A forward bias PN junction will act as a/an Closed Switch.
A PN junction consists of a p-type semiconductor region and an n-type semiconductor region, which are joined together to form a diode.
When a positive voltage is applied to the p-type side and a negative voltage is applied to the n-type side, the diode is said to be forward-biased.
In this condition, the potential barrier between the p-type and n-type regions is reduced, and the majority carriers (holes in the p-type region and electrons in the n-type region) are able to flow across the junction and combine, allowing current to flow through the diode.
Since a forward-biased PN junction allows current to flow through it, it acts as a closed switch.
When the PN junction is reverse-biased, the potential barrier is increased, and current cannot flow through the diode. In this condition, the diode behaves as an open switch.