A transistor is called a current control device because it allows the flow of current through it to be controlled by the current or voltage applied to its base terminal.
In a bipolar junction transistor (BJT), which is a common type of transistor, the base current has a significant influence on the collector current. By changing the base current, the amount of current flowing from the collector to the emitter can be regulated.
For example, in an NPN transistor, a small increase in the base current causes a much larger increase in the collector current. This amplification or control of current makes transistors essential in various electronic circuits for functions such as amplification of weak signals and switching.
The relationship between the base current and the collector current is described by the transistor's current gain. The ability to precisely control the collector current based on the base current input gives transistors their role as current control devices, enabling the design of complex and efficient electronic systems.