What is transistor ? What are the different types ?
A transistor is a three lagged semi conductor device. Basically a transistor means (transfer - resister).
Whether the transistor is pnp or npn it resembles two diodes (back to back). The one of the left is called emitter diodes, and the one on the right is the collector diode. Since two types of charges are involved transistor : are classified as bipolar devices.
Biasing the transistor :
Emitter - Base - Forward Bias.
Collector - Base - Reverse Bias.