FORWARD BIAS

Suppose that the bias at the base of an npn transistor is positive relative to the emitter, starting at small levels and gradually increasing. This is forward bias. If this bias is less than forward breakover, no current flows. However, when the voltage reaches forward breakover, the E-B junction conducts current.
Despite reverse bias at the base-collector (B-C) junction, the emittercollector (E-C) current, more often called collector current and denoted IC, flows when the E-B junction conducts. A small rise in the positive-polarity signal at the base, attended by a small rise in the base current IB, causes a large increase in IC. This is the principle by which a bipolar transistor can amplify signals.