A MOSFET, or metal oxide semiconductor field effect transistor, is one of the most important devices in modern electronics. It acts like an electrically controlled switch or variable resistor, using a gate voltage to control current between the source and drain. MOSFETs are the building blocks of digital logic, memory, power converters, and microprocessors.
Their importance comes from the fact that they can switch very fast while using very little control power.
In an N-channel MOSFET, a thin insulating oxide separates the gate from the semiconductor body, so almost no steady current flows into the gate. When the gate voltage is high enough, its electric field attracts electrons into the channel region and forms a conducting path from source to drain. Enhancement-mode MOSFETs are normally off and need gate voltage to create a channel, while depletion-mode MOSFETs already have a channel that gate voltage can weaken or shut off.
In digital circuits, this control lets a small voltage signal represent a 0 or 1 and drive switching action in logic gates.
Key Facts
- Gate current is ideally zero because the gate is insulated by the oxide layer.
- An N-channel enhancement MOSFET turns on when VGS > Vth.
- Drain current flows from drain to source by conventional current, while electrons move from source to drain.
- Ohmic region condition for an N-channel MOSFET: VDS < VGS - Vth.
- Saturation region condition for an N-channel MOSFET: VDS >= VGS - Vth.
- In ideal saturation, ID = 1/2 k(VGS - Vth)^2, where k depends on device geometry and materials.
Vocabulary
- MOSFET
- A MOSFET is a transistor that uses an electric field from an insulated gate to control current between source and drain.
- Gate
- The gate is the control terminal that creates an electric field across the oxide layer to turn the channel on or off.
- Source
- The source is the terminal that supplies charge carriers to the channel in normal operation.
- Drain
- The drain is the terminal that collects charge carriers after they pass through the channel.
- Threshold voltage
- Threshold voltage is the minimum gate-to-source voltage needed to form a conducting channel in an enhancement-mode MOSFET.
Common Mistakes to Avoid
- Treating the gate like a normal current input is wrong because the oxide layer blocks steady DC gate current in an ideal MOSFET.
- Confusing VGS with VDS is wrong because VGS controls whether the channel forms, while VDS pushes current through the formed channel.
- Assuming every MOSFET is normally off is wrong because enhancement-mode devices are normally off, but depletion-mode devices can conduct with VGS = 0.
- Ignoring the body or substrate is wrong because the body affects threshold voltage, parasitic diodes, and how the MOSFET must be connected in real circuits.
Practice Questions
- 1 An N-channel enhancement MOSFET has Vth = 2.0 V. If VGS = 5.0 V, what is the overdrive voltage VGS - Vth, and is the channel formed?
- 2 A MOSFET has k = 0.020 A/V^2 and operates in ideal saturation with VGS = 4.0 V and Vth = 1.0 V. Use ID = 1/2 k(VGS - Vth)^2 to find ID.
- 3 Explain why a MOSFET can be used as a logic switch even though almost no steady current flows into its gate.