Zero Bias:
Whenever the base is not connected to anything, or whenever it is at similar potential as the emitter, the bipolar transistor is at zero bias. Under this situation, that is termed cutoff, no significant current can flow via a p-n junction unless the forward bias is at least equivalent to the forward breakover voltage. For silicon, the critical voltage is around 0.6 V; and for germanium, it is 0.3 V.
With the zero bias, the emitter-base (E-B) current IB is zero, and the E-B junction does not conduct. This avoids current from flowing in the collector unless a signal is injected at the base to change the circumstances. This signal should have a positive polarity for at least portion of its cycle, and its peaks should be enough to overcome the forward breakover of the E-B junction for at least a part of the cycle.