When the Collector Current in a Transistor Is Zero the Transistor?

The switched (controlled) current goes between the collector and emitter. The controlling current goes between the base and emitter. When a transistor has zero current through it, it is said to be in a state of cutoff (fully nonconducting).

What happens when the collector current is zero?

If there is no base current, then the collector current would be zero as well. Under this condition, there is no current in any of the junctions, and therefore all junction voltages are zero.

What happens when the base current of a transistor is zero?

If the base current is zero, the collector current is zero. When transistor is in cutoff region both the junctions are reverse biased and transistor becomes an open circuit.