PN junction diode

In this article we will learn what is forward bias and reverse bias? what is diffusion current and drift current? what is avalanche breakdown?

Before going to this question, know about PN junction semiconductor and how it's formed?

       It's a combination of P type semiconductor with N type semiconductor. It's formed when a P type semiconductor is joined a N type semiconductor. P type semiconductors have holes, which is positively charged and N type semiconductor have electrons, which is negatively charged. Where P type and N type joints and form a junction, this junction is known as PN-junction. The electrons near to the junction is jump from N to P and holes near to the junction is jump from P to N. This phenomena creates a space charge region as shown in below figure.

       Now, the electrons are available at P region and holes are available at N region and the electric field is created in this space charge region because of the movement of the holes and electrons. The direction of electric field is N type region to P type region. Some electrons moves back from P to N in the space region and some holes moves back from N type to P type space region. This continues to happen till equilibrium is reached at this region. This movement of holes and electrons in space region gives rise to a current and this current is known as diffusion current.

let's start with first question, what is forward bias and reverse bias? 

        In forward bias, the positive terminal of battery is connected towards the P region of the PN junction and the negative terminal of the battery is connected towards the N region of the PN junction. The positive terminal repulse the holes towards the junction in the P region and the negative terminal repulse the electrons towards the junction in N region. This results in the junction to shrink. Due to this depletion region or space charge region narrows down. 

        If the voltage in forward bias, is able a specified range, this electrons in the N regions drifts through the junction and migrates to the P region and the holes in the P region drifts through the junction and migrates to the N region. Now, because of this movements of electrons and holes, current starts following through the circuit and this current is known as the drift current.

        In reverse bias, the negative terminal of battery will be connected towards the P type semiconductor and the positive terminal of the battery will be connected to the n type of semiconductor. Here, holes is attracted to the negative terminal of battery and electrons are attracted to the positive terminal of battery. This results in increase the depletion region. Now in this condition, PN junction behaves like an insulator and it will not allow any current to flow in the circuit. But in particular condition, if the battery voltage is above a reverse bias breakdown voltage. The electrons and holes breakdown thorough the PN junction and cross, resulting in the current to flow through the circuit. This breakdown is called as avalanche breakdown. In this process, the current following through the PN junction is very high and ultimately the PN junction gets damaged due to overheating caused by the excess amount of current. 

This is how PN junction semiconductor works.

VI characteristic of PN junction diode:

It is a relationship between current through the diode and applied voltage. It is represented as a graph. X axis represents as a voltage and Y axis represents current through the diode. Right side of this graph is forward voltage region and diode is in forward biased. Left side is reverse voltage region and diode is in reverse biased.

A diode has non-linear characteristic. When voltage is positive, current exponentially increases until reaches value Vd, which is depends on built-in electric field of depletion region. Vd is 0.7V for silicon and 0.3V for germanium diodes. For voltage greater than Vd, current rapidly increases and is limited only by resistance in circuit.

When voltage is negative, very small amount of current will flow through the diode, this current is known as leakage current or reverse saturation current. Reverse saturation current is few micro ampere and is practically negligible. But after reverse breakdown, current is increases.

So, working of diode is divided into three regions;

1) Forward region

2) Reverse region

3) Breakdown region