The MOSFET

The metal-oxide semiconductor field-effect transistor (MOSFET) is the most fundamental component of the contemporary digital  circuits. Its main advantages are its integration density,low power consumption  and ease of manufacturing which makes it possible to realize large complex circuits in a economical fashion.

Cross section of NMOS transistor             

Fig shows a cross section of a typical n-channel MOS transistor (NMOS) . Heavily doped n-type source and drain regions are implanted into lightly doped p-type substrate(often called the body). A thin layer of silicon dioxide(SiO₂) is grown over the region between the source and drain and is covered by a conductive material most often polysilicon forms the gate of the transistor . Neighbouring devices are insulated using a thick layer of SiO₂ (called field oxide) and a reverse biase d np- diode formed by adding an extra p⁺  region , called the channel stop implant(or field implant).

NMOS can be considered to act as a switch . When a voltage is applied to the gate that is larger then the threshold voltage V_T. , a conducting channel is formed b/w drain and source.

In NMOS transistor , current is carried by electrons moving through an n-type channel b/w source and drain. This is  in contrast with p-n junction , where currnt is carried by both holeds and electrons.

MOS devices made by using an n-type substrate and p⁺ drain and source regions is called a PMOS , in such a device current is carried by holes moving through the p-type channel.

Figure below  shows the circuit symbols for various MOS transistors.

Circuit symbols for MOS transistors

It should be noted that if the fourth terminal is not shown it is assumed that the substrate is connected to the appropriate supply.

First we will analyze the static behaviour of the MOS device using a NMOS device , all the arguments remain valid for PMOS devices as well.