BJT bias requirement: For proper active-region operation of a bipolar transistor, the base–emitter (BE) junction must be forward biased. Which other junction must be reverse biased?

Introduction:Correct DC biasing of a BJT establishes the operating (Q) point in the active region. This enables linear amplification. Knowing which junctions must be forward or reverse biased is a foundational concept in analog electronics.

Given Data / Assumptions:

• BJT operated in active region for amplification.
• Base–emitter (BE) junction forward biased.
• Second PN junction bias must be identified.

Concept / Approach:

In active mode: the BE junction is forward biased to inject carriers from emitter into base; the collector–base (CB) junction is reverse biased to sweep carriers from the base into the collector. This separation of biases provides transistor action with current gain.

Step-by-Step Solution:

Verification / Alternative check:

Examining output characteristics (IC vs VCE) shows that with BE forward and CB reverse bias, the device exhibits near-constant IC for a given IB, the signature of active mode.

Why Other Options Are Wrong:

• collector–emitter junction: VCE is a terminal-to-terminal potential; active-mode condition is specifically CB reverse bias.
• base–emitter junction: This is already forward biased; reverse biasing it would move the device to cutoff or reverse-active region.
• emitter–substrate junction: Not a standard discrete BJT terminal pair in circuit bias discussions.
• base–collector junction (wording): Equivalent to collector–base; among duplicates, we choose the explicit correct phrasing.

Common Pitfalls:

• Confusing saturation (both junctions forward) with active region.
• Overlooking that reverse-active mode swaps roles and is rarely used in designs.

Final Answer:

collector–base junction