pn - junction diode or semiconductor diode
 
 
INTRODUCTION
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A pn - junction diode is an electronic device formed from a p-type and an n-type substance semiconductor. A semiconductor diode has the property of one way conduction i.e. it allows electric current to flow in only one direction.
FABRICATION OF pn-JUNCTION
 
A pn-junction is fabricated by placing a small amount of indium on a plate or wafer of n -type germanium. Indium on heating at 550oC melts and diffuses through a small part of the n-type germanium. Indium being a p-type impurity, converts the part of the n-type germanium to p-type material. Thus a junction is formed between p-type section and an n-type section of germanium.
A brass-base is used to fix the pn-junction to which leads are attached as shown:
The whole apparatus is sealed in a glass tube or a metallic tube.
WORKING OF pn-JUNCTION DIODE
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As we know that a p-type substance has excess of mobile positive charge or holes and n-type substance has an excess of negative charge or electrons, the electrons from n-type and holes from p-type sections flow across the junction and combine. In this way a layer of positive charges is formed on the n-type and a layer of negative charges on p-type material.
Due to induction of these layers a potential barrier is now developed across the junction and further flow of charges is prevented from one side to the other.
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