BDX53C is a three layer NPN device within the working range, the collector current IC is a function of the base current IB, a change in the base current giving a corresponding amplified change in the collector current for a given collector emitter voltage VCE.

BDX54C is a three layer PNP device within the working range, the collector current IC is a function of the base current IB, a change in the base current giving a corresponding amplified change in the collector current for a given collector emitter voltage VCE.