MOS transistor and its manufacturing technology

The MOS transistor is by far the most widespread device in today's semiconductor production, being the basic component of CMOS (Complementary MOS) technology, which alone accounts for over 70% of the world's integrated circuit production.

Several acronyms, with varying degrees of justification, are used in the literature to describe the MOS (Metal Oxide Semiconductor) transistor: MOSFET (MOS Field Effect Transistor), IGFET (Insulated Gate Field Effect Transistor) and MOST (Metal Oxide Semiconductor Transistor).

The MOSFET transistor is characterized by the fact that the gate, through the electric field effect, controls the carrier density in the device channel, and thus the current intensity in the channel. The channel is connected on either side to two highly doped regions, between which a voltage is applied to cause the current to flow. This transistor is just one element in a larger family of FETs (Field Effect Transistors) that use the field effect to modulate current intensity in a channel. In fact, there are three groups in the FET family:

• the IGFET, whose field effect comes from a gate insulated from the channel by a thin dielectric; currently, the dielectric used almost exclusively is SiO₂ oxide, hence the name MOSFET ; however, it should be noted that other dielectrics, such as Si₃N₄, a SiO₂/Si₃N₄ stack or a SiO₂/Ta₂O₅ stack, can also be used;

• the JFET (Junction FET), whose field effect comes from a reverse-biased PN junction; if the P region is the gate, it modulates the thickness of the channel (N region) and, therefore, its current by extending the space charge zone under the effect of the junction's reverse bias;

• MESFET (Metal Semiconductor FET), whose field effect comes from a reverse-biased Schottky junction (metal-semiconductor), with the metal gate operating in a similar way to the JFET gate.

In this article about the MOS transistor (IGFET or MOSFET), we'll look at how it works and the technology involved in its manufacture. Operation is described at three levels of difficulty:

• a qualitative level, giving a good intuition of the transistor's operating principle;

• a "simplified model" level, to describe the operation of an ideal transistor;

• a "complete model" level, to understand all parasitic and secondary effects, and analyze their impact on transistor characteristics.

The technology section covers all the elements of transistor architecture and their technological realization. Various transistor architectures (e.g. surface channel, buried channel, heterostructure channel, etc.) are detailed, as well as the assembly of a complete technology.