ZERO BIAS

When the base is not connected to anything, or when it is at the same potential as the emitter, a bipolar transistor is at zero bias. Under this condition, which is called cutoff, no appreciable current can flow through a p-n junction unless the forward bias is at least equal to the forward breakover voltage. For silicon, the critical voltage is 0.6 V; for germanium, it is 0.3 V.
With zero bias, the emitter-base (E-B) current IB is zero, and the E-B junction does not conduct. This prevents current from flowing in the collector unless a signal is injected at the base to change the situation. This signal must have a positive polarity for at least part of its cycle, and its peaks must be sufficient to overcome the forward breakover of the E-B junction for at least a portion of the cycle.