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notes-326-set3

notes-326-set3 - ELEC 326 Kartik Mohanram Handed out on...

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ELEC 326 Kartik Mohanram Handed out on Thursday, August 27 th , 2009 1 Physical properties of gates Over the next 1–2 lectures, we will discuss some of the physical characteristics of integrated circuits. It reviews and expands on material covered in Elec 220 and introduces concepts from ELEC 261/305 as well. We will review the structure of MOS transistors, develop a very simple structural model for gates that demonstrates how they work, introduce some special types of gates, including transmission, tri-state, and open collector gates, discuss some of the differenced between real and ideal gates, discuss some of the physical properties of gates, understand how timing, voltage, and current properties of real gates are described, look at analysis and synthesis of gate networks, and conclude with an introduction to Verilog and its use in specifying gate network behavior. Use this list as a starting point to explore chapters 2, 3, and 4 in the textbook. 2 MOSFET transistors Metal-oxide-semiconductor field effect transistors (MOSFETs) are the transistors most widely used in integrated cir- cuits today. The name is due to: the device structure – a sandwich of a metal conductor, an oxide insulator, and a semiconductor substrate and the way it works – an electric field controls the flow of current through the device. Although early MOSFET transistors used metal for the first layer, current ones use a polysilicon material (a conductor with somewhat more resistance than a conductor), which is easier to fabricate 2.1 n-Channel MOSFET transistors With no voltage between the gate terminal and the substrate, there are two junctions between the two n regions and the p region. This acts like two oppositely connected diodes, and no current can flow between the source and the drain. Application of a positive voltage between the gate terminal and the substrate creates an electric field that drives holes out of the region under the gate, creating a channel of n-type material that connects the source and drain terminals. 1
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Current is due to electron movement Tap analogy Sub-threshold, linear, and saturation regions of operation Standard notation that you will encounter includes supply voltage V DD , gate-to-source voltage V GS , drain-to- source voltage V DS , and threshold voltage V T , n 2.2 p-Channel MOSFET transistors The p and n regions are reversed from the n-Channel device. Application of a voltage on the gate terminal that is negative relative to the substrate creates a p channel beneath the gate and charge flow is due to hole movement. 2
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2.3 Complementary MOSFETS (CMOS) n-channel and p-channel transistors can be fabricated on the same substrate as shown above The following symbols are used to represent MOSFET transistors in circuit diagrams: Gate Terminal Drain Terminal Source Terminal n -Channel MOSFET Symbol Gate Terminal Drain Terminal Source Terminal p -Channel MOSFET Symbol
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