The word "Transistor" is a shortening of "Transconductance Resistor". This is a solid state device in which the resistance property of its output is controlled by an input. Principal uses of a transistor are in switching and amplification. The transistor is also the basic building block of integrated circuits.

A Transistor is made from an element with semiconductor properties. Such elements are in group 4 of the periodic table, and include germanium and silicon; such elements have a tetrahedral crystal structure. Interestingly enough, carbon is in the same group, but although the tetrahedral form of carbon may have semiconductor properties, this is actually diamond. It is much cheaper to make transistors out of silicon.


The existing technologies before semiconductors for signal amplification were thermionic valves - vacuum tubes, and for switching were relays. In the 1930s, Bell Lab's director of research, Mervin Kelly realised that the technology behind telephone exchanges needed improvement. The valves were troublesome because they generated huge amounts of heat, were prone to warm up problems, and required maintenance. He was aware of certain chemical elements with strange electrical conductive properties, which were later to be called semiconductors.

After the end of the second world war, Kelly assembled a team of research scientists with a view to replacing the thermionic valves with solid state semiconductors. Bill Shockley was selected as team leader, together with Walter Brattain and John Bardeen.

Interestingly, to solve the problem of amplification, Shockley focused on using an electric field to control flow of current through semiconductor material - later to be called the field effect. He was working alone while developing this, mainly at home.

At the same time, Brattain and Bardeen were investigating the properties of impure semiconductor material. They made an accidental discovery that when semiconductors were coated with a different element, the result had some interesting electrical properties. They had discovered that impurities changed the behaviour; they went on to develop this into the P-N junction.

In 1947, they made the first working "point contact transistor", and demonstrated this to Shockley. Shockley was furious, and shortly after this, the team split up. Both types of transistor became very important. Brattain and Bardeen's transistor formed the basis of the bipolar junction transistor (BJT), and Shockley's, the field effect transistor (FET: types of FET include the JFET and MOSFET).


Bipolar transistors are relatively low impedance devices. The input current is proportional to the output current. The ratio of these two currents is called the gain or HFE. BJTs do not have a linear characteristic, hence the incremental gain, written Hfe (what change of output happens as a result of a change of input), is different depending on the starting current. Fortunately, the middle of the curve is straight, which is good news if we want to use the device for amplification. To achieve this, amplifier cicuits use bias resistors to provide the quiescent current necessary to operate in the middle of the graph.

The output of a BJT is two terminals referred to as the emitter and collecter, and the third, input terminal is called the base. The current flowing between emitter and base controls the current flowing between emitter and collecter. The direction of the currents depends whether the device is NPN or PNP. In a PNP device, the current flows from emitter to base, and emitter to collecter. for NPN, the current flows the other way, i.e. towards the emitter.


Field effect transistors are high impedance devices. The input voltage controls the output current. The high impedance nature means that FET circuitry can be used to measure voltages without draining current. This lends the technology to such applications as solid state memory. Although FETs can be used for amplification, their most common use is in switching; FETs are the building blocks behind the CMOS microtechnology of modern computers. Early chips such as TTL did use bipolar technology, but MOSFET technology proved more suitable, with lower power dissipation and higher density of transistors on a chip.

The output terminals of a FET are the source and drain, and the input terminal is called the gate.

Identifying the terminals

Transistors come in a variety of cases and shapes. However, the case styles are numbered, such as TO92, TO18 etc. which are standard across the industry. The case is never completely symmetrical, and there will always be some identifying mark such as a painted dot, or a nick in the plastic case. To identify the pins, you need to look up the part number in a technical reference - electronics vendors provide this information free of charge.