Final answer:
The center material of a junction transistor is known as the base, which controls the electric current from emitter to the collector when base voltage is applied.
Step-by-step explanation:
The center material of a bipolar junction transistor (BJT) is called a base. A junction transistor is composed of three parts: the emitter, base, and collector. The base is a thin p-type semiconductor sandwiched between two n-type semiconductors (for an n-p-n transistor) or two p-type semiconductors (for a p-n-p transistor).
The base voltage applied to the base acts as a valve to control the electric current flowing from the emitter to the collector. When a positive terminal is connected to the base, a small base current (IB) is allowed to flow, which in turn controls a much larger collector current (IC), thus amplifying the input signal.