12-02-09-09-09-34-1257-mugaa08.pdf - UNIT II MOSFET...

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UNIT II MOSFET TRANSISTORS Contents: 2.1 Basic MOSFET structure 2.2 MOSFET operation 2.3 Types of MOSFET 2.4 Modes of operation 2.5 MOS Capacitor 2.6 MOSFET SPICE model 2.7 CMOS 2.8 CMOS Transfer characteristic 2.9 Voltage Transfer curve 2.10 Latchup problem and prevention Objective: To define and draw the symbols for MOSFET. To understand the formation of conducting channel between source and drain of MOSFET. To write the equations governing the output and transfer characteristics of n and p-channel MOSFETs. To determine the drain current in the three operational modes of MOSFET. To find the capacitance of a MOS capacitor. To derive the I-V relationship of MOSFET. To list MOSFET SPICE parameters and their relationship with MOSFET terms. To illustrate the important characteristics of CMOS devices and its working principle. To plot the CMOS transfer characteristic and voltage transfer curve. To describe the parasitic structure which disturbs proper functioning of the circuits leading to even destruction. 2.1 Basic MOSFET STRUCTURE MOSFET(Metal Oxide Semiconductor Field Effect Transistor). In this section, we'll study the basic structure of MOSFET. The metaloxidesemiconductor field-effect transistor(MOSFET, MOS-FET, or MOS FET) is a device used for amplifying or switching electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The
channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common. The 'metal' in the name is now often a misnomer because the previously metal gate material is now often a layer of polysilicon (polycrystalline silicon). Aluminium had been the gate material until the mid 1970s, when polysilicon became dominant, due to its capability to form self-aligned gates. Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates. IGFET is a related term meaning insulated-gate field-effect transistor, and is almost synonymous with MOSFET, though it can refer to FETs with a gate insulator that is not oxide. Another synonym is MISFET for metalinsulatorsemiconductor FET. 2.2 Types of MOSFETMOSFETs are divided into two types viz. p-MOSFETand n-MOSFETdepending upon its type of source and drain. Fig. 3.21: p-MOSFET Fig. 3.22: n-MOSFET Fig. 3.23: c-MOSFETThe combination of a n-MOSFETand a p-MOSFET(as shown in figure 3.21) is called cMOSFETwhich is the mostly used as MOSFET transistor.Circuit symbols A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at

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