The BASE - The lead responsible for activation of the transistor.
The COLLECTOR - The lead which is the positive part of transistor.
The EMITTER - The lead which is the negative part of transistor.
Figure 1 shows the symbol of an NPN transistor.
Figure 1 shows an NPN transistor which can be used as a type of switch. A small current or voltage at the base will yields for a larger voltage to flow through the other two leads ie,from the collector to the emitter.
Figure 2 is based on an NPN transistor. When the switch is in ON state, a current passes through the resistor to the base of the transistor. The transistor then permits the current to flow from the +9 volts to the 0vs, and the lamp glows.
The transistor needs to get a voltage at its ‘base’ portion and until this happens the lamp does not glows.
As transistors can be damaged easily by a high voltage / current, the resistor is used to protect the transistor. Transistors are the essential component in electronic circuits. Transistors can sometimes used to amplify a signal. For oscillators also transistor plays the key role.