Saturday, September 4, 2010

p-n diode

A P-N diode
The most frequently encountered  semiconductor electronic device  is the diode. It is a two terminal device consisting of a P-N junction formed either in germanium or silicon crystal. P-N junction is also sometimes called as crystal diode as the P-N junction is directly grown out of a crystal . A diode is a unidirectional device  it permits a easy flow to the current passing from one side only and offers a high resistance when the direction of the current is opposite.
  It is because , on the formation of the p-n junction some of the holes from the P-type material tend to diffuse across the boundary into N-type material and some of the free electrons similarly diffuse into the P-type material as shown  where blue circles are the holes and green circles are the free electron

this happens because of the concentration of holes is higher on P-type than on N-type and theconcentration of free electrons is higher on N-type than at P type . this process is called diffusion , the region across the junction is completely immobilized which leads to the formation of a depletion layer , transition layer or potential barrier.
The thickness of the depletion region is of the order of 1 micron (millionth of a meter ,10^(-6) meters )  .

  • Forward biasing- When a positive voltage is applied across the P-type semiconductor and negative terminal connected to the N-type semiconductor , then the diode is said to be in forward bias . the holes of P-type being positive charge repel away from the supplied positive charge and move towards the junction and similarly the free electrons from N-type also are driven towards the junction , In other words the forward potential establishes an electric field opposite to the potential barrier and hence the width of the depletion layer is reduced , and current could pass through it(diode) easily.

  • Reverse biasing - When the negative terminal is connected to the P-type semiconductor and the positive terminal to the N-type , the diode is in reverse bias . Because the applied reverse voltage establishes a electric field which acts in the same direction as that of potential barrier , thus making the depletion layer wider and offering high resistance to the current flowing through it .
Diodes are mainly used in rectifiers(AC to DC converters), clippers and as a safety device (to block the flow of current in reverse order),