A diode is said to be an Ideal Diode when it is forward biased and acts like a perfect conductor, with zero voltage across it. Similarly, when the diode is reversed biased, it acts as a perfect insulator with zero current through it. The V-I characteristics of the Ideal diode are shown in the figure below.
A Real diode contains barrier potential V0 (0.7 V for silicon and 0.3 V for Germanium) and a forward resistance RF of about 25 ohms. When a diode is forward biased and conducts a forward current IF flows through it which causes a voltage drop IFRF in the forward resistance. Hence, the forward voltage VF applied across the real diode for conduction, has to overcome the following.
- Potential barrier
- Drop in forward resistance
For Silicon diode, the equation becomes as shown below.
For Silicon diode the equation becomes
The V-I characteristic of the Real diode is shown below.
For all the practical purposes, a diode is considered to be an open switch when reversing biased. It is because the value of reverse resistance is so high (RR > 100 MΩ) that is considered to be an infinite for all practical purposes.
The equivalent circuit of the real diode under forwarding bias condition is shown below.