Semiconductor diode basics

Semiconductor :

A pn junction is known as a semi-conductor or crystal diode.The outstanding property of a crystal diode to conduct in on direction only permits it to be used as a rectifier.

A semiconductor is a substance which has resistivity in between conductor and insulators

A crystal or semiconductor diode represented by the schematic symbol shown in fig

(i) If arrow head of diode symbol is positive with respect to bar of the symbol,the diode is forward biased.

(ii) If the arrow head of the diode symbol is negative with respect to bar the diode is reverse  biased.

A crystal diode has two terminals.When it is connected in a circuit,one thing to decide is whether the diode is forward or reverse biased.If the external circuit is trying to push conventional current in the direction of arrow,the diode is forward biased.On the other hand,if the conventional current is trying to flow opposite to arrow head.

Properties of semiconductor:

(i)The resistivity of semi conductor is less than an insulator but more than a conductor.

(ii)Semiconductor have negative temperature coefficients of resistance which means the resistance of a semiconductor decreases with increase in temperature .

(iii)When suitable metallic impurity is added to a semiconductor,its current conducting properties changes appreciably.

Commonly used Semiconductors :

There are many semiconductor available but few of them have a practical application in electronics.The most frequently used materials are Germanium(Ge) and silicon(Si).
Crystal diode Rectifiers :
The supply voltage varies sinusoidal and has a frequency of 50 Hz.It is used for heating and electric motors.When such D.C supply is required the mains ac supply is rectified by using crystal diodes.The following two rectifiers circuits can be used ,
(i)Half wave rectifier 
(ii)Full wave rectifier