Control nature of BJT versus MOSFET Which statement correctly classifies the controlling quantity of a bipolar junction transistor (BJT) and a metal–oxide–semiconductor field-effect transistor (MOSFET)?

Introduction / Context:
Knowing whether a device is current- or voltage-controlled is crucial for gate/base drive design, driver power dissipation, and dynamic performance in power electronics. BJTs and MOSFETs represent two fundamentally different control paradigms.

Given Data / Assumptions:

• BJT: requires base current proportional to collector current (via beta) to maintain conduction.
• MOSFET: gate forms an insulated capacitor; conduction is governed by gate-to-source voltage exceeding threshold.
• Steady-state viewpoints; transient gate charging for MOSFET acknowledged but does not change classification.

Concept / Approach:

A BJT is a transconductance device where collector current is controlled by base-emitter current and voltage; practically, the base must be driven with current (current-controlled). A MOSFET channel is formed by an electric field; the gate is insulated and ideally draws negligible steady current, so the device is voltage-controlled, with drive power mainly due to charging/discharging gate capacitances during switching.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets specify base drive current for BJTs, while MOSFET drivers are rated by gate charge (Q_G) and allowable V_GS—consistent with current- vs voltage-control distinctions.

Why Other Options Are Wrong:

• Both current-controlled or both voltage-controlled: contradict device physics and practical drive requirements.
• BJT voltage-controlled and MOSFET current-controlled: reversed from reality.

Common Pitfalls:

Confusing instantaneous charging current of MOSFET gates (during switching) with steady-state control nature; the control variable remains V_GS.

Final Answer:

BJT is current-controlled and MOSFET is voltage-controlled.