1. Semiconductors Summary – The PN Junction Theory
A P-N junction diode is a two electrode device that is formed by placing a layer of P-type
semiconductor material against a layer of N-type semiconductor material in order to form a junction.
The p side is known as the anode or the positive terminal while the n side is known as the cathode or
the negative terminal.
When a p-n junction is formed, the density gradient causes the electrons to diffuse from the n-side
to the p-side and the holes to diffuse from the p-side to the n-side. This causes the formation of ions
near the junction which creates a barrier potential that inhibits any further diffusion of the carriers
when a state of equilibrium is reached. The direction of this barrier potential is from N-side to the P-
side. The region near the junction becomes completely devoid of charge carriers and this region is
therefore termed as depletion or space charge region.
The importance of a P-N junction diode lies in how it behaves under different biasing conditions.
When forward biased i.e. the p-side of the diode is connected to the positive terminal of the battery
and the n-side is connected to the negative terminal, the diode behaves almost like an ideal
conductor. Similarly when the polarities on the battery are reversed i.e. in condition of reverse bias,
the diode behaves close to a perfect insulator with very little reverse leakage current.
During forward bias conditions, once the small bias potential in the diode is overcome, the current
increases very rapidly in the diode with any further increment in voltage and is only controlled by
the outer circuit elements. The diode is thus acting very close to an ideal conductor. Now in reverse
bias conditions, the majority carriers on both sides of the junction get depleted because of the
action of the battery. This causes the diode to behave very close to a perfect insulator with only a
small reverse leakage current flowing because of the flow of the minority carrier.
When using a P-N junction diode in a circuit, the most important parameters that should be
considered are:-
The forward knee voltage that depends on the barrier potential of the diode Reverse Breakdown
voltage or Peak Inverse Voltage (PIV) which is the reverse bias voltage at which the junction breaks
down and large current flows in the circuit. This can cause permanent damage to the diode.
Maximum temperature the diode can withstand without damage.
The range of applications in which these diodes are used today is too vast to imagine. Ranging from
rectifiers to LED TV to voltage regulators, these diodes are used almost everywhere in electronics.