How does a transistor control the flow of electric current?
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.
A transistor controls the flow of electric current by acting as a switch or an amplifier, using a small input signal to regulate a much larger current between two other terminals. The mechanism depends on the type of transistor:12
Bipolar Transistors
In a bipolar transistor, current flows between the emitter and collector. By applying a small input voltage or current to the base, the device modulates the flow of charge carriers (electrons or holes) across the junctions. Because the base region is very narrow, most carriers injected from the emitter pass through the base to the collector, allowing a small base current to control a significantly larger collector current.32
Field-Effect Transistors (FETs)
Field-effect transistors, such as the MESFET or JFET, function as voltage-controlled resistors. They consist of a conductive channel between a source and a drain. A third terminal, the gate, creates a depletion region that expands or shrinks based on the applied gate voltage. This expansion or contraction changes the channel's cross-sectional area, thereby increasing or decreasing the current flow between the source and drain.4