Emitter Current in a Transistor:

The emitter current in a transistor refers to the flow of charge carriers (electrons or holes) from the emitter terminal to the base terminal. In a bipolar junction transistor (BJT), which is the most common type of transistor, the emitter current is primarily constituted by electrons.

Explanation of the Correct Answer:

The correct answer to the given question is option 'C', which states that the emitter current is constituted by electron current. This means that the flow of electrons from the emitter to the base forms the emitter current in a BJT.

Working Principle of a BJT:

To understand why the emitter current consists of electron current, let's briefly discuss the working principle of a BJT. A BJT has three terminals - emitter, base, and collector. It is made up of two pn junctions, namely the emitter-base (EB) junction and the base-collector (BC) junction.

The BJT operates in two modes - the active mode and the saturation mode. In the active mode, the emitter-base junction is forward-biased, while the base-collector junction is reverse-biased.

Flow of Charge Carriers:

In the active mode, the forward bias on the emitter-base junction causes a large number of electrons (majority carriers) to flow from the n-type emitter to the p-type base region. These electrons constitute the emitter current.

The base region is very thin, and due to the presence of a reverse bias, only a small number of electrons are able to cross the base-collector junction and reach the collector terminal. These electrons are known as the minority carriers.

Conclusion:

In conclusion, the emitter current in a BJT consists of electron current. The forward bias on the emitter-base junction allows a large number of electrons to flow from the emitter to the base, forming the emitter current. The base-collector junction is reverse-biased, causing only a small number of minority carriers to flow from the base to the collector. Therefore, it is the electrons that primarily constitute the emitter current.