The triode region in a MOSFET refers to one of its operational modes where the transistor behaves similarly to a triode vacuum tube. In this region, the MOSFET operates with a linear relationship between the drain current (I_D) and the drain-source voltage (V_DS), similar to a resistor. The transistor is typically in saturation when operating in this region, meaning the drain-source voltage is less than the gate-source voltage minus the threshold voltage (V_GS – V_th). In the triode region, the drain current is controlled primarily by the gate-source voltage (V_GS) and the drain-source voltage (V_DS).
The term “triode region” originates from vacuum tube terminology, where a triode tube also exhibits a linear relationship between the anode current and anode voltage when operating in a similar mode. In both cases, the term “triode” denotes the linear relationship between current and voltage characteristics.
MOSFETs generally operate in three distinct regions: cutoff, triode (or ohmic), and saturation. In the cutoff region, the transistor is essentially off, and very little or no current flows between the drain and source. In the saturation region, the MOSFET behaves like a closed switch, allowing a significant amount of current to flow between the drain and source without much change in voltage. The triode region lies between these two extremes, where the MOSFET operates in a linear mode, similar to a resistor, with current varying linearly with voltage.
The ohmic or triode region is significant in MOSFET circuits where precise linear control of current is necessary, such as in analog amplifiers and other linear applications. Understanding and controlling the transistor’s operation in this region is essential for designing circuits that require precise voltage-current relationships and ensuring optimal performance across varying operating conditions.