Biasing of a diode

Basically a diode can be biased in two ways-forward biased and reverse biased mode.

• FORWARD BIASED p-n JUNCTION DIODE:-

 The above image shows the connection type for forward biased diode.Remember that in forward biasing the positive terminal of the battery is connected to p-type and negetive terminal of the battery to n-type.The holes in the p-side are repelled by the positive terminal of the battery while the electrons in the n-side are repelled by the negetive terminal of the battery causing some electrons and holes to enter the depletion region(To know more about depletion region go to-http://dutta60.blogspot.in/2014/07/p-n-junction-diode.html ).Thus the width of the potential barrier as well as the width of the depletion region is reduced as a result of which large number of majority charge carriers can diffuse through the junction.Holes travelling to the right and electrons travelling to the left constitute current in the same direction .So we can say that the resultant current is the sum of currents produced due to holes and electrons.After this an interesting thing happens that is after crossing the junction the holes become minority carriers in the n-region resulting in an injected minority current,and the same is valid for electrons also.Diffusion current can be of substantial amount as the amount of  carriers is large,on the other hand the generation of  carriers also depends upon the degree of biasing.Now for each electron and hole recombination,an electron from the negetive terminal of the battery enters the n-region and after that it drifts towards the juncion.Also in the p-region an electron breaks bond in the crystal as a result of which a hole is created and this hole drifts towards the junction but the electron enters the positive terminal of the battery.So we can conclude that current in the external circuit is due to the movement of electrons,current in p-region is due to holes,and current in n-region is due to electrons.

• REVERSE BIASED p-n JUNCTION DIODE:-

  The connection here is exactly opposite of what the connection is done for forward-biased mode.The negetive terminal of the battery is connected to the p-side and the positive terminal to the n-side.The holes of the p-side are attracted towards negetive terminal while the electrons in the n-side are attracted towards the positive terminal.As a consequence of which the depletion width is increased due to the increase in uncompensated ions.Now it becomes more difficult for majority carriers to cross over as the resistance offered by the depletion region is enormous .But the movement of the minority carriers is helped by the potential barrier.The rate of generation of minority carriers at a particular temperature is constant as a result of which the current remains the same whether or not voltage is increased or decreased .This current is called the reverse saturation current which is very small.

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