Describe the operation of a BJT transistor in active mode and the role of the base-emitter junction.

• A. In active mode, BE junction forward-biased, BC reverse-biased; collector current roughly β times base current; BE acts as diode controlling current.
• B. BC forward biased; BE reverse biased; collector current is independent of base current.
• C. BE and BC both forward biased; collector current equals base current.
• D. BE reverse biased; transistor acts as open circuit.

Answer: (A)

In active mode, the base-emitter junction is forward-biased, acting like a small forward-biased diode that injects carriers from the emitter into the base. At the same time, the base-collector junction is reverse-biased, which sweeps those injected carriers into the collector rather than letting them recombine in the base. This setup creates a current gain: the collector current is controlled by the small base current and is approximately equal to beta times the base current, where beta is the transistor’s current gain. The base-emitter diode’s forward action sets the base current (and with it, the emitter current), so it indirectly governs the amplified collector current. The collector-emitter current is largely independent of the collector voltage as long as the collector-base junction remains reverse-biased and Vce stays high enough to avoid saturation (aside from small variations due to the Early effect).

If the base-collector junction were forward-biased, or the base-emitter junction reverse-biased, the device would not operate in this active amplification region—either it would push toward saturation or turn off, and the collector current would not follow the simple Ic ≈ βIb relationship.