It is now over 70 years since the first transistor was introduced on December 23 in 1947.The first electronics device to be introduced is called the diode. It is the simplest of semiconductor devices.but it's play a very lively role in electronics systems ,having characteristics that closely match those of a simple switch.it will appear in a range of applications that closely match those of a simple switch. It is an electronics device. that is to allow an electric current to flow in one direction. Because diode works in the forward direction (That is called forward bias). In reverse bias direction there is no electric current flow (That is called reversed bias) .Thus in the diode current flows from +ve to -ve.

IDEAL DIODE
The term ideal will be used frequently in this text as new devices are introduced.In a simple way we can say that the ideal diode is an open circuit The Ideal diode is a two terminal device having the symbol fig (a) and characteristics fig(b)


The characteristics of an ideal diode are those of a switch that can conduct current in only one direction.The ideal diode,therefore,is an open circuit in the region of non conduction

PRACTICAL DIODE
In case of practical diode there is a voltage drop of about 0.7(For silicon material) and there is a voltage drop of about 0.3 (For germanium). The voltage drop in circuit analysis. when it is forward bias. The characteristics of practical diode is shown below.