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CREA - SEMICONDUCTOR TEST EQUIPMENTS

The Enhancement Mosfet

In a junction field-effect transistor, the effective size of the channel is controlled by an electric field applied to the channel through a pn junction. A basically different field-effect device is obtained by using a metal gate electrode separated by an oxide layer from the semiconductor channel. This metal-oxide-semiconductor (MOS) arrangement allows the channel characteristics to be controlled by an electric field established by applying a voltage between the gate and the body of the semiconductor and trasmitted through the oxide layer. Such a device called a MOSFET or MOS transistor. Its importance is underscored by the fact that more ICs are fabricated with MOS devices than with any other kind of semiconductor device. There are two types of MOS transistors. The depletion MOSFET has a behavior similar to that of the JFET; at zero gate voltage and a fixed drain voltage, the current is a maximum and then decreases with the applied gate potential (of the proper polarity). The second kind of device, called the enhancement MOSFET, exhibits no current at zero gate voltage and the magnitude of the output current increases with an increase in the magnitude of the gate potential. Both types can exists in either the p- or n- channel variety. We consider the charateristics of an n- channel enhancement type in this and the succeeding section.

The Enhancement Mosfet Structure

A simplified structure of an n- channel enhancement MOSFET are commonly referred as NMOS and PMOS transistors.
The region between source and drain is the channel, wich is covered by a thin silicon dioxide (SiO2) layer. The gate is formed by the metal electrode placed over the oxide layer. At present, MOSFET fabrication technology utilizes a polysilicon conducting layer for the gate rather than the metal gate. The physicals principle wich govern MOSFET operation, however, are the same for both types of gate.
The metal area of the gate, in conjunction with the insulating dielectric oxide layer and the semiconductor channel, form a parallel-plate capacitor. The insulating layer of silicon dioxide is the reason why this device is also called the insulated-gate-field effect-transistor (IGFET). This layer results in an extremely high input resistance (10 MOhms to 100.000 MOhms) for the MOSFET.Manufacturers of IC MOS transistor do not provide curves of the volt-ampere characteristics. Where needed or desiderable, these curves are generated from the analytic expressions for MOSFET behavior in each region of operation.

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