Types of Transistors and How They Work

Transistor is a semiconductor device invented by John Bardeen, Walter Brattain and William Shockley in 1947 and they received the Nobel Prize in Physics for this invention in 1956. It is a device used to amplify and switch electrical signals. Large number of transistors can be included in a single chip known as an integrated circuit, which are the building blocks of modern computers and many other solid state devices. Most of the transistors are currently made from silicon while germanium was used earlier. Field effect transistors have only one kind of charge carrier while bipolar junction transistors may have two kinds of charge carriers. In comparison to vacuum tubes, transistors are much smaller and require less power, often working at much lower voltages.

Currently two basic types of transistors in use are the field-effect transistor (FET) and bipolar junction transistor (BJT). Terminals of the FET are named as gate, source and drain. Voltage given to the gate can control the current between source and drain in case of FET. Terminals of BJT are named base, collector and emitter. Current at the base terminal, which flows between the base and emitter, can control or switch a much larger current between the collector and emitter. That is how BJTs are used for amplification and switching.

BJTs are generally grouped into two – NPN and PNP, depending on the types of semiconductors in them. N-type semiconductor has excess of electrons while P-type semiconductor has a deficit of electrons or holes, which are considered as positive charge carriers and hence the name. These two types are made by mixing small amounts of other materials into the silicon chip, by a process known as doping. There is difference in the amount of doping between various layers of NPN and PNP transistors. Emitter is heavily doped compared to other two layers. Collector is typically doped ten times lighter than the base.

FET is also sometimes called a unipolar transistor as they have only a single type of charge carriers. They can be N-channel or P-channel, depending on the type of semiconductor used. They are named as field effect transistors as they use an electric field to control the current. Two types of FETs are the junction FET or JFET and metal oxide semiconductor FET or MOSFET. Voltage applied to the gate terminal controls the conductivity between the drain and source. Though Julius Edgar Lilienfeld, proposed the concept of a field-effect transistor in 1925, a working prototype could not be made then and had to wait for further development of semiconductor technology.