A transistor is a semiconductor device used to amplify or switch electronic signals. Transistors are broadly divided into three types: bipolar transistors (bipolar junction transistors: BJTs), field-effect transistors (FETs), and insulated-gate bipolar transistors (IGBTs).
A bipolar transistor is a type of transistor that uses both electrons and holes as charge carriers. Two types of bipolar transistor are manufactured: npn and pnp.
A field-effect transistor is a unipolar device constructed with no pn junction in the main current-carrying path. Also, two types of field-effect transistor are manufactured: N-channel and P-channel.
An IGBT consists of a voltage-driven MOSFET followed by a high-current transistor.
Typical schematic symbols of these transistors are shown below: