Working of a PN junction diode:
if an external potential is applied to the terminals of PN junction, it will alter the potential between the P and N-regions. This potential difference can alter the flow of majority carriers, so that the PN junction can be used as an opportunity for the diffusion of electrons and holes. If the voltage applied decreases the width of the depletion layer, then the diode is assumed to be in forward bias and if the applied voltage increases the depletion layer width then the diode is assumed to be in reverse bias. If the width of depletion layer do not alters then it is in the zero bias state.
Forward Bias: External voltage decreases the built-in potential barrier.
Reverse Bias: External voltage increases the built-in potential barrier.
Zero Bias: No external voltage is applied.
Forward Biased Diode
With the externally applied voltage, a potential difference is altered between the P and N regions.
When positive terminal of the source is connected to the P side and the negative terminal is connected to N side then the junction diode is said to be connected in forward bias condition. Forward bias lowers the potential across the PN junction.
Reverse Biased Diode
When positive terminal of the source is connected to the N side and the negative terminal is connected to P side, then the junction diode is said to be connected in reverse bias condition.
In this type of connection majority charge carriers are attracted away from the depletion layer by their respective battery terminals connected to PN junction
V-I Characteristics of PN Junction Diode
In the current–voltage characteristics of junction diode, from the first quadrant in the figure current in the forward bias is incredibly low if the input voltage applied to the diode is lower than the threshold voltage (Vr). The threshold voltage is additionally referred to as cut-in voltage.
Once the forward bias input voltage surpasses the cut-in voltage (0.3 V for germanium diode, 0.6-0.7 V for silicon diode), the current spectacularly increases, as a result the diode functions as short-circuit.
The reverse bias characteristic curve of diode is shown in the fourth quadrant of the figure above. The current in the reverse bias is low till breakdown is reached and therefore the diode looks like as open circuit. When the reverse bias input voltage has reached the breakdown voltage, reverse current increases spectacularly.