The oldest and simplest transistors are bipolar junction transistors (BJT), which comprise slices of N-type and P-type semiconductors. If the slices are arranged N-P-N, it is called an NPN transistor. If the slices are arranged P-N-P, it is called a PNP transistor.
A current can flow all the way across either type of transistor, but not easily. If a current is applied to the middle slice, it can have a large effect on the trans-transistor current. Due to this effect, transistors can act as high-speed switches in computers or as amplifiers in radios and phones.
The circuit symbols for these transistors consists of three lines coming together. One of the lines ends in a short bar. Both of the other lines terminate on the other side of the short bar (one of them has an arrowhead). Often, the juncture of the three lines is drawn inside a circle. The distinction between NPN and PNP is marked by the arrowhead; it points in for PNP, and out for NPN.
Field effect transistors (FETs) work like BJTs, except that the fields the components produce have an effect on the flow of holes and electrons. FETs are thin slabs of one semiconductor type inlaid with dots of other semiconductor types. The difference in size between the semiconductors creates the field effects.
The circuit symbol for an FET is a circle or oval where several lines converge. One of these lines (the gate) ends in a short bar. All the other converging lines terminate on the other side of the short bar. The gate is the inlaid slice of semiconducting material.
Some of the newer transistors either give off light or are affected by light; these are especially interesting to scientists who want to build computers that work on light instead of electricity. Insulated gate bipolar transistors (IGBTs) are made out of four semiconductor layers. They are cheap, fast and easy to manufacture, and can handle high voltages. IGBTs are a key component in electric vehicles.
The circuit symbol for an IGBT is a gate that consists in a short right-angled turn. None of the other components actually touch the gate. One of the most fantastic features of the new transistor is the single electron transistor. This tiny transistor is a switch that can be activated by a single electron. According to MIT physicist M. A. Kastner, these devices portend a future of even smaller electronic devices.