Consider a P type material and N type material are brought together to form a PN Junction Diode.
Free electrons on the N-side and free holes on the P-side can initially wander across the junction. When a free electron meets a free hole it can 'drop into it'. So far as charge movements are concerned this means the hole and electron cancel each other and vanish.
Here the connection is Forward Bias. When the Switch is closed, the charge carriers start crossing the Depletion Zone.