When forward biased, BJT can be turned on by injecting a positive current into the base. The base current must be maintained for as long as the switch is to be on.
The switch can be turned off by withdrawing a current from the base. The base current must be maintained for as long as the switch is to be off.
BJT is a Current Controlled Switch. This implies that a considerable amount of power is required from the drive circuit to turn the device on and off.
Fig. 7-7 shows the symbol for an NPN BJT and the - characteristic of a BJT.
BJTs are not capable of withstanding reverse voltages. BJTs have the advantage of low onstate voltage drop.