When and why is a $p-n$ junction called forward biased? Describe the changes that occur under such bias.

(N/A) $p-n$ junction is said to be forward biased when the positive terminal of an external battery is connected to the $p$-side and the negative terminal is connected to the $n$-side of the junction,as shown in figure $(a)$.
The circuit symbol representation is shown in figure $(b)$.
When forward bias is applied,the external voltage opposes the built-in potential barrier $(V_0)$.
As a result,the effective barrier height is reduced to $(V_0 - V)$,and the width of the depletion region decreases.
Because the depletion region has very few charge carriers,its resistance is high,causing most of the applied voltage to drop across it,while the voltage drop across the bulk $p$ and $n$ regions is negligible.
This reduction in barrier height allows majority charge carriers to cross the junction more easily,leading to a significant increase in current flow,as illustrated by the potential energy diagram in figure $(c)$.