MOSFETs are often referred to as voltage-controlled devices because the primary mechanism for controlling the current through them is the voltage applied to the gate terminal relative to the source terminal. In a MOSFET, the gate-source voltage (V_GS) determines the electric field across the gate oxide, which in turn controls the channel conductivity between the drain and source terminals. By varying V_GS, the MOSFET can modulate the current flow through the channel, making it responsive to changes in applied voltage and acting as a voltage-dependent current source.
In contrast, bipolar junction transistors (BJTs) are typically considered current-controlled devices because the base current (I_B) controls the conduction between the emitter and collector terminals. The base current modulates the minority carriers (electrons or holes) injected into the base region, which in turn affects the collector current (I_C). The amount of current amplification (β) of a BJT is primarily determined by the ratio of collector current to base current, making it inherently current controlled.
BJTs can also operate in voltage-controlled mode to some extent, especially in applications where feedback mechanisms or biasing arrangements control the base-emitter voltage (V_BE). However, their fundamental operation relies on current control due to the physics of minority carrier injection and amplification processes within the transistor structure.
The distinction between MOSFETs as voltage-controlled and BJTs as current-controlled devices arises from their different physical structures and operating principles. MOSFETs rely on the electric field created by the gate voltage to control the channel conductivity directly, whereas BJTs use base current to control the emitter-collector current flow indirectly through carrier injection and transistor gain mechanisms. These characteristics determine how each device responds to control signals and influences their applications in various electronic circuits.