CMOS Transistors and Boolean Logic Gates

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CMOS Transistors and Boolean Logic Gates University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell CMOS Transistors Need circuits to represent 2 discrete values 1,0 for binary representations True, False for Boolean logic Let high voltage (Vdd) represent 1, or true Let low voltage (0 volts or gnd) represent 0, or false If we have some switches to control whether or not these voltages can propagate through a circuit, we can build a computer with them Note, the earliest digital computers were electromechanical, made out of relays, so this is hardly a new idea Our switches will be CMOS transistors University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 2 Two kinds of transistors N-type P-type source s 1 (Vdd) 1 (Vdd) gate g drain d s s 1 (Vdd) 1 (Vdd) g g d d University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 3 Two kinds of transistors N-type P-type source s 0 (gnd) 0 (gnd) g g drain d s s 0 (gnd) 0 (gnd) g g d d University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 4 How they work as switches polysilicon gate N-type W tox L SiO gate oxide n+ n+ 2 (good insulator, !ox = 3.9) p-type body When gate is not at higher voltage than source • no excess electrons in channel under gate • so no current can flow • switch is open V = 0 V gs + g + gd - - s d n+ n+ p-type body b University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 5 How they work as switches When 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 V > V gs t V = V + g + gd gs - - s d V = 0 N-type n+ n+ ds p-type body b V > V gs t V > V > V + g + gs gd t - - I s d ds CMOS transistor pictures from n+ n+ 0 < V < V -V UT ECE VLSI course slides ds gs t p-type body b University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 6 CMOS circuit rules Never create a path from Vdd to gnd Don’t pass weak values N-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 transistors Pass weak 0’s (Vth), strong 1’s (Vdd) Use P-type transistors only to pass 1’s (p to positive) Never leave a wire undriven Make sure there’s always a path to Vdd or gnd University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 7 Example CMOS gate - inverter Truth table Circuit In Out 0 1 1 0 Note how all 3 design rules are obeyed Circuit amplifies weak input 1 or 0 University of Texas at Austin CS310H - Computer Organization Spring 2010 Don Fussell 8.

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