All About Diodes part-5

Principle and Operation: 

The possible configurations for a diode are:  

1.      Open circuited

2.      Short circuited

3.      Forward biased

4.      Reverse biased

1.      Open circuited: In open circuited condition, the current that flows through the diode is zero (I = 0). The potential barrier at the PN junction remains the same as  created in the diode fabrication.

2.      Short circuited: In short circuited condition, the sum voltage in the loop must be zero. So it is assumed, that the potential barrier at the PN junction is compensated by the potential drops at the metal semiconductor junctions. The holes supplied by the n-region must be driven to the p region which is physically impossible. The similar discussion applies to the electron current in the n-region. 

Conclusion: The potential barrier height cannot be measured directly by a multimeter.

3.      Forward bias: In forward bias condition, higher or positive potential is applied at the anode and negative or lower potential is applied at the cathode of a diode. The positive potential at anode repels the holes in p-region towards n-region while negative potential at the cathode repels electrons in n-region towards p-region. Thus, the height of the potential barrier reduces. The depletion region disappears when the applied voltage equals to the potential barrier and a large current flows through the diode. The voltage required to drive the diode into a state of conduction is called as the ‘Cut in/Offset/Threshold/Firing voltage’. The current is of considerable magnitude as it is dominantly constituted by the majority charge currents that is the hole current in the p-region and the electron current in the n-region. The current that flows from anode to cathode is limited by the crystal bulk resistance, recombination of charges and the ohmic contact resistances at the two metal semiconductor junctions.  The current is restricted to mille Amperes order.   

4.      Reverse Bias: In reverse bias condition,  the higher or positive potential is applied at the cathode and negative or lower potential is applied at the anode. The negative potential at anode attracts the holes in p-region that are away from the n-region while positive potential at the cathode attracts electrons in n-region that are away from the p-region. The applied voltage increases the height of the potential barrier. The current flows dominantly due to the minority charge currents that is the electron current in p-region and the hole current in n-region. Thus a constant current of negligible magnitude flows in the reverse direction which is called as the ‘Reverse saturation current’. Practically, the diode remains in a state of non – conduction. The reverse saturation current is of the order of microamperes in a germanium diode or nanoamperes in a silicon diode  If the reverse voltage exceeds the limit  of ‘breakdown/zener/Peak inverse/Peak reverse voltage’, the potential breakdown that occurs leads to a large reverse current.

 source: engineersgarage.com