The transistor is a three-terminal solid state device formed by combining three layers of doped semiconductor such as npn or pnp.
Transistors have the important property that the current at one pair of terminals can be controlled by a much smaller current at the other pair.
Hence transistors are the basis of the electronic signal amplifier.
This can be considered as two pn junctions back-to-back. If we bias the two junctions the first junction will be forward biased and the second reverse biased. Thus we will obtain a large current in the first circuit and a small current in the second circuit with the junctions acting independently.
The three layers are called the emitter E, base B and collector C.
To produce a transistor we need the two junctions to interact. In order to achieve this the base region must be made very thin. Also the emitter region must be much more heavily doped than the base. This device produces an interaction under bias.
Under forward bias there will be a large flow of majority carriers from the emitter to the base - these are primarily electrons from the heavily doped emitter.
Because the base region is very thin the majority of the electrons from the emitter will flow directly into the base-collector circuit and fall down the potential gradient into the collector.
Thus the current in the base-emitter circuit is very small compared with that in the base-collector circuit.
Transistors are usually made by successive diffusions of p and n-type dopants into the basic n-type collector region. Note how small they are.
Transistors are usually operated in the grounded base configuration.
Note the circuit symbol for the solid state transistor.
With this circuit we can measure the IV characteristics of the transistor.
The transistor's performance is usually described by two parameters
= ratio of the currents flowing in the collector and emitter leads. ? N is typically between 0.95 and 0.99
= ratio of the currents flowing in the collector and base leads
? N is a measure of the current amplification by the transistor and is often in the range of 20 to 500.
Amplification occurs because the small base current (which flows in the emitter/base junction) produces a large collector current.
The collector current IC is a function of both the collector voltage and the base current.
The value of ? N can be determined from these characteristic curves.
Note that the current/voltage characteristics of a transistor are temperature dependent with ? N increasing with temperature.
The transistor should not be operated at currents and voltages above this curve.
Because of their ability to amplify a current transistors are widely used as amplifiers in electronic circuits.
Field Effect Transistors are an alternative design with a lower impedance. These are available in pn and MOSFET designs.