For current to flow in an NPN silicon transistor's emitter-collector junction, the base must be:

In an NPN silicon transistor, for current to flow through the emitter-collector junction, the base must be at least 0.7 volts positive with respect to the emitter. This requirement arises from the characteristics of the silicon material used in the transistor.

Silicon transistors operate based on the principle that the base-emitter junction should be forward-biased to allow current to flow. When the base voltage exceeds the emitter voltage by a threshold of approximately 0.7 volts, it allows for a sufficient flow of charge carriers (holes from the base and electrons from the emitter), which enables the transistor to conduct. This positive voltage is necessary to overcome the potential barrier created by the p-n junction between the base and the emitter.

Thus, a base voltage that is at least 0.7 volts higher than the emitter ensures that the junction is properly forward-biased, allowing the device to function effectively in amplifying or switching applications.