Grades 9–12 · Computing
How can a transistor switch without moving parts?
A transistor can control current without opening a mechanical contact. In a field-effect transistor, an electric field changes how easily charge can travel through a semiconductor. One important example is the enhancement n-channel MOSFET, a building block of many electronic circuits.
In the diagramAn electric field creates a conducting channelSimplified n-channel MOSFET cross-sectionGate: 2 V relative to sourceInsulatorSourceDrainSilicon body−Channel formed: a drain–source path conducts.Gate stays insulated • geometry and voltages are illustrative
Its gate is a conductor separated from the semiconductor by an insulating layer. Applying a suitable positive voltage between gate and source attracts electrons toward the region below the gate. Enough attraction creates a conducting channel between source and drain. With a drain–source voltage also applied, charge can move through that path. The gate does not swing like a door, and electrons do not need to flow continuously through its insulator to hold the ideal state.
The diagram represents this behavior with an invented threshold: a gate–source voltage above 1.5 volts turns the ideal path on. Real current changes continuously and depends on the device and its voltages; an actual off transistor also has some leakage. The simple switch picture captures control, rather than every electrical detail.
By connecting many transistors, engineers build logic gates, memory and processors. Their arrangement makes one set of electrical signals control another, so the physical motion of charge carries out operations on encoded information.
About this explanation
Starting knowledge: Understand voltage, current, conductors, insulators and the idea of an electric field.
Sources
- Intel — The Transistor, Explained
- Toshiba Electronic Devices & Storage — Structure and Operation of MOSFET