CMOS Transistors and Gates

1. CMOS Transistors and Gates

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/r4) Hydraulic pictures from http://hyperphysics.phy-astr.gsu.edu University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 2

3. Volume flow rate in m3/sec, etc. Current flow rate in coulombs/sec = amps Hydraulic analogy • Voltage  water pressure • Current  flow rate University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 3

4. Hydraulic analogy • Resistance  related to length and radius of pipe (kL/r4) • Ground  reservoir University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 4

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

6. 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 CS310 - Computer Organization Spring 2009 Don Fussell 6

7. 1 (Vdd) 1 (Vdd) 1 (Vdd) 1 (Vdd) Two kinds of transistors N-type P-type source s gate g drain d s s g g d d University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 7

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

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 9

10. 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 N-type CMOS transistor pictures from UT ECE VLSI course slides University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 10

11. 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 CS310 - Computer Organization Spring 2009 Don Fussell 11

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