Understanding FET Operating Modes

A Field-Effect Transistor (FET) is a type of transistor that uses an electric field to control the flow of current. FETs have different operating modes, often called regions, depending on the voltages applied to their terminals (gate, drain, and source).

These operating modes determine how the FET behaves and are crucial for its use in electronic circuits, whether for amplification or switching.

Different Operating Modes of a FET

FETs, particularly MOSFETs (Metal-Oxide-Semiconductor FETs), typically operate in three main regions:

• Cut-off Region: In this region, the voltage applied to the gate is not sufficient to create or sustain a conducting channel between the source and drain. Consequently, very little current flows through the device. This mode represents the "switched off" state of the transistor, acting like an open switch.
• Ohmic Region (or Triode/Linear Region): When the gate voltage is high enough to form a channel, and the voltage between the drain and source (V_DS) is relatively low, the channel behaves like a resistor. The current flowing through the device is approximately proportional to V_DS. In this mode, the FET acts like a voltage-controlled resistor. For some switching applications, the FET operates in this low-resistance ohmic region when "on".
• Saturation Region: When the gate voltage is high enough and the V_DS voltage increases beyond a certain point (the pinch-off voltage), the conducting channel near the drain becomes narrower or "pinched off". In this saturation region, the drain current becomes relatively constant and independent of V_DS, primarily controlled by the gate voltage. This mode is often used for amplification or as a current source. For digital switching applications, the FET is often driven into saturation (for higher voltages) or deep into the ohmic region when it is in the "switched on" state because these regions allow significant current flow compared to the cut-off state.

FET Switched On Condition

The question asks about the mode where the FET is in a "switched on condition". This condition implies that the FET is allowing significant current to flow between the drain and source terminals, behaving like a closed switch or a conductor.

Comparing the modes:

• Cut-off is the "switched off" state (very low current).
• Both Ohmic and Saturation are conducting states where significant current flows.

While both ohmic and saturation regions represent a conducting state, the saturation region is often considered the "on" state in digital logic applications, especially when considering higher drain-source voltages, as it provides a substantial current path. In many contexts, driving a MOSFET into saturation by applying a sufficiently high gate voltage signifies turning the device "on".

Therefore, the saturation mode corresponds to a condition where the FET is significantly conductive, often described as being in a "switched on condition", particularly in switching applications.