How does a MOSFET transistor turn ON?

In an enhancement-mode NMOS transistor, applying a positive gate voltage (VGS) above the threshold voltage (VTH) creates an electric field that attracts minority electrons to the silicon surface directly under the gate dielectric. This forms a conductive n-type inversion channel connecting the source and drain, allowing current to flow when a drain-to-source voltage (VDS) is present.

What is the function of the gate oxide in a MOSFET?

The gate oxide is an ultra-thin insulating dielectric layer (such as silicon dioxide or high-k hafnium dioxide) positioned between the gate electrode and the silicon substrate. It prevents DC electrical current from leaking into the gate while allowing the vertical electric field to control channel conductivity.

What is the difference between conventional current and electron flow in an NMOS?

Negatively charged electrons drift from the source into the channel and toward the positively biased drain. By engineering convention, conventional electrical current (ID) is defined in the opposite direction of negative charge movement, flowing from the drain into the source.

Interactive 3D MOSFET Explorer — Learn Transistor Structure

👆 Drag to rotate • Pinch to zoom • Click parts to inspect

Components:

⚡ Gate Voltage (VGS) Control ● ON — Channel Formed

Gate-to-Source Voltage (VGS) 1.20 V

Vth = 0.7V

0.0 V (Cutoff) 1.0 V 2.0 V (Strong)

ON State: VGS > VTH (1.20V > 0.70V). An electric field pulls mobile electrons into the channel, bridging Source and Drain. Current flows under drain-to-source bias.

Gate (G)

Control Electrode

💡 Simple Explanation

The gate controls whether a conductive path forms between the source and drain, acting like an electrical valve handle.

🔬 Device Physics

In an NMOS transistor, applying a sufficiently positive gate-to-source voltage (VGS ≥ VTH) induces a vertical electric field across the gate dielectric, creating an n-type electron inversion channel.

Material:	Polysilicon / Metal
Role:	Controls channel conductivity
Current (I):	Near-zero DC leakage (IG ≈ 0)

How a MOSFET Works

The fundamental principle of field-effect switching in three steps:

Apply Voltage to the Gate

The gate electrode is isolated from the silicon substrate by an ultra-thin insulating dielectric (gate oxide). Applying a positive voltage (VGS) establishes a vertical electric field across this dielectric.

Inversion Channel Forms

When VGS exceeds the threshold voltage (VTH), the electric field repels positive holes into the substrate and attracts mobile electrons to the surface, creating an n-type inversion layer connecting Source and Drain.

Current Flows Under Drain Bias

With the conductive electron bridge active, applying a voltage between Drain and Source (VDS) causes electrons to drift across the channel, sustaining conventional drain current (ID) from Drain to Source.

Educational & Scientific Note: This interactive 3D model illustrates simplified enhancement-mode NMOS operation. The threshold voltage (VTH = 0.7 V) is an illustrative textbook value chosen for clarity; real-world transistors exhibit threshold voltages determined by gate dielectric thickness, work function engineering, channel doping, and operating temperature (e.g. 0.2 V – 0.4 V in modern FinFET/GAA technologies). This tool is a conceptual learning model and does not compute quantitative SPICE current-voltage equations.

How a MOSFET Works

Apply Voltage to the Gate

Inversion Channel Forms

Current Flows Under Drain Bias

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