University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell CMOS Transistors and Gates

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 2 Simple electronics Ohm’s Law - V = IR voltage (V) equals current (I) times resistance (R) Hydraulic Analogy Charge  liquid Current  flow rate Voltage  water pressure Resistance  related to length and radius of pipe (kL/r 4 ) Hydraulic pictures from

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 3 Hydraulic analogy Voltage  water pressure Current  flow rate Volume flow rate in m 3 /sec, etc.Current flow rate in coulombs/sec = amps

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 4 Hydraulic analogy Resistance  related to length and radius of pipe (kL/r 4 ) Ground  reservoir

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 5 Hydraulic analogy Resistances in series Resistances in parallel

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 6 CMOS Transistors Need circuits to represent 2 discrete values 1,0 for binary representations True, False for Boolean logic Let high voltage (V dd ) 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 CS310 - Computer Organization Spring 2009 Don Fussell 7 1 (Vdd) Two kinds of transistors N-type 1 (Vdd) P-type 1 (Vdd) source drain gate s d g s d g s d g

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 8 0 (gnd) Two kinds of transistors N-type 0 (gnd) P-type 0 (gnd) s d g s d g s d g source drain g

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 9 How they work as switches N-type When gate is not at higher voltage than source no excess electrons in channel under gate so no current can flow switch is open

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 10 How they work as switches N-type When V gs > V th,the threshold voltage excess electrons attracted into channel current flows and switch is closed drain voltage cannot be more than source voltage = V g -V th this is at most V dd -V th V dd -V th is still considered a 1, but a weak 1 if source voltage is 0, then drain voltage is too, so 0 still strong CMOS transistor pictures from UT ECE VLSI course slides

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 11 CMOS circuit rules Never create a path from V dd to gnd Don’t pass weak values N-type transistors pass weak 1’s (V dd - V th ) 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 (V th ), strong 1’s (V dd ) 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 V dd or gnd

University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 12 Example CMOS gate - inverter InOut Truth table Circuit Note how all 3 design rules are obeyed Circuit amplifies weak input 1 or 0