Metal oxide effect transistors (MOSFETs), are one of the most popular and common types of transistors found in modern electronics. They are often numbered in the billions in today's modern CPUs and integrated circuits. A MOSFETs can be categorized as a voltage controlled switch. They can also be used to perform logic operations and to create amplifiers. They are the basic building block used to create some of the most powerful computers, data processing and computational machines ever created.  MOSFETs are popular because they are relatively cheap to build in mass quantities on silicon (Si) wafers.

A MOSFETs consists of a metal gate sitting on top of an insulator known as an oxide. Silicon dioxide or SiO₂ has been a common oxide used in previous process technologies. A MOSFETs also has two n-type wells, which contain doped n-type material embedded in a p-type silicon substrate. There are also p-type MOSFETs as well, where the well type and substrate are just polar opposite than that of the n-type MOSFETs.  A length, known as the channel length, which is a key dimension in a MOSFETs, physically separates the two n-type wells. The two wells are called the drain and source.  This separation between the two n-wells is where an electric field is formed when a voltage is applied to the metal gate.  Once the voltage is applied to the gate an electric field is formed and creates an inversion such that positive charge appears on the metal gate, and negative charge is lined up along the channel. Once this inversion is created by the electric field and negative charges are built up along the channel length, a conduction channel is established, and now current or electrons can flow from the drain to the source.  In this state the transistor is on, like a switch, allowing current to flow, which previously would not allow the flow of current. This is why MOSFETs are considered a voltage-controlled switch.  The current that flows is called Ids. Once a voltage known as Vgs is removed from the gate, the channel will close off, the current will not conduct and the transistor is off.

MOSFETs do not behave a as perfectly linear device and have different states of operation. Depending on the use and application, different states can be desirable or undesirable.   A common state is called saturation, where regardless of applying more voltage to the gate, the device will not conduct any more current.