For a properly biased BJT in the common-collector (emitter follower) or common-emitter family, the collector-base junction must be reverse-biased. Which other junction must be forward-biased for normal active-region operation?

Introduction / Context:
Proper BJT biasing places the device in its active region so it can amplify. This requires specific polarities across the internal PN junctions. Knowing which junction is forward-biased versus reverse-biased is fundamental to understanding transistor action and small-signal models.

Given Data / Assumptions:

• BJT intended to operate in the forward-active region.
• Collector-base (C-B) junction is reverse-biased.
• We must select which other junction is forward-biased.

Concept / Approach:
In forward-active operation, the base-emitter (B-E) junction is forward-biased, injecting carriers from the emitter into the base. The collector-base (C-B) junction is reverse-biased to sweep carriers from the base into the collector, enabling transistor action and gain.

Step-by-Step Reasoning:
1) Forward-active condition: B-E forward bias (approximately 0.6–0.7 V for silicon).2) Reverse bias C-B junction to collect injected carriers efficiently.3) Under these conditions, collector current is approximately β times base current.4) Therefore, the required forward-biased junction is the base-emitter junction.

Verification / Alternative check:
Examine transistor I-V curves: forward biasing B-E produces exponential base/emitter currents; reverse-biased C-B gives the large-signal region where Ic is controlled by B-E injection, consistent with active mode.

Why Other Options Are Wrong:
Collector-emitter: this is not a PN junction and is not forward-biased in active mode.

Collector-base/base-collector: must be reverse-biased, not forward.

Cathode-anode: diode terminology; not a BJT junction designation.

Common Pitfalls:
Confusing saturation (both junctions forward-biased) with active mode. Saturation is useful for switching but not for linear amplification.

Final Answer:
base-emitter