pn Diode Currents

p‒n Diode Currents

• It is indicated earlier that when a p‒n junction diode is forward biased a large forward current flows, which is mainly due to majority carriers. The depletion region near the junction is very very small, under forward biased condition.

• In forward biased condition holes get diffused into n‒side from p‒side while electrons get diffused into p‒side from n side.

• So on p‒side, the current carried by electrons which is diffusion current due to minority carriers, decreases exponentially with respect to distance measured from the junction. This current due to electrons, on p‒side which are minority carriers is denoted as I_np.

• Similarly holes from p‒side diffuse into n‒side carry current which decreases exponentially with respect to distance measured from the junction. This current due to holes on n‒side, which are minority carriers is denoted as I_pn.

• If distance is denoted by x then,

I_np (x) = Current due to electrons in p‒side as a function of x

I_pn (x) = Current due to holes in n‒side as a function of x

• At the junction i.e. at x = 0, electrons crossing from n‒side to p‒side constitute a current, I_ap (0) in the same direction as holes crossing the junction from p‒side to n‒side constitute a current, I_pn(0).

• Hence the current at the junction is the total conventional current I flowing through the circuit.

 I = I_pn(0) + I_np (0)           .. (2.8.1)

• Now I_pn (x) decreases on n‒side as we move away from junction on n‒side. Similarly I_np (x) decreases on p‒side as we move away from junction on p‒side.

• But as the entire circuit is a series circuit, the total current must be maintained at I, independent of x.

• This indicates that on p‒side there exists one more current component which is due to holes on p‒side which are the majority carriers. It is denoted by I_pp(x) and the addition of the two currents on p‒side is total current I.

 I_pp (x) = Current due to holes in p‒side.

• Similarly on n‒side, there exists one more current component which is due to electrons on n‒side, which are the majority carriers. It is denoted as I_nn(x) and the addition of the two currents on n‒side is total current I.

  I_nn(x) = Current due to electrons in n‒side.

 On p‒side, I = I_pp(x) + I_np(x)                   …………(2.8.2)

On n‒side, I = I_nn(x) + I_pn(x)                     ……… (2.8.3)

• These current components are plotted as a function of distance in Fig. 2.8.1.

• The current I_pp decreases towards the junction, at the junction enters the n side and becomes I_pn which further decreases exponentially.

• Similarly the current I_nn decreases towards the junction, at the junction enters the p‒side and becomes I_np which also further decreases exponentially.

Key Point: In forward bias condition, the current enters the p‒side as a hole current and leaves the n‒side as an electron current, of the same magnitude.

• So sum of the currents carried by electrons and holes at any point inside the diode is always constant equal to total forward current I. But the proportion due to holes and electrons in constituting the current varies with the distance, from the junction.

Review Question

1. Explain the current components in a p‒n junction diode.