LectureNotes_6 - Metal Oxide Semiconductor Field Effect...

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Metal Oxide Semiconductor Field Effect Transistors Recall that the bipolar junction transistor (BJT) is a current controlled device; that is the base current controls the amount of collector current. The FET is a voltage controlled device in which the voltage at the gate terminal controls the amount of current through the device. The MOSFET uses an insulated gate to control conduction in the channel. The insulation is an extremely thin layer (<1 m) of glass (typically (SiO 2 ). The figure below is an over view of the FET family showing the various types available. Field-effect Transistors Junction (JFET) Insulated gate (MOSFET) Depletion Depletion Enhancement p -channel n -channel p -channel n -channel p -channel n -channel The enhancement-type MOSFET is the most widely used FET. We will study its structure and physical operation. Device Structure The figure below shows the physical structure of the n -channel enhancement-type MOSFET. The transistor is fabricated on a p-type substrate. The substrate provides physical support for the device Two heavily doped n-type regions (source and drain are created in the substrate. A thin layer of silicon dioxide (SiO2) of thickness t ox (typically 2-50 nm), an excellent electrical insulator is grown on the surface of the substrate, covering the area between the source and the drain regions.
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Metal is deposited on top of the oxide layer to form the gate electrode of the device. Metal contacts are also made to the source region, the drain region and the substrate (also known as the body ). The name of the device (metal-oxide-semiconductor FET) is derived from its physical structure. However the name has become a general one and is used also for FETs that do not use metal for the gate electrode. Most modern MOSFETs are fabricated using a process known as silicon-gate technology in which a certain type of silicon, called polysilicon, is used to form the gate electrode. Observe that the substrate forms pn junctions with the source and drain regions. In normal operations these pn junctions are kept reverse-biased at all times.
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Since the drain will be at a positive voltage relative to the source, the two pn junctions can be effectively cut off by simply connecting the substrate terminal to the source terminal. Assume above to be the case in the description of the MOSFET operation. Therefore the substrate will be considered as having no effect on device operation and the device will be treated as a three terminal device. Operation with No Gate Voltage With no bias voltage applied to the gate, two back-to-back diodes exist in series between drain and source. One diode is formed by the pn junction between the n+ drain region and the p- substrate and the other is formed by the pn junction between the p-type substrate and the n+ source region. These back-to-back diodes prevent current conduction from drain to source
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This note was uploaded on 02/23/2011 for the course ELETRONICS P24L taught by Professor Learymyers during the Spring '11 term at University of the West Indies at Mona.

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LectureNotes_6 - Metal Oxide Semiconductor Field Effect...

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