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

For a Bipolar Junction Transistor (BJT) to operate in the "Active Region"—where it conducts current from the collector to the emitter—specific biasing conditions must be met at its two PN junctions. 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).