Question

Which of the following is a unipolar transistor?
A) p-n-p transistor
B) n-p-n transistor
C) Field-effect transistor
D) Point contact transistor

Answer 1

A unipolar transistor is a transistor that uses only one type of charge carrier i.e. either electrons or holes for conduction from one part to another part within the transistor.

Complete step by step solution:
p-n-p transistor: p-n-p transistor is a bipolar junction transistor(BJT). As the name suggests, these transistors are constructed using one n-type semiconductor between two p-type semiconductors. The collector and emitter of these transistors are the two p-type semiconductors where the base is the n-type one. So, clearly, they are not unipolar transistors. These transistors use a small base current with a negative base voltage to produce a larger emitter-collector current.

n-p-n transistor: n-p-n transistor is also a bipolar junction transistor(BJT). As the name suggests, these transistors are constructed using one p-type semiconductor between two n-type semiconductors. The collector and emitter of these transistors are the two n-type semiconductors where the base is the p-type one. So, clearly, they are not unipolar transistors. These transistors use a small base current with a positive base voltage to produce a larger emitter-collector current.

Field-effect transistor: Field-effect transistors(FET) are the example of a unipolar transistor. Unlike BJTs, these transistors are made up of only one type of semiconductor (either p-type or n-type). FETs have three terminals, known as a drain, source, and gate. Current, controlled by the voltage applied at the gate flows from drain to source. In depletion mode, the depletion region is increased to restrict the current flow while in enhanced mode increased gate voltage increases current flow, hence conductivity.

Point contact transistor: Unlike BJTs such as np-n or p-n-p these transistors only have one p-n junction. Here, a little positive charge is applied at its base. So, the electrons rush to the collector from the emitter. Meanwhile, holes start traveling to the ground. From its working process, you can notice that both types of careers are used here. Hence, this is not a unipolar transistor.

The unipolar transistor is (C) Field effect transistor.

Note:
Transistors are generally used either as a switch or to amplify the current. This question is based upon finding the nature of transistors and how it works. For the unipolar transistor, the transistor must be made up of one type semiconductor. Both the name of p-n-p and n-p-n transistor suggests that they can’t be unipolar and for the point-contact transistor, one p-n junction is used. Hence, the Field effect transistor is the correct answer.