In this article, we will learn What is forward bias and reverse bias in PN Junction? What is diffusion current and drift current? What is an avalanche breakdown?
Before going to this question, know about the PN junction semiconductor and how it’s formed?
PN Junction Semiconductor
It’s a combination of a P-type semiconductor with an N-type semiconductor. It’s formed when a P-type semiconductor is joined an N-type semiconductor.
P-type semiconductors have holes, which is positively charged and N-type semiconductors 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 are jumped from N to P and holes near to the junction is the jump from P to N. This phenomenon creates a space charge region as shown in the below figure.
Now, the electrons are available at the P region and holes are available at the 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 the electric field is the N-type region of the P-type region. Some electrons move back from P to N in the space region and some holes move back from N-type to P-type space region.
This continues to happen till equilibrium is reached in this region. This movement of holes and electrons in the space region gives rise to a current and this current is known as diffusion current.
let’s start with the first question, What is forward bias and reverse bias?
In forward bias, the positive terminal of the 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 the 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, these 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 movement of electrons and holes, current starts flowing through the circuit, and this current is known as the drift current.
In reverse bias, the negative terminal of the battery will be connected towards the P-type semiconductor and the positive terminal of the battery will be connected to the N-type semiconductor.
Here, holes are attracted to the negative terminal of a battery, and electrons are attracted to the positive terminal of the battery. This increases 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 a particular condition, if the battery voltage is above a reverse bias breakdown voltage.
The electrons and holes breakdown through the PN junction and cross, resulting in the current flow through the circuit. This breakdown is called an avalanche breakdown. In this process, the current flowing 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 the current through the diode and applied voltage. It is represented as a graph. The X-axis represents a voltage and Y-axis represents the current through the diode.
The right side of this graph is forward voltage region and a diode is in forward biased. The left side is the reverse voltage region and the diode is in reverse biased.
A diode has a non-linear characteristic.
When voltage is positive, current exponentially increases until it reaches value Vd, which depends on the built-in electric field of the 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 the circuit.
When voltage is a negative, a 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 a few microamperes and is practically negligible. But after a reverse breakdown, a current is increased.
So, working of a diode is divided into three regions;
1) Forward region
2) Reverse region
3) Breakdown region