Question

A NPN transistor conducts when

• A. Both collector and emitter are positive with respect to the base
• B. Collector is positive and the emitter is at the same potential as the base
• C. Collector is positive and the emitter is negative with respect to the base
• D. Both collector and emitter are negative with respect to the base

Hint:

Remember the "Forward-Reverse" rule for Active mode: The Input (Emitter-Base) is Forward biased, and the Output (Collector-Base) is Reverse biased.

Answer 1

The Correct Option is C

1. The NPN Structure: An NPN transistor consists of a P-type layer (Base) sandwiched between two N-type layers (Emitter and Collector).

2. Biasing the Junctions:

• Emitter-Base (E-B) Junction: This must be

forward biased. For an N-type emitter and P-type base, this means the base must be more positive than the emitter (or the emitter negative with respect to the base).

• Collector-Base (C-B) Junction: This must be

reverse biased. For an N-type collector and P-type base, this means the collector must be more positive than the base.

3. Why this configuration?: Forward biasing the E-B junction allows electrons to flow from the N-type emitter into the P-type base. Because the base is very thin and lightly doped, most of these electrons are then swept across the C-B junction by the strong positive potential on the collector, creating the collector current ($I_C$). Therefore, the transistor conducts when the collector is positive (relative to the base) and the emitter is negative (relative to the base).