The invention of the bipolar junction transistor in 1948 by Shockley, Bardeen, and Brattain paved the way for the subsequent arrival of the integrated circuit, which revolutionized the semiconductor industry.  The bipolar transistor is a three-terminal device consisting of 3 layers of alternating n- and p-type materials referred to as the emitter, base, and collector.  Its structure basically consists of two back-to-back diodes, one between the emitter and base and the other between the base and collector.

There are two types of bipolar transistor:  the NPN and the PNP.  In the NPN transistor, the base is composed of a p-type material and is sandwiched by an n-type emitter and an n-type collector.  In the PNP, the base is n-type while the emitter and collector are p-type.

The bipolar transistor works by yielding a high collector current Ic when a relatively small current Ib is forced into its base.  Since Ib is relatively much smaller than Ic, a small variation in Ib results in a much larger variation in Ic.  This, in effect, is current amplification, with the current gain known as the beta of the transistor. The currents going into and out of the emitter, base, and collector follow Kirchoff's current law:  Ib=Ie-Ic.  Since Ic is much greater than Ib, Ie is very close in value to Ic.  In short, a large current flows from the emitter to the collector of a transistor whenever the base receives some input current.  The transistor is therefore very useful as a switch or as an amplifier.

How the transistor operates (and therefore used) depends greatly on how it is electrically stimulated, or biased.  The transistor may be operated in three different regions:  saturation, cut-off, and active.  A transistor is said to be saturated if both its base-collector and base-emitter junctions are forward biased.  Under this mode, the transistor is already completely 'on', i.e., the collector current is already very high and no longer increases appreciably even if more current is fed into the base.