BJT Transistor Biasing
If you consider an NPN BJT to be two diodes in series opposition with their anodes connected together, then the collector is the cathode of one diode, the emitter is the cathode of the other, and the base is the junction of the two anodes.

For an NPN BJT, then, it means that the base must be positive with respect to the emitter and the collector must be positive with respect to the base.