115A_1_EE115A_W10-Chap6

115A_1_EE115A_W10-Chap6 - Chapter 6 Physics of MOS...

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Chapter 6 Physics of MOS Transistors h 6.1 Structure of MOSFET h 6.2 Operation of MOSFET 273 h 6.3 MOS Device Models h 6.4 PMOS Transistor h 6.5 CMOS Technology h 6.6 Comparison of Bipolar and CMOS Devices
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Chapter Outline CH 6 Physics of MOS Transistors 274
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Metal-Oxide-Semiconductor (MOS) Capacitor CH 6 Physics of MOS Transistors 275 h The MOS structure can be thought of as a parallel-plate capacitor, with the top plate being the positive plate, oxide being the dielectric, and Si substrate being the negative plate. (We are assuming P-substrate.)
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Structure and Symbol of MOSFET CH 6 Physics of MOS Transistors 276 h This device is symmetric, so either of the n+ regions can be source or drain.
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State of the Art MOSFET Structure CH 6 Physics of MOS Transistors 277 h The gate is formed by polysilicon, and the insulator by Silicon dioxide.
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Formation of Channel CH 6 Physics of MOS Transistors 278 h First, the holes are repelled by the positive gate voltage, leaving behind negative ions and forming a depletion region. Next, electrons are attracted to the interface, creating a channel (“inversion layer”).
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Voltage-Dependent Resistor CH 6 Physics of MOS Transistors 279 h The inversion channel of a MOSFET can be seen as a resistor. h Since the charge density inside the channel depends on the gate voltage, this resistance is also voltage-dependent.
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Voltage-Controlled Attenuator CH 6 Physics of MOS Transistors 280 h As the gate voltage decreases, the output drops because the channel resistance increases. h This type of gain control finds application in cell phones to avoid saturation near base stations.
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MOSFET Characteristics CH 6 Physics of MOS Transistors 281 h The MOS characteristics are measured by varying V G while keeping V D constant, and varying V D while keeping V G constant. h (d) shows the voltage dependence of channel resistance.
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L and t ox Dependence CH 6 Physics of MOS Transistors 282 h Small gate length and oxide thickness yield low channel resistance, which will increase the drain current.
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CH 6 Physics of MOS Transistors 283 h As the gate width increases, the current increases due to a decrease in resistance. However, gate capacitance also
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This note was uploaded on 01/08/2011 for the course EE 115 taught by Professor Kaiser during the Spring '09 term at UCLA.

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115A_1_EE115A_W10-Chap6 - Chapter 6 Physics of MOS...

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