When the p-n junction diode is reverse biased the width of the depletion region increases and the charge carriers are blocked by the depletion layer. The resistance of the depletion layer is large because of more width of depletion layer. Under reverse biasing, the diode offers very large resistance to the electric current. This resistance is called the reverse resistance.
In reverse biased, only small amount of electric current due to minority carrier flows through the diode. Thus, the resistance of the diode must be infinite in reverse biased but, practically it does not have infinite resistance because of the current flowing through the depletion layer due to minority charge carriers.
There are two types of resistance of p-n junction diode.
Forward Resistance of Diode
The resistance of the forward biased diode is called forward diode resistance. The forward resistance can be further grouped into two category.
Static Resistance or DC Forward Resistance of Diode
Solved Example on Forward Resistance of diode
Calculate the DC resistance of the diode of the following V-I curve.
a) ID = 2 mA
b) ID = 20 mA
c) VD = -10 Volts
The forward resistance of the diode decrease with increase in the forward biasing voltage.
Dynamic Resistance or AC Forward Resistance of Diode
The resistance offered by the diode when AC is applied to the diode is called AC resistance or dynamic resistance. The current flows in both the direction when AC voltage is applied.
The V-I curve of p-n junction diode as as given below.
Formula of AC resistance or Dynamic Resistance of Diode
Reverse Resistance of Diode
When reverse bias is applied to p-n junction diode, the width of the depletion layer increase and it offers higher resistance to the flow of charge carriers.
The reverse resistance of p-n diode is in order of mega ohms. The reverse resistance is very large compared to the forward resistance of the diode.
The static reverse resistance of the diode can be calculated by following mathematical expression.
Formula of Reverse DC Resistance of Diode
The dynamic reverse resistance of the diode is as follows.