MOSFET Basics and Functionality

A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a fundamental building block in modern electronics, especially in digital circuits like microprocessors and memory, and also in analog applications. It is a voltage-controlled device where the voltage applied to the gate terminal controls the current flow between the source and drain terminals.

MOSFET as a Capacitor

The structure of a MOSFET inherently includes various capacitances. These capacitances are formed between different terminals due to the physical separation of conductors by a dielectric (insulating) material, primarily the gate oxide. The most significant capacitances within a MOSFET include:

• Gate-Source Capacitance (\(C_{GS}\)): The capacitance between the gate and the source terminal.
• Gate-Drain Capacitance (\(C_{GD}\)): The capacitance between the gate and the drain terminal.
• Gate-Bulk Capacitance (\(C_{GB}\)): The capacitance between the gate and the substrate (bulk) terminal.

The overall gate capacitance of a MOSFET is a combination of these components, and its value is not constant but varies depending on the operating conditions, particularly the voltage applied to the gate.

Voltage Controlled Capacitor Mechanism

The core reason a MOSFET can function as a voltage-controlled capacitor lies in how the gate voltage (\(V_{GS}\)) influences the charge distribution and the effective distance between the gate and the channel/substrate. Let's consider an N-channel MOSFET for explanation:

• Accumulation Region (\(V_{GS}\) < \(V_{FB}\)): When a negative voltage is applied to the gate (or voltage less than flat-band voltage, \(V_{FB}\)), holes accumulate at the silicon-oxide interface. In this region, the gate capacitance is effectively the capacitance of the gate oxide, \(C_{ox}\), in parallel with the capacitance of the bulk silicon. The capacitance remains relatively constant.
• Depletion Region (\(V_{FB}\) < \(V_{GS}\) < \(V_{T}\)): As the gate voltage increases, holes are pushed away from the interface, creating a depletion region. The width of this depletion region increases with increasing gate voltage. Since capacitance is inversely proportional to the distance between the plates (\(C = \frac{\epsilon A}{d}\)), the increasing depletion width effectively increases the distance, thus reducing the overall capacitance between the gate and the bulk.
• Inversion Region (\(V_{GS}\) > \(V_{T}\)): When the gate voltage exceeds the threshold voltage (\(V_{T}\)), an inversion layer (channel) forms. The channel is now a conductive path, and the capacitance is primarily between the gate and this inversion layer. In strong inversion, the capacitance again approaches the gate oxide capacitance (\(C_{ox}\)) as the channel acts as an extension of the source/drain.

Because the effective capacitance between the gate and the semiconductor (channel/bulk) changes significantly with the applied gate-to-source voltage (\(V_{GS}\)), the MOSFET effectively acts as a capacitor whose capacitance value can be controlled by varying the gate voltage. This property makes MOSFETs useful in various analog circuit applications, such as voltage-controlled oscillators (VCOs), tuners, and filters, where tunable capacitance is required.