Describe NPN transistor operation in the active region and the role of the base-emitter junction.

• A. In active region, V_BE ≈ 0.0 V; collector current independent of base current.
• B. In active region, V_BE ≈ 0.7 V, base current controls collector current; I_C ≈ β I_B; emitter-base junction behaves as diode.
• C. In active region, V_BE ≈ 1.2 V; no relation between base and collector current
• D. In active region, V_BE ≈ 0.7 V; collector current independent of base current.

Answer: (B)

In the active region, an NPN transistor operates as a current amplifier, so the base current controls the collector current. The base-emitter junction must be forward-biased, typically around 0.6–0.7 V for silicon, to inject carriers from the emitter into the base. Because the base-collector junction is reverse-biased, most of these carriers are swept into the collector, producing the collector current. The relationship is I_C ≈ β I_B, where β is the transistor’s current gain, so the amount of collector current scales with the base current. The base-emitter junction behaving like a diode is what enables this injection and forward conduction.

That’s why the correct statement notes V_BE about 0.7 V, that the base current controls the collector current, and that I_C ≈ β I_B, with the emitter-base junction acting as a diode. The other options misstate the forward-bias level or the dependence of collector current on base current.