EC-6.7 The MOS Capacitor and the MOSFET
You can explain what happens under the gate as its voltage changes, say what the threshold voltage is, and describe the transistor's two operating regions and what scaling does to them.
Before:EC-1. Circuit AnalysisEC-5. Physics and Materials for ElectronicsUnlocks:S4-1. Analog CircuitsS5-4. Linear and Digital Integrated CircuitsPE1-1. VLSI TechnologyPE1-2. Fiber Optic CommunicationPE2-1. CMOS Analog and Digital IC DesignEC-12. Electrical Energy, Machines and Power Electronics
Almost every transistor ever manufactured is this one, so it is worth meeting properly rather than as a symbol. The device is a capacitor whose lower plate is semiconductor, and the whole trick is that changing the plate voltage changes what the semiconductor underneath is: first emptied of its own carriers, then populated with the opposite kind. That inverted layer is the channel, and the threshold is simply the voltage at which it appears. Once that is clear the two operating regions are obvious rather than memorised.
Work through these
A capacitor with a semiconductor plate, and the three states underneath
Depending on gate voltage the surface under the oxide accumulates its own majority carriers, is depleted of them, or inverts to the opposite type. Those three states are the whole device, and the third one is the useful one.
NPTEL: Introduction to Semiconductor Devices · CourseThreshold voltage, and what it actually is
The threshold is the gate voltage at which the inverted layer becomes strong enough to conduct meaningfully between source and drain. It depends on the oxide thickness, the doping and the gate material, all of which are process choices.
The channel, and why the transistor is a voltage-controlled device
The gate is separated from the channel by an insulator, so it draws no steady current and controls purely by its field. That is the fundamental difference from the bipolar device and the reason digital logic uses this one.
NPTEL: Solid State Devices · CourseLinear and saturation regions, and where pinch-off comes from
At low drain voltage the channel behaves as a controlled resistor, and as drain voltage rises the channel narrows at the drain end until it pinches off and current stops increasing. Which region a circuit uses depends on whether it is switching or amplifying.
MIT OpenCourseWare 6.012: Microelectronic Devices and Circuits · CourseThe body effect, and what happens when transistors are stacked
A source held above the body raises the threshold, so a transistor sitting on top of another conducts less well than the same device alone. This is why a logic gate with many stacked inputs is slower than one with few.
Scaling, and the effects that stop being negligible when the device shrinks
Making everything smaller was straightforward for decades, and then leakage through the gate, the threshold falling with channel length, and velocity saturation each stopped being ignorable. This is where the fifth-semester integrated-circuit option picks the device up.
Sign in to keep your progress.
Free resources
We haven't checked most of these for screen reader use yet.
Links last checked 3 Sept 2026.
Stuck here?
Ask a mentor. A real person answers, and they can see exactly which topic you're on. Usually within a couple of working days.
Checking your session…
Topics shown in module order.