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Chapter 12: Transistors Back To Chapter

12.16: Characteristics of MOSFET

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Characteristics of MOSFET

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The essential parameters of MOSFETs include the channel length, the channel width, the oxide thickness, the junction depth, and the substrate doping.

With zero gate voltage, the source-to-drain electrodes form two back-to-back p-n junctions, allowing only a reverse-leakage current from the source to the drain. This is known as the cutoff region.

A positive bias at the gate converts the MOS structure to form a surface inversion layer or n-channel between two n-plus regions.

With minor drain voltage, electrons flow from the source to the drain through the conducting channel, acting as a resistor, with its conductance adjustable via the gate voltage. This is the linear region where the drain current is proportional to the drain-source voltage.

Each MOSFET has a threshold voltage, the minimum gate-source voltage necessary to create a conducting path between the source and the drain.

As the drain voltage increases, it saturates, reducing the inversion layer thickness near the edge of the channel length to zero. This pinch-off point marks the start of the saturation region, where the drain current remains constant despite an increasing drain voltage.

12.16: Characteristics of MOSFET

Metal-oxide-semiconductor field-effect Transistors, or MOSFETs, play a critical role in electronic circuits. They are primarily utilized for amplifying and switching signals.

Various vital parameters influence their functionality, which is crucial for theory and electronics applications. First, channel dimensions, precisely length, and width, are pivotal. The size of these channels affects the transistor's ability to carry current and switching speeds; shorter channels typically enable quicker operation. Next, the oxide thickness, the silicon dioxide layer separating the gate from the channel, modulates the gate's control over the channel. Thinner oxides increase the gate capacitance, enhancing this control.

Another significant factor is junction depth and substrate doping, which adjust the MOSFET's threshold voltage and control leakage currents. Doping modifies the semiconductor's properties by introducing impurities.

Operationally, MOSFETs exhibit three distinct regions based on the gate-to-source voltage.

• In the Cutoff Region, the transistor is off, showing no conductive path between the source and drain, which restricts current to minimal reverse leakage.
• The Linear Region emerges once V_GS exceeds the threshold and V_DS (drain-to-source voltage) is low. Here, the transistor acts like a variable resistor where the drain current ( I_D) is directly proportional to V_DS, modulated by V_GS.
• In the Saturation Region, I_D plateaus despite V_DS increases due to the pinch-off effect, where the channel near the drain narrows, restricting further current flow.

These operating conditions determine how MOSFETs are implemented in circuits, especially where precise electronic control is required.

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Keywords: MOSFET Channel Dimensions Oxide Thickness Junction Depth Substrate Doping Threshold Voltage Cutoff Region Linear Region Saturation Region Drain Current Gate-to-source Voltage Drain-to-source Voltage Electronic Circuits Amplifying Switching

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