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  Concept of MOS Transistor and its Operating modes

The MOS transistor:

The most fundamental element in the design of large scale integrated circuit is the transistor. For the processes we will illustrate, the type of transistor accessible is the Metal Oxide Semiconductor Field Effect Transistor (or MOSFET). Such transistors are formed as a “sandwich'' comprising of a semiconductor layer, generally a slice, or wafer, from a single crystal of the silicon; a layer of silicon dioxide (that is, the oxide) and a layer of metal. Such layers are patterned in a way that allows transistors to be made in semiconductor material (that is, the “substrate''); a diagram exhibiting a typical (or idealized) MOSFET is as shown in figure below. Silicon dioxide is an excellent insulator, therefore a very thin layer, usually only a few hundred molecules thick, is needed. In reality, the transistors which we will employ do not use metal for their gate regions; however rather use polycrystalline silicon. Polysilicon gate FET's have substituted virtually all of the older devices employing metal gates in large scale integrated circuits. (Both polysilicon and metal FET's are sometimes termed to as IGFET's --- insulated gate field effect transistors, as the silicon dioxide beneath the gate is an insulator).

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The transistor comprises of three regions, the “source'', the “gate'' and the “drain''. The region as the gate region is in fact a “sandwich'' comprising of the underlying substrate material, that is a single crystal of semiconductor material (generally silicon); a thin insulating layer (generally silicon dioxide); and an upper metal layer. The electrical charge or current, can flow from the source to drain based on the charge applied to gate region. The semiconductor material in source and drain region are “doped'' with a different kind of material than in the region beneath the gate, therefore an NPN or PNP type structure exists among the source and drain region of a MOSFET. If the source and drain regions are doped with N-type material and the substrate doped with P-type material. Then such a transistor is termed as N channel MOSFET. When they were doped with P-type material, and substrate doped with the N-type material then the device would be termed as P channel MOSFET.

Operating modes:

•    Accumulation
•    Depletion
•    Inversion

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