Question

For transistor action, which of the following statements is correct ?

• A. Base, emitter and collector regions should have same doping concentrations.
• B. Base, emitter and collector regions should have same size.
• C. Both emitter junction as well as the collector junction are forward biased.
• D. The base region must be very thin and lightly doped.

Answer 1

The Correct Option is D

The given question addresses the fundamental principles of transistor action in semiconductor devices, which is an integral aspect of Physics and Electronics used in various government exams. To resolve the question, let’s analyze the core concept related to how a transistor operates, specifically focusing on the base region, emitter junction, and collector junction.

Let's examine each option: 

1. Base, emitter and collector regions should have the same doping concentrations.
   This is not true for transistor action. In practice, the doping concentrations of these regions are purposefully different to optimize the transistor's performance:
   • The emitter is heavily doped to inject a large number of carriers into the base.
   • The base is lightly doped and very thin to ensure most of the injected carriers from the emitter reach the collector.
   • The collector is moderately doped and larger in area to collect carriers efficiently.
2. Base, emitter and collector regions should have same size.
   This is incorrect. Each region of a transistor has a specific size and doping purposefully chosen:
   • The collector is usually larger than the base and emitter to manage the heat and collect carriers.
   • The base is very thin to reduce recombination of charge carriers.
3. Both emitter junction as well as the collector junction are forward biased.
   This is incorrect. For transistor action to occur:
   • The emitter-base junction is forward biased to inject carriers into the base.
   • The collector-base junction is reverse biased to attract majority carriers from the base.
4. The base region must be very thin and lightly doped.
   This statement is correct. The thin and lightly doped base allows efficient transfer of carriers from the emitter to the collector, which ensures transistor action. Only limited recombination of the charge carriers occurs in the base ensuring a high current gain.

Thus, the correct answer is: The base region must be very thin and lightly doped. This setup is essential for the effective operation of the transistor, allowing the minority carriers from the emitter to traverse the base and reach the collector with minimal recombination.