ELEC 326Kartik MohanramHanded out on Thursday, August 27th, 20091 Physical properties of gatesOver the next 1–2 lectures, we will discuss some of the physical characteristics of integrated circuits. It reviews andexpands 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 transistorsMetal-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 conductorwith somewhat more resistance than a conductor), which is easier to fabricate2.1 n-Channel MOSFET transistors• With no voltage between the gate terminal and the substrate, there are two junctions between the two n regionsand 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 drivesholes out of the region under the gate, creating a channel of n-type material that connects the source and drainterminals.1• 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 VDD, gate-to-source voltage VGS, drain-to-source voltage VDS, and threshold voltage VT,n2.2 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 beneaththe gate and charge flow is due to hole movement.22.3 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 TerminalDrain TerminalSource Terminaln-ChannelMOSFET SymbolGate TerminalDrainTerminalSource Terminalp-ChannelMOSFET Symbol• The ideal normally closed switches that we will encounter soon are models for the MOSFET transistorsNormally Closed SwitchControlTerminal CDataTerminalDataTerminalGateTerminalDrainTerminalSourceTerminalGateTerminalDrainTerminalSourceTerminalNormally Open SwitchControlTerminal CDataTerminalDataTerminal3 GatesA gate can be thought of as a simple electronic circuit (a system) that realizes a logical operation.• The direction of information flow is from the input terminals to the output terminal.• The number of input and output terminals is finite and they carry binary-valued signals (i.e, 0 and 1). More onthis when we discuss noise margins in digital circuits.• The transformation of input signals to output signals can be modeled as a logical operation.3.1 Truth tables• Since there is a finite number of input signal combinations, we can represent the behavior of a gate by simplylisting all of its possible input configurations and the corresponding output signal. Such a list is called a truthtable.3• The use of the symbols H/1 and L/0 usually correlates with the high and low voltages and is the positive logicconvention that we will use for the rest of this course.• Recall the standard gates that you encountered in 220, their symbols, and their truth tables.• Gates with more than 3 inputs:– And gates: The output is 1 if and only if ... ?– Or gates: The output is 1 if and only if ... ?– Nand gates: The output is 0 if and only if ... ?– Nor gates: The output is 0 if and only if ... ?– Exclusive-or (XOR) gates: The output is 1 if and only if ... ?– Equivalence (XNOR) gates: The output is 1 if and only if ... ?– Comments on the logical symbols and logical connectives.3.2 Exercise:• Determine howmany differenttwo-input gates there can be. In other words, how many unique Boolean functionsof two inputs can you construct?• How many different three-input gates? Alternately, how many unique Boolean functions of three inputs can youconstruct?• Generalize this4 Building a structural model for gatesThis section presents a very simple structural model of gates based on an idealized device called a logic switch.4.1 Logic switches• Definition: A logic switch is a three-terminal device that is used to control the connection of two points in acircuit.Normally Closed SwitchNormally Open SwitchControlTerminal CDataTerminalDataTerminalControlTerminal CDataTerminalDataTerminal• Switch states:– Closed: Switch conducts between its data terminals.– Open: No conduction between the data terminals.– When C=0, the switch is in its normal state.– When C=1, the switch is in its active or on state.– When C=1, the arm is pulled towards the C input.4• Ideal switch assumption:– 0 resistance when closed, ∞ resistance when open.– 0 delay switching between open and closed.• Real electronic switches are implemented with transistors, and they fail to meet the ideal switch assumptions inthat:– Resistance in the open state is high, but not infinite.– Resistance in the closed state is low, but not zero.– Time required to change states is greater than zero.4.2 Exercise:• The following switch network conducts between its two terminals just in case both the signals X and Y are 1.XY• Design three two-terminal networks, each with two control signals X and Y, such that the networks conduct onlyif:– Network 1: X is 1 or Y is 1.– Network 2: X is 1 and Y is 0.– Network 3: X and Y are equalLet us look at switch implementations of inverters, nands, and nors.4.3 General