A p-n junction diode permits current to flow in only one direction. This limits its applications to rectification and detection.
A more useful semiconductor device is a bipolar junction transistor. The invention of transistor by John Bardeen, Walter Brattain and William Shockley in 1948 at Bell laboratory in USA revolutionized the electronic industry.
The transistors find many any varied uses ranging from gas lighter to toys to amplifiers, radio sets and television. In the form of switching device, these can be used to regulate vehicular traffic on the roads. They form key elements in computers, space vehicles, power systems in satellites and communication.
A transistor is basically a silicon or germanium crystal containing three alternate regions of p and n-type semiconductors. These three regions are called emitter (E), base (B) and collector (C). The middle region is the base and the outer two regions are emitter and collector. The emitter and collector are of the same type (p or n) and collector is the largest of the three regions.
The base terminal controls the current flowing between the emitter and the collector. This control action gives the transistor an added advantage over the diode, which has no possibility of controlling the current flow. Depending on the type of doping, the transistors are classified as n-p-n or p-n-p. In general, the level of doping decreases from emitter to collector to base.
In case of a n-p-n transistor, the majority carriers (electrons) from the emitter are injected into base region. Since base is a very lightly doped thin layer, it allows most of the electrons injected by the emitter to pass into the collector. Being the largest of three regions, the collector dissipates more heat compared to the other two regions.
The arrow head indicates the direction of flow of conventional current.
In a n-p-n transistor, the emitter current is due to flow of electrons from emitter to base, and the conventional current flows from base to emitter and hence the arrow head points out from the base. In case of p-n-p transistor, the emitter current comprises flow of holes from emitter to base. Thus, the conventional current flows from emitter to base.
Since transistors are bipolar devices, their operation depends on both the majority and minority carriers.