Fig.1.1(a)
The applied voltage mostly drops across the depletion region and the voltage drop across the p-side and n-side of the junction is negligible.(This is because the resisitance of the depletion region-a region where there are no charges-is very high compared to the resistance of n-side and p-side). The direction of the applied voltage[V] is opposite to the built-in potential V0.As a result,the depletion layer width decreases and the barrier height is reduced [Fig. 1.1(b)].The effective barrier height under forward bias is [V0-V].
Fig.1.1(b)
If the applied voltage is small, the barrier potential will be reduced only slightly below the equilibrium value, and only a small number of carriers in the material-those that happen to be in the uppermost energy levels-will possess enough energy to cross the junction.So the current will be small.If we increase the applied voltage significantly, the barrier height will be reduced and more number of carriers will have the required energy.Thus the current increases.
Due to the applied voltage, electrons from n-side cross the depletion region and reach p-side(Where they are minority carries).Similarly, holes from p-side cross the junction and reach the n-side(Where they are minority carries).This process under forward bias is known as minority carrier injection.At the junction boundary, on each side, the minority carrier concentration increases signifcantly compared to the locations far from the junction.
Due to this concentration gradient, the injected electrons on p-side diffuse from the junction edge of p-side to the other end of p-side.Likewise, the injected holes on n-side diffuse from the junction edge of n-side to the other end of n-side[Fig. 4].This motion of charged carriers on either side gives rise to current.The total diode forward current is sum of hole diffusion current and conventional current due to electron diffusion.The magnitude of this current is usually in mA.
Fig. 4
