Q1Analog Electronics

In a Field Effect Transistor (FET), the electric field produced by the gate-source voltage ($V_{GS}$) controls the width of the conducting channel and hence the drain current $I_D$. The drain current is related to $V_{GS}$ by the square-law equation:

The gate terminal is either reverse-biased (JFET) or insulated by an oxide layer (MOSFET), so the gate current $I_G\approx 0$. This means the input (gate) presents an extremely high impedance, and the device is controlled purely by voltage rather than current. Hence FET is called a voltage-operated (or voltage-controlled) device, in contrast to the BJT which is a current-controlled device.