From DaphneWiki

Transistors

Electronics can easily flow from the p-type base to the n-type collector as long as the base-emitter junction is forward biased (meaning the base is at a higher voltage than the emitter).

In a way, a transistor is like a variable, adjustable resistor.

Four modes: Saturation (short circuit), cut-off (open circuit), active (current from collector to emitter is proportional to current flowing into the base), reverse-active (like active except current flowing in reverse)

Voltages from base to emitter (Vbe) and base to collector (Vbc) set the transistor's mode.

Mode	Condition
Reverse active	Vc < Vb < Ve
Saturation	Ve < Vb, Vc < Vb; Vc must be slightly greater than Ve
Cut-off	Ve > Vb, Vc > Vb
Forward active	Vc > Vb > Ve