BC550 is a NPN Transistor, hence we need to provide a small amount of positive voltage to the base of the transistor to turn it on.
Package Type: TO-92
Transistor Type: NPN
Max Collector Current(IC): 100mA
Max Collector-Emitter Voltage (VCE): 45V
Max Collector-Base Voltage (VCB): 50V
Max Emitter-Base Voltage (VEBO): 5V
Max Collector Dissipation (Pc): 500 miliWatt
Max Transition Frequency (fT): 150 MHz
Minimum & Maximum DC Current Gain (hFE): 110 – 800
Max Storage & Operating temperature Should Be: -65 to +150 Centigrade