P-N Junction Diode

A P-N junction diode is a 2- terminal semiconductor device. The positive terminal refers as anode and the negative terminal refers as cathode. It acts as a one way switch i.e., allows flow of current in forward bias and restrict the flow of current in reverse bias condition. A diode offers very high resistance ideally (infinity) in reverse bias condition. Unlike a resistor, a diode does not behave linearly with respect to the applied voltage. Instead, it has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.

Construction:

A diode consists of P type and N type semiconductor materials. P-type material has significant number of holes i.e. positive charge carriers in majority whereas N-type semiconductor material has significant number of electrons i.e. negative charge carriers in majority.

When P-type and N-type semiconductor materials are brought in contact, few electrons diffuse from N- region to P-region and few holes diffuse from P region to N-region ( this movement of free electrons and holes occur near the junction because electrons and holes available near the junction only have enough energy to move across the junction). As majority of holes and electrons can not move across the junction due to not having enough energy required for movement across the junction, a depletion layer is created between the P and N region. The formation of depletion region results in an electric field inside the depletion region opposing the diffusion of carriers (opposes the flow of electrons from n-side to p-side and holes from p-side to n-side), known as the 'Potential Barrier'. Thus, an external supply is required for conduction.

Working

Forward biasing:

In forward bias, the positive terminal of the battery is connected to P-region of the diode and negative terminal of the battery is connected to N-region of the diode. When the battery voltage is applied, the positive of the battery pumps the holes towards the junction and negative of the battery pumps the electrons towards junction. This happens due to Coulomb's law of repulsion. Because of it the free electrons and holes near the junction try to cross the junction. This apply pressureon the depletion region and make it shrink. As the voltage of the battery is increased, the depletion layer will reduce and as battery voltage reaches the threshold voltage (cut off voltage of depletion layer), free electrons and holes cross the junction, depletion layer disappears and current start flowing i.e. the diode starts conducting. 

Reverse Biasing:

In reverse bias, the negative terminal of the battery is connected to P-region of the diode and positive terminal of the battery is connected to N-region of the diode. As the battery voltage is increased, the free electrons near the junction in N region and holes near the junction in P-region move away from the junction ( due to Coulomb's law of attraction). This will increase the depletion region. Because of it depletion layer widens. Further increase in supply voltage increase the depletion region. Thus, blocks movement of free electrons and holes across the junction I.e. blocks the flow of current.

Key Points:

In forward biased condition, depletion layer offers very low resistance.

InReverse Biased condition, a very high resistance is offered by the junction.

Knee voltage or cut-off voltage or threshold voltage is the voltage at which a diode starts conducting i.e., current starts flowing rapidly.

In reverse biased condition, a very small current flows due to minority charge carriers called reverse leakage current.

Breakdown voltage: A voltage at which reverse current starts flowing rapidly, leads to destruction of diode.

Cut off voltage of a diode is a voltage at which diode starts conducting significantly in forward bias I.e. current starts increasing rapidly.

Cut off voltage of a diode depends on the diode materials.

For Si, 0.7 volt

For Ge, 0.3 volt

Diffusion current flows because of the movement of holes abd free electrons from areas of high concentration where they are the majority carriers to areas of low concentration where they become minority carriers. This process occurs until they are uniformly distributed. This process known as Diffusion and the current flows due to this process is called diffusion current.

Drift current is the current flow in the diode due to the applied electric field. Diffusion current fliws due to the diffusion of charge carriers because of carrier concentration.

Diode i-v Characteristic:

Applications:

The major applications of diodes are in rectification and as a one way switch.

References: