What is the function of a bipolar junction transistor (BJT)?

A bipolar junction transistor (BJT) functions primarily as an amplifier of current. It achieves this by utilizing a small input current at the base terminal to control a larger current flowing from the collector to the emitter. This property of current amplification is fundamental to the operation of BJTs in various electronic applications, allowing them to be used in amplifiers, signal modulation, and switching.

The design of a BJT, which consists of three semiconductor regions (the emitter, base, and collector), enables it to control current flow effectively. As a result, even a tiny change in the base current can induce a proportionately larger change in the collector-emitter current, making the BJT a key component in signal processing and amplification tasks.

The other choices do not correctly represent the primary function of a BJT. While a BJT might interact with AC and DC signals, its main role is not to convert one into the other. It doesn't regulate temperature in circuits, nor is it used for storing electric charge, as its design and operation do not facilitate these processes.