A bipolar transistor consists of three layers separated by a semi-conductor material. There are two types the NPN and the PNP. Both types perform the same function, however they are different in polarity. A transistor resembles two diodes connected end to end. While you cannot actually make a transistor with two diodes it is a good analogy. In an NPN transistor, if the base and the emitter are the same voltage it prevents the E-B junction from conducting. It is referred to as Zero Bias and will not let current flow between the emitter and the collector. In order for it to conduct the voltage at the base will have to be greater than that at the emitter. At some point the voltage will be high enough for current to flow from the emitter to the collector. The voltage at which it starts to conduct is called the breakover voltage. When the base voltage(Eв) is positive, it is referred to as forward biased. Likewise if it is negative it is reverse biased. The base voltage is normally reverse biased as long as the voltage is less than the supply voltage. When a transistor is reverse biased, and the voltage reaches such a point that it over comes the reverse bias and the breakover point, an avalanche breakdown occurs. When the collecter current versus(Iс) the base voltage(Eв) curve reaches a point in which it levels off the transistor will then be saturated. The transistor at that point is wide open and conducting at its maximum. For a PNP transistor is just the opposite. If you substitute positive for negative and negative for positive a PNP will conduct the same as an NPN.
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