Modes of Operation of MOSFET
The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is one of the most widely used transistors in modern electronics due to its efficiency, high input impedance, and fast switching capabilities. The MOSFET operates in different modes depending on the applied voltage across its terminals: Source (S), Gate (G), and Drain (D). The behavior of the MOSFET is determined by the voltages applied to the Gate relative to the Source (V_GS) and the Drain relative to the Source (V_DS). These modes define the MOSFET’s operational characteristics and dictate its function in various applications such as amplification, switching, and signal processing. The key modes of operation for MOSFETs are Cutoff, Triode (Linear), Saturation, and Subthreshold regions.
Cutoff Mode
The cutoff mode occurs when the Gate-to-Source voltage (V_GS) is below the threshold voltage (V_th). In this region, the MOSFET behaves like an open switch, meaning that there is no current flowing from the Drain to the Source (I_D = 0). The device is in an “off” state, and the MOSFET does not conduct any current regardless of the voltage applied to the Drain. This mode is utilized when the MOSFET is intended to be in the “off” state in digital circuits, particularly for switching purposes, where no current flow is desired.
Triode (Linear) Mode
The triode mode, also known as the linear mode, occurs when the Gate-to-Source voltage (V_GS) exceeds the threshold voltage (V_th), and the Drain-to-Source voltage (V_DS) is small (V_DS < V_GS – V_th). In this mode, the MOSFET operates as a variable resistor. The current between the Drain and Source (I_D) is controlled by the voltage applied to the Gate, and the device acts like a linear resistor whose resistance is inversely proportional to V_GS. The triode mode is often used in analog circuits where the MOSFET is employed for linear amplification or as a voltage-controlled resistor.
In this region, the Drain current is given by the following equation:
I_D = K[(V_GS - V_th)V_DS - (V_DS^2)/2]
Where K is a constant that depends on the MOSFET’s parameters, such as mobility, oxide capacitance, and channel length.
Saturation Mode
The saturation mode, also known as the active mode, is the most commonly used mode for MOSFETs in switching and amplification applications. In this region, the Gate-to-Source voltage (V_GS) is greater than the threshold voltage (V_th), and the Drain-to-Source voltage (V_DS) is greater than the difference between V_GS and V_th (V_DS > V_GS – V_th). In this mode, the MOSFET is fully “on” and the Drain current (I_D) becomes relatively independent of V_DS. The current is mainly controlled by V_GS, and the MOSFET operates as a constant current source. The saturation mode is essential in digital circuits and amplifiers where MOSFETs act as switches or current amplifiers.
The current in saturation mode can be expressed as:
I_D = (K/2) * (V_GS - V_th)^2
This relationship indicates that the Drain current depends quadratically on the difference between V_GS and the threshold voltage, making it suitable for high-speed digital switching and amplification tasks. In this mode, the MOSFET is fully saturated and operates as a current amplifier.
Subthreshold (Weak Inversion) Mode
The subthreshold mode, also known as weak inversion mode, occurs when the Gate-to-Source voltage (V_GS) is below the threshold voltage (V_th). In this mode, the MOSFET still conducts a small current, but the current is exponentially dependent on V_GS. The current in this region is very small and is a result of the thermal generation of minority carriers in the channel. The subthreshold mode is used in low-power applications, particularly in circuits designed for ultra-low power consumption, such as in mobile devices where energy efficiency is crucial.
The Drain current in the subthreshold region is governed by an exponential function:
I_D = I_D0 * exp((V_GS - V_th) / nV_T)
Where I_D0 is the current for V_GS = V_th, V_T is the thermal voltage, and n is a factor related to the subthreshold slope. This mode is critical in designing circuits where power consumption must be minimized at the expense of speed.
Reverse Mode (Body Effect)
In addition to the primary modes of operation, the MOSFET can also exhibit reverse operation due to the body effect (also known as the back-gate effect). This effect occurs when a reverse bias is applied between the body (or substrate) and the Source terminal, which can increase the threshold voltage (V_th) and reduce the channel’s conductivity. The body effect becomes significant when there is a voltage difference between the body and the Source, and it is commonly observed in MOSFETs with a separate body terminal in integrated circuits (ICs). The body effect is typically avoided or minimized in well-designed circuits.
In summary, the MOSFET operates in different modes based on the voltages applied to its terminals, each with distinct characteristics and applications. The cutoff mode is used for “off” switching, the triode mode for linear amplification, the saturation mode for high-speed switching and current amplification, and the subthreshold mode for ultra-low-power applications. Understanding the various modes of operation of MOSFETs is crucial for designing efficient and reliable electronic circuits, as each mode offers unique advantages depending on the application.
