Analog Electronics

Charge distribution of diode

Normal (un-biased) state:

Apply the relation given below

                                n * p = ni2 at constant temperature    (Mass action law)

Now we apply the above relation to p-type:

p i.e. the concentration of majority carriers (holes) is larger as doping of p-side is high and we have the value of ni2as constant at fixed temperature. Hence from the above relation we find that number of minority carriers ( electrons) is less in p-type material while as doping of n-side is normal, hence number of majority carriers (i.e. electrons) in n-side is not large with the value of ni2as constant and hence number of minority carriers is larger as compared to that in p-side. We depict the above as below:

Npo is defined as the concentration of minority carriers in N-type material i.e. holes and Pno is defined as the concentration of minority carriers in P-type material i.e. electrons when diode is in un-biased state (i.e. diode is neither forward biased or reverse biased).

Forward Biased state:

When diode is forward biased, the majority carriers of both sides cross the junction and after reaching the other side, the charge carriers start combining. So holes from p-side start moving towards n-side and electrons from n-side start moving to p-side. When holes enter the n-side they become the minority carriers and just at the junction there would be high concentration of holes in n-side as the recombining has just started. Also all the holes can not recombine at the junction. Hence when we move away from the junction in the n-side, the concentration of holes is decreasing as more and more holes are recombining. This is also shown in the figure below. Similarly in the p-side, concentration of the electrons is high near the junction and it starts decreasing as we move away from the junction in the p-side.

The red curve shows the level of concentration of minority carriers at different distances on the both sides of junction and the shaded blue part shows the increase in the concentration of minority carriers after forward biasing the diode. There is a difference in the peak level of minority carriers as we have the difference in the doping level of both sides.

Reverse Biased state:

When we reverse biase any diode, the minority carriers from both sides cross the junction and then recombine after reaching the other side. Hence the holes from n-side move towards p-side and after reaching p-type material become majority carriers. These holes combine with minority carriers of p-side i.e. electrons. So the minority carriers at junction i.e. holes in the n-side which are near junction would immediately cross the junction on reverse biase and other holes move slowly. Similar to the above, electrons of p-side move to n-side. Hence the concentration of minority carriers falls on both sides as shown below:

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