What is forward bias and reverse bias ?

Explanation

When the positive terminal of the battery is connected to p side of junction and negative terminal connected to N side of junction such a connection is known as forward as forward bias as shown in figure (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 resistance of the depletion region is very high due to no charges presents in this region.
The direction of the applied voltage (V) is opposite to the built-in potential `(V_(0))` as a result the depletion layer width decreases and the barrier height is reduced due to V > > `V_(0)`. This is shown in figure (c ).

The effective barrier height under forward bias is `V_(0)-V`.
If the external battery is not connected to the p-n junction then the effective depletion barrier.s height `V_(0)` is shown as 1 in figure (c ).
When the applied voltage is low, the height of effective barrier potential shows by 2 and the applied voltage is large the height of effective barrier potential shows by 3.
If we increase the applied voltage, the barrier height will be reduced and more number of carriers will have the required energy, so the current increases.
Due to the applied voltage, electrons from n-side cross the depletion region and reach p-side where they are minority carriers. Similarly, holes from p-side cross the junction and reach the n-side where they are minority carriers.
The process under forward bias is known as minority carrier injection.
At the junction boundary, on each side, the minority carrier concentration increases compared to the locations far from the junction.
Due to the concentration gradient, the injected electrons on p-side diffuse from the junction edge of p-side to the other end of p-side, similarly the injected holes on n-side diffuse from the junction edge of n-side to the other end of n-side.
This motion of charged carriers on either side give 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.
Forward bias junction has low resistance and external battery voltage is 1.5 V.