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Bipolar junction transistors (BJTs) are one of the most widely used types of transistors in application fields such as amplifiers, switches, and oscillators. They are often referred to as "single" transistors because of their simple construction and one-to-one design. BJTs are made up of three main components: an emitter, collector, and base. BJTs can be used in two configurations: as a voltage-controlled current source, or as a current-controlled voltage source. Each configuration has its own unique characteristics and benefits, as well as drawbacks.

The base-emitter junction in a BJT is responsible for the current flow characteristics of the device. It\'s an area between two semiconductor materials, usually composed of P-type and N-type materials, that forms a diode. The diode is the switching element in the BJT and controls the current between the emitter and collector. When a voltage is applied to the base-emitter junction, the diode allows current to flow from the emitter to the collector side. This current is directly proportional to the amount of voltage applied.

In a voltage-controlled current source, the amount of current that can flow from the emitter to the collector side is controlled by the amount of voltage applied to the base-emitter junction, or the “base current”. This voltage acts as a gate for the current and can vary from a few hundred millivolts to several volts. When the base current is raised, the current that is allowed to flow through the transistor increases as well. This kind of BJT is commonly used in amplifiers.

In a current-controlled voltage source, the amount of voltage that can be applied is controlled by the amount of current flowing from the emitter to the collector side. Typically, the amount of voltage produced is much lower than when using a current-controlled current source. As with the voltage-controlled current source, the amount of current flowing through the emitter and collector is directly related to the voltage applied to the base-emitter junction. This type of BJT is commonly used as a switch.

Finally, a BJT can be used as an oscillator. In this configuration, the current that is allowed to flow through the transistor is constantly changing. This causes the voltage across the base-emitter junction to oscillate, creating an alternating current. This AC current can be used to produce an output signal that can be used for things like driving a loudspeaker, generating a radio signal, or providing an interface for a digital circuit.

Bipolar junction transistors offer significant advantages over other types of transistors in certain application fields. They have a simple construction and can be used in various configurations, depending on the application. They are also relatively inexpensive and very reliable, making them one of the most popular types of transistors for a wide range of applications.