Bipolar Junction Transistor

The bipolar junction transistor (BJT) is a three-element (emitter, base, and collector) device made up of alternating layers of n- and p-type semiconductor materials joined metallurgically. The transistor can be of pnp type (principal conduction by positive holes) or of npn type (principal conduction by negative electrons), as shown in Fig. 3-1 (where schematic symbols and positive current directions are also shown). The double subscript notation is utilized in labeling terminal voltages, so that, for example, Vbe symbolizes the increase in potential from emitter terminal E to base terminal B. For reasons that will become apparent, terminal currents and voltages commonly consist of superimposed dc and ac components (usually sinusoidal signals).

The table below presents the notation for terminal voltages and currents:

CURRENT RELATIONSHIPS

The two pn junctions of the BJT can be independently biased, to result in four possible transistor 

operating modes as summarized in Table 3-3. A junction is forward-biased if the n material is at a lower 

potential than the p material, and reverse-biased if the n material is at a higher potential than the p 

material.

Saturation- With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. (Base higher than emitter, but collector is not higher than base.)

Cut-off-  In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current flow, which corresponds to a logical "off", or an open switch. (Base lower than emitter, but collector is higher than base. It means the transistor is not letting conventional current to go through collector to emitter.)

Inverse- The inverse mode is a little-used, inefficient active mode with the emitter and collector interchanged. (Base lower than emitter, collector lower than base: reverse conventional current goes through transistor.)

Linear/Active- describes transistor operation in the region to the right of saturation and above cutoff (Base higher than Emitter, Collector higher than Base)

Sources:

Schaum's Outline of Electronic Devices and Circuits

Wikipedia