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UT CS 310 - CMOS Transistors and Boolean Logic Gates

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University of Texas at Austin CS310H - Computer Organization Spring 2010 Don FussellCMOS Transistors and BooleanLogic GatesUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 2CMOS TransistorsNeed circuits to represent 2 discrete values1,0 for binary representationsTrue, False for Boolean logicLet high voltage (Vdd) represent 1, or trueLet low voltage (0 volts or gnd) represent 0, or falseIf we have some switches to control whether or not thesevoltages can propagate through a circuit, we can build acomputer with themNote, the earliest digital computers were electromechanical, madeout of relays, so this is hardly a new ideaOur switches will be CMOS transistorsUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 31 (Vdd)Two kinds of transistorsN-type1 (Vdd)P-type1 (Vdd)1 (Vdd)sourcedraingatesdgsdgsdgUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 40 (gnd)Two kinds of transistorsN-type0 (gnd)P-type0 (gnd)0 (gnd)sdgsdgsdgsourcedraingUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 5How they work as switchesN-typen+ n+p-type bodyWLtoxSiO2 gate oxide(good insulator, !ox = 3.9)polysilicongate+-Vgs = 0n+ n++-Vgdp-type bodybgsdWhen gate is not at higher voltage than source• no excess electrons in channel under gate• so no current can flow• switch is openUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 6How they work as switchesN-typeWhen Vgs > Vth ,the threshold voltage• excess electrons attracted into channel• current flows and switch is closed• drain voltage cannot be more than source voltage = Vg-Vth• this is at most Vdd-Vth• Vdd-Vth is still considered a 1, but a weak 1• if source voltage is 0, then drain voltage is too, so 0 still strong+-Vgs > Vtn+ n++-Vgd = Vgs+-Vgs > Vtn+ n++-Vgs > Vgd > VtVds = 00 < Vds < Vgs-Vtp-type bodyp-type bodybgsdbgsdIdsCMOS transistor pictures fromUT ECE VLSI course slidesUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 7CMOS circuit rulesNever create a path from Vdd to gndDon’t pass weak valuesN-type transistors pass weak 1’s (Vdd - Vth)N-type transistors pass strong 0’s (gnd)Use N-type transistors only to pass 0’s (n to negative)Conversely for P-type transistorsPass weak 0’s (Vth), strong 1’s (Vdd)Use P-type transistors only to pass 1’s (p to positive)Never leave a wire undrivenMake sure there’s always a path to Vdd or gndUniversity of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 8Example CMOS gate - inverter0110OutInTruth tableCircuitNote how all 3 design rules are obeyedCircuit amplifies weak input 1 or


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