Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage.

The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics).

Then bipolar transistors have the ability to operate within three different regions:

• 1. Active Region - the transistor operates as an amplifier and Ic = β.Ib
• 2. Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
• 3. Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0

The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development.

There are two basic types of bipolar transistor construction, PNP and NPN, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made.