Microelectronic Transistors: Basics,Structure And Operation Of Mosfet

4. Microelectronic Transistors

4.a Structure and Operation of a MOSFET

The metal-oxide semiconductor field effect transistor (MOSFET) is one of the most common types of transistor in microelectronic digital circuits. Properly designed MOSFET circuits use very little power and are economical to fabricate. As shown in the below figure the field effect transistor has three terminals which are called source, drain and gate. The name metal-oxide-semiconductor field effect transistor stems from its constituent materials. MOSFETs are built upon a crystalline substrate of the doped semiconductor silicon. Pure silicon is very poor conductor, so impurities, such as boron or arsenic, are introduced into the silicon to create an excess of mobile positive or negative charges. Negatively doped (N-doped) silicon contains free electrons that are able to move through the bulk semiconductor. Positively doped (P-doped) silicon are commonly known as holes, which act as positive charges that move freely through the material.

A metal electrode separated from the semiconductor below by an insulating oxide barrier serves as the gate of the MOSFET, whose voltage and associated electric field controls the flow of current from the source to the drain. This is why the device is called a field-effect transistor. Very little current can flow when the voltage on the gate is low, since the region between source and drain contains few mobile negative charges.