Ideal Diode & Real Diodes & their Differences

Ideal Diode and Real Diode

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.

  

An Ideal diode also acts like a switch. When the diode is forward biased it acts like a closed switch as shown in the figure below.

Whereas, if the diode is reversed biased, it acts like an open switch as shown in the figure below.

 

Real Diode

A Real diode contains barrier potential V₀ (0.7 V for silicon and 0.3 V for Germanium) and a forward resistance R_F of about 25 ohms. When a diode is forward biased and conducts a forward current I_F flows through it which causes a voltage drop I_FR_F in the forward resistance. Hence, the forward voltage V_F applied across the real diode for conduction, has to overcome the following.

·         Potential barrier

·         Drop in forward resistance

 i.e.,

For Silicon diode, the equation becomes as shown below.

 

For Germanium 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 (R_R > 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.

This circuit shows that a real diode still acts as a switch when forward biased, but the voltage required to operate this switch is V_F, that is

 

Ideal diodes	Practical diodes
Ideal diodes act as perfect conductor and perfect insulator.	Practical diodes cannot act as perfect conductor and perfect insulator.
Ideal diode draws no current when reverse biased.	Practical diode draws very low current when reverse biased.
Ideal diode offers infinite resistance when reverse biased.	Practical diode offers very high resistance when reverse biased.
It cannot be manufactured.	It can be manufactured.
It has zero cut-in voltage.	It has very low cut-in voltage.
Ideal diode has zero voltage drops across its junction when forward biased.	It has very low voltage drop across it, when forward biased.