When an external voltage is applied across the diode opposite the inbuilt barrier potential VB , it is said to be forward biased. That is forward bias means p–side of the diode at higher potential with respect to the n–side of the diode as shown in Fig.(a). To apply forward bias we connect p– side to the positive terminal of the battery and n–side to the negative terminal. As a result, the depletion layer width decreases and the barrier height is reduced as shown in the figure.(b). The effective barrier width under forward bias is (V0 –V).
If the applied voltage is small, the barrier potential will be reduced only slightly below the equilibrium (VB) value, so the current will be small. If we increase the applied voltage significantally, the barrier is ultimately eliminated and current increases exponentially. It is due to this reason that junction resistance is very low for the forward bias. For an ideal diode it should be zero forward bias resistance.
Hence in forward based junction, the applied electric field E dominates the barrier field EB . As a result, the majority charge carriers (holes in p–side and electrons in n–side) are pulled towards the junction. This motion of charge carriers on either side give rise to the current. The total forward current is sum of hole diffusion current and current due to free electron diffusion. The magnitude of this current is usually in mA.
