Diodes

Written By: 
Nikhil Agnihotri

 

What is a Diode?
A Diode is the simplest two-terminal unilateral semiconductor device.  It allows current to flow only in one direction and blocks the current that flows in the opposite direction. The two terminals of the diode are called as anode and cathode. The symbol of diode is as shown in the figure below. 
 
Symbol of a Diode

 

The characteristics of a diode closely match to that of a switch. An ideal switch when open does not conduct current in either directions and in closed state conducts in both directions. The characteristic of a diode is as shown in the figure below.                                               
Characteristics of a Diode

 

 

Ideally, in one direction that is indicated by the arrow head diode must behave short circuited and in other one that opposite to that of the direction of arrow head must be open circuited. By ideal characteristics, the diodes is designed to meet these features theoretically but are not achieved practically. So the practical diode characteristics are only close to that of the desired.    
Diode characterstics

 

How Diodes work?
The diode operates when a voltage signal is applied across its terminals. The application of a DC voltage to make the diode operate in a circuit is called as ‘Biasing’. As already mentioned above the diode resembles to that of a one way switch so it can either be in a state of conduction or in a state of non conduction. The ‘ON’ state of a diode is achieved by ‘Forward biasing’ which means that positive or higher potential is applied to the anode and negative or lower potential is applied at the cathode of the diode. In other words, the ‘ON’ state of diode has the applied current in the same direction of the arrow head. The ‘OFF’ state of a diode is achieved by ‘Reverse biasing’ which means that positive or higher potential is applied to the cathode and negative or lower potential is applied at the anode of the diode. In other words, the ‘OFF’ state of diode has the applied current in the opposite direction of the arrow head.
 
During ‘ON’ state, the practical diode offers a resistance called as the ‘Forward resistance’.  The diode requires a forward bias voltage to switch to the ‘ON’ condition which is called Cut-in-voltage. The diode starts conducting in reverse biased mode when the reverse bias voltage exceeds its limit which is called as the Breakdown voltage. The diode remains in ‘OFF’ state when no voltage is applied across it.   

 

A simple p-n juction diode is fabricated by doping p and n type layers on a silicon or germanium wafer. The germanium and silicon materials are prefered for diode fabrication because:
·         They are available in high purity.  
·         Slight doping like one atom per ten million atoms of a desired impurity can change the conductivity to a considerable level.
·          The properties of these materials change on applying heat and light and hence it is important in the devlopment of heat and light sensetive devices.
 
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