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The depletion region decreases and conduction of majority carriers starts.
Forward bias refers to the application of voltage across the device such that the electric field at the junction is reduced. By applying a positive voltage to the p-type material and a negative voltage to the n-type material, an electric field with opposite direction to that in the depletion region is applied across the device. Since the resistivity of the depletion region is much higher than that in the remainder of the device (due to the limited number of carriers in the depletion region), nearly all of the applied electric field is dropped across the depletion region. The net electric field is the difference between the existing field in the depletion region and the applied field (for realistic devices, the built-in field is always larger than the applied field), thus reducing the net electric field in the depletion region. Reducing the electric field disturbs the equilibrium existing at the junction, reducing the barrier to the diffusion of carriers from one side of the junction to the other and increasing the diffusion current. While the diffusion current increases, the drift current remains essentially unchanged since it depends on the number of carriers generated within a diffusion length of the depletion region or in the depletion region itself. Since the depletion region is only reduced in width by a minor amount, the number of minority carriers swept across the junction is essentially unchanged.
In forward biasing, the Battery terminal's -ve side is connected to the negative side of the PN junction (n-side). and the +ve side of the battery terminal is connected to the positive side of the PN junction (P side).
Since positive and positive repel each other & negative and negative also repel each other (Opposites attract & alike poles reject each other) the depletion region will reduce in size to enable electron hole flow.
The depletion region totally decreses and conduction starts.
