction diode is very simple. You just need to take a battery whose values can be varied from o to V volts , connect its positive terminal to the p-side of pn junction diode and then connect the negative terminal of battery to the n-side of the pn junction diode.

If you have done upto this, the forward bias circuit of pn junction diode is complete. Now all we need to do is understand how the pn junction diode behaves when we increase the voltage levels from 0 to say 10 volts or volts.

We have learned that if we apply an external voltage higher than the barrier potential of pn junction diode, it will start conducting, which means it will start passing current through it. Lets get a voltmeter and ammeter and connect it to the forward biased circuit of pn junction diode.

A simple circuit diagram is shown below, which has a pn junction diode, a battery in picture it is not shown as variable. Note:- Assume that the pn junction diode is made from Silicon.

The reason is difference in barrier potential for a diode made from Germanium and Silicon. For a silicon diode — barrier potential is 0. What we are going to do is, vary the voltage across diode by adjusting the battery. We start from o volts, then slowly move 0. Finally after taking the readings, just plot a graph with voltmeter readings on X-axis and corresponding Ammeter readings on Y axis.

Join all the dots in graph paper and you will see a graphical representation as shown below. We talked a lot about barrier potential but did we ever mention its value?

From the graph, we observe that the diode does not conduct at all in the initial stages. From 0 volts to 0. This means the diode has not started conducting current through it. From 0. From this data what you can infer? The barrier potential of silicon diode is 0. The diode starts conducting at 0. It conducts current linearly with increase in voltage applied across the 2 terminals provided the applied voltage crosses barrier potential. We have seen the characteristics of pn junction diode through its graph.

What really happens inside the diode during the forward bias? We know a diode has a depletion region with a fixed barrier potential. This depletion region has a predefined width, say W. This width will vary for a Silicon diode and a Germanium diode.

The width highly depends on the type of semiconductor used to make pn junction, the level of doping etc. When we apply voltage to the terminals of diode, the width of depletion region slowly starts decreasing. The reason for this is, in forward bias we apply voltage in a direction opposite to that of barrier potential.

We know the p-side of diode is connected to positive terminal and n-side of diode is connected to negative terminal of battery. So the electrons in n-side gets pushed towards the junction by force of repulsion and the holes in p-side gets pushed towards the junction.

As the applied voltage increases from 0 volts to 0. This means depletion region vanishes at 0. This results in increased diffusion of electrons from n-side to p-side region and the increased diffusion of holes from p-side to n-side region. This is another interesting factor, to explain. As the voltage level increases, the electrons from n-side gets pushed towards the p-side junction. Similarly holes from p-side gets pushed towards the n-side junction. Now there arises a concentration gradient between the number of electrons at the p-side junction region and the number of electrons at the region towards the p-side terminal.

A similar concentration gradient develops between the number of holes at the n-side junction region and the number of holes at region near the n-side terminal.

This results in movement of charge carriers electrons and holes from region of higher concentration to region of lower concentration. This movement of charge carriers inside pn junction gives rise to current through the circuit. Why should we reverse bias a pn diode?

The reason is, we want to learn its characteristics under different circumstances. By reverse biasing, we mean, applying an external voltage which is opposite in direction to forward bias. So here we connect positive terminal of battery to n-side of the diode and negative terminal of the battery to p-side of the diode.

This completes the reverse bias circuit for pn junction diode. Now to study its characteristics change in current with applied voltage , we need to repeat all those steps again. Connect voltmeter, ammeter, vary the battery voltage, note the readings etc etc.

Finally we will get a graph as shown. Here the interesting thing to note is that, diode does not conduct with change in applied voltage. The current remains constant at a negligibly small value in the range of micro amps for a long range of change in applied voltage. When the voltage is raised above a particular point, say 80 volts, the current suddenly shoots increases suddenly.

We connected p-side of diode to negative terminal of battery and n-side of diode to positive terminal of battery. So one thing is clear, we are applying external voltage in the same direction of barrier potential.

The electrons at n-side will get pulled from junction region to the terminal region of n-side and similarly the holes at p-side junction will get pulled towards the terminal region of p-side. As width of depletion region increases, it results in increasing the electric field strength.

The reverse saturation current is the negligibly small current in the range of micro amperes shown in graph, from 0 volts to break down voltage. How it occurs? We know, as electrons and holes are pulled away from junction, they dont get diffused each other across the junction. What remains is the drift due to electric field. This reverse saturation current is the result of drifting of charge carriers from the junction region to terminal region.

This drift is caused by the electric field generated by depletion region. At breakdown voltage, the current through diode shoots rapidly. Even for a small change in applied voltage, there is a high increase in net current through the diode. For each pn junction diode, there will be a maximum net current that it can withstand.

If the reverse current exceeds this maximum rating, the diode will get damaged. To conclude about pn junction characteristics, we need to get an answer to the first question we have raised — What is the use of pn junction?

From the analysis of both forward bias and reverse bias, we can arrive at one fact — a pn junction diode conducts current only in one direction — i. During forward bias, the diode conducts current with increase in voltage. During reverse bias, the diode does not conduct with increase in voltage break down usually results in damage of diode. Where can we put this characteristics of diode into use? Hope you got the answer!

Its in conversion of alternating current to direct current AC to DC. So the practical application of pn junction diode is rectification! Author jojo. Faith N. I am clearly understood to read it……. For this crystal clear explanation….. I really got something from it….

PN Junction Diode Characteristics – Explained in Detail with Graphs

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