Semiconductor Fundamentals · Topic 11 of 13
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MOSFET
The voltage-controlled switch that makes up modern logic.
What you’ll learn
- The four MOSFET terminals
- How gate voltage forms a channel
- Threshold voltage and switching
Why it matters
The MOSFET is the workhorse transistor — billions of them form a modern processor.
Explanation
A MOSFET (metal-oxide-semiconductor field-effect transistor) has a gate, source, drain, and body. A thin insulating oxide separates the gate from the channel region between source and drain.
When the gate voltage exceeds the threshold voltage (Vth), it attracts carriers to form a conducting channel, turning the device on. Below Vth it is essentially off. Because the gate is insulated, control takes almost no steady current.
Visual explanation
Key terminology
- Gate oxide
- The thin insulator between gate and channel.
- Threshold voltage (Vth)
- Gate voltage at which the channel forms.
- Channel
- The conducting path between source and drain.
Formula
Drain current grows with gate overdrive and the width/length ratio.
Example
Common mistakes
Watch out for:
- Thinking the gate draws current — it's capacitive, drawing current only while switching.
- Confusing NMOS (electron channel) with PMOS (hole channel).
Real-world application
FinFET and gate-all-around (GAA) transistors are 3D evolutions of the MOSFET for advanced nodes.
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MOSFET
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CMOS
Semiconductor Fundamentals
Pairing NMOS and PMOS transistors for low-power digital logic.
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