A transistor is a type of a semiconductor device that can be used to both conduct and insulate electric current or voltage. A transistor basically acts as a switch and an amplifier.

A typical transistor is composed of three layers of semiconductor materials or more specifically terminals which helps to make a connection to an external circuit and carry the current. A voltage or current that is applied to anyone pair of the terminals of a transistor controls the current through the other pair of terminals. There are three terminals for a transistor. They are;

Base: This is used to activate the transistor.
Collector: It is the positive lead of the transistor.
Emitter: It is the negative lead of the transistor.

BC547 is a Bipolar Junction Transistor, abbreviated as BJT.
It is an NPN transistor.
BC547 has two working states which are:

In a forward biased state, the collector and emitter are connected allowing current to pass through. While in a reverse biased state, it works as an open switch and doesn’t allow the current to pass.

BC547 is normally used for:

Current Amplification.
Fast switching.

Breadboard Hardware Circuits

Transistor