Transistors can be operated in the switching mode. If base current IB is zero transistor is in an ON state behaves as a switch. If the base current IB is sufficient to drive the transistor into saturation, them the transistor behaves as closed switch. The transistor is a current driven device. The base current determines whether it is in ON state or OFF state. To keep the device in ON state there should be a sufficient base current.
NPN transistor circuit diagram input characteristics.