Biasing of pn junction

Biasing of pn junction

A p-n junction can be connected to an external battery by two ways. 

The method of connecting the p-n junction to an external battery is known as biasing. 

There are two types of biasing, forward basing and reverses biasing. 

Forward Bias

If p terminal of a p-n junction is connected to positive terminal of an external battery, and n terminal of it is connected to negative terminal of battery then such type of biasing is known as forward bias. It is clear from the figure given below.

In forward bias the current flowing in the circuit is due to majority charge carriers and therefore the resistance is very small and hence the current flowing in the circuit is of the order of milli ampere.

In forward biasing as we increase the potential across the junction, the value of current in the circuit increases very small initially, but after a certain value of potential the depletion region breaks due to high potential across the junction and the value of current increases suddenly.

The value of positive potential or the value of potential in forward bias after which the current increases suddenly is known as knee voltage.

The value of resistance in forward bias is of the order of kilo ohm.

The graphical relation between the forward bias voltage applied to p-n junction diode and the forward current passing through the p-n junction is known as forward characteristic.

The forward characteristic curve of p-n junction along with its circuit diagram is shown in the figure given below

In the central figure the red circles show the electrons and the white circle show the holes. So it is clear from figure that the holes are majority charge carriers in p region, whereas the electrons are majority charge carriers in n region. It means the electrons and holes are minority charge carriers in p and n regions respectively.

Reverse Bias

If p terminal of a p-n junction is connected to negative terminal of an external battery, and n terminal of it is connected to positive terminal of battery then such type of biasing is known as reverse bias. It is clear from the figure given below.

In reverse bias the current flowing in the circuit is due to minority charge carriers and therefore the resistance is very high and hence the current flowing in the circuit is of the order of micro ampere.

In reverse biasing as we increase the potential across the junction, the value of current in the circuit remain almost constant or a very small amount of current increase initially. Since this current is of the order of micro ampere, so we can say there is no current flowing in the circuit initially.

But after a certain value of negative potential, which we applied across the junction, all the covalent bond breaks and the value of current increases abruptly with a very small change of negative potential.

The value of negative potential across the p-n junction or the value of potential applied across the p-n junction in reverse bias after which the value of current increase abruptly is known is breakdown voltage.

The value of resistance in reverse bias is of the order of mega ohm.

The graphical relation between the reverse bias voltage applied to p-n junction diode and the reverse current passing through the p-n junction is known as reverse characteristic.

The reverse characteristic curve of p-n junction along with its circuit diagram is shown in the figure given below

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