The correct answer is: The base has the least concentration of impurity.
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers. It is composed of three semiconductor regions: the emitter, the base, and the collector. The emitter and collector regions are heavily doped with impurities, while the base region is lightly doped.
The emitter and collector regions are connected to external circuits, while the base region is connected to a small voltage source. When a voltage is applied to the base-emitter junction, a small current flows through the base region. This current causes a much larger current to flow through the emitter-collector junction. This is because the emitter and collector regions are heavily doped, and the base region is lightly doped.
The base region is the control element of the BJT. The current through the base region controls the current through the emitter-collector junction. This makes the BJT a very versatile device that can be used as an amplifier, a switch, or a logic gate.
The concentration of impurity in the base region is important because it determines the current gain of the BJT. The current gain is the ratio of the collector current to the base current. A higher current gain means that the BJT can amplify a smaller signal into a larger signal.
The base region is lightly doped so that the current through the base region is small. This small current controls the much larger current through the emitter-collector junction. This makes the BJT a very efficient device.
The other options are incorrect because they do not accurately describe the concentration of impurity in the three regions of a BJT.