Which component conducts electricity from a negative emitter to a positive collector when its base voltage is made positive?

In an NPN (Negative-Positive-Negative) transistor, the emitter is N-type material, the base is P-type, and the collector is N-type. To turn the transistor on and allow current to flow from the negative emitter to the positive collector, the base (P-type) must be biased positively relative to the emitter. A PNP transistor operates with opposite polarities, requiring a negative voltage at the base relative to the emitter. A triode vacuum tube has a grid, plate, and cathode, not a base, emitter, and collector. A varactor is a variable capacitance diode, not a switching or amplifying transistor.