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

The question describes the standard biasing and current flow for a PNP bipolar junction transistor. In a PNP transistor, the emitter is composed of P-type material (positive), the base is N-type (negative), and the collector is P-type. For current to flow through this device, the emitter-base junction must be forward-biased. This is achieved by making the base negative relative to the emitter. When this happens, holes (positive charge carriers) are injected from the emitter through the thin base and collected at the negative collector. An NPN transistor operates with the exact opposite polarities: its base must be positive relative to its negative emitter to conduct. Vacuum tubes and varactors do not match this specific three-terminal, polarity-dependent description.