What defines a basic semiconductor device that controls the flow of electric current?

A basic semiconductor device that controls the flow of electric current is defined as a junction diode. A junction diode allows current to flow in one direction while blocking it in the opposite direction, which is fundamental in creating control over electric current in circuits. This behavior is due to the p-n junction formed within the diode, where p-type and n-type semiconductor materials meet.

In practical applications, this characteristic of allowing current to pass only in one direction is essential for rectification processes in power supplies and signal processing. Thus, the junction diode plays a crucial role in controlling the flow of electric current within electronic devices.

The other options listed do not fit this definition accurately: a capacitor stores electrical energy but does not control current flow; a transistor is a more complex device that can amplify or switch electronic signals but is not the simplest form of control; while an oscillator generates repetitive signals, it does not inherently control current flow itself.