Do transistors fundamentally behave as voltage sources, or are they better modeled as controlled current sources within typical operating regions?

Introduction / Context:
Proper device modeling is crucial for analog design. The statement that transistors are “basically voltage sources” is misleading. Most transistor behaviors are captured by controlled current source models in their active regions.

Given Data / Assumptions:

• BJT in forward-active region: collector current controlled by base-emitter voltage/current.
• MOSFET in saturation region: drain current controlled by gate-source voltage.
• Small-signal models linearize around a bias point.

Concept / Approach:

In BJTs, Ic ≈ β * Ib or more precisely Ic depends exponentially on Vbe; the device is modeled as a transconductance element (current source) controlled by Vbe, with output resistance ro. In MOSFETs, Id ≈ k * (Vgs − Vth)^2 in saturation; again, a transconductance-controlled current source model with finite output resistance fits. Neither device inherently enforces a fixed voltage (the hallmark of a voltage source).

Step-by-Step Solution:

Verification / Alternative check:

Load-line analysis on a transistor shows the operating point determined by the intersection of device I–V curves (current-source-like) and the external load line, not by an internal fixed voltage source behavior.

Why Other Options Are Wrong:

• BJT vs MOSFET distinction does not change the current-source modeling.
• “Saturation” in MOSFETs still yields controlled current with finite output resistance; not a voltage source.
• Small-signal AC models also treat the device as a transconductance (current) source.

Common Pitfalls:

Confusing the presence of Vbe or Vgs thresholds with the device acting as a voltage source. These voltages control current; they do not supply fixed voltage to the load.

Final Answer:

False