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3) Transistor MOSFET
A MOSFET transistor is a substrate of doped semiconductor material in which, by diffusion of doping techniques, creating two islands separated by opposite type an area on which grows a dielectric layer topped by a layer of conductor. MOSFET transistors are divided into two main types depending on how you made doping:

• nMOS Type: p-type substrate n-type diffusions

• pMOS type, substrate type and n-type diffusions p.

dissemination areas are called source (source) and drain (drain), and between them is the door (gate).
The MOSFET transistor has three operating states:

cutting State

When the gate voltage is identical to that of the substrate, the MOSFET is in a state of non-conduction: no current flows between source and drain while applying a potential difference between them. Also called the isolated mosfet junction of two components.

linear Driving

Al polarize the door with a negative voltage (pMOS) or positive (nMOS) creates a region of deplexing in the region separating the source and drain. If this stress becomes large enough, appear minority carriers (electrons in pMOS, nMOS voids) in the region deplexing that will result in a raceway. The transistor then passes conducting state, so that a potential difference between source and drain will result in a current. The transistor behaves as a resistance controlled by the gate voltage.

saturation tension When between drain and source exceeds a certain limit, the conduction channel under the door suffered a bottleneck in the vicinity of the drain and disappears. The current between source and drain is not interrupted, as it is due to the electric field between the two, but is independent of the potential difference between the two terminals.

Mathematical models

• For a type induced channel MOSFET in linear region n:

where in which b is the width of the channel, μ n the electron mobility, ε is the electric permittivity of the oxide layer, L the channel length and W the thickness of oxide layer.

• When the transistor operates in the saturation region, the formula becomes this:

These formulas are a simple model of operation of MOSFET transistors, but do not take into account a number of second-order effects, such as:

• speed Saturation: The relationship between gate voltage and drain current does not grow quadratically in short channel transistors.
  
• body Effect: The tension between source and substrate modifies the threshold voltage results in raceway
  
• channel length modulation.

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