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ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems. Day 10: September 29, 2010 MOS Transistors Details. Last Time. Focused on I vs V relationships Effective resistance Drive. Today. Capacitance Gate Source/Drain Contact More threshold dependence V DS.
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ESE370:Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 10: September 29, 2010 MOS Transistors Details
Last Time • Focused on I vs V relationships • Effective resistance • Drive
Today • Capacitance • Gate • Source/Drain Contact • More threshold dependence • VDS
Theme • Refining model • Exploring next level of complexity
Capacitance gate drain src channel • First order: looks like a capacitor • Today: • Like resistance, it is not constant • Capacitance not just to src (drain)
Threshold • Threshold decreases with VDS VT VDS
Capacitance • Argued looked like a capacitor to the channel • …but the channel isn’t really one of our terminals • Don’t connect directly to it.
Capacitance • Four Terminals • How many combinations • 4 things taken 2 at a time
Capacitances • GS, GB, GD, SB, DB, SD
Moving Plates? • What is distance from gate to conductor? • Depletion? • Strong Inversion?
Overlap • What is the capacitive implication of gate/src and gate/drain overlap?
Overlap • Length of overlap?
Capacitance in Strong Inversion(easy case) • Looks like parallel plate Gate – Channel • What is CGC? • What is CGB?
Capacitance in Strong Inversion • Looks like parallel plate Gate – Channel • What is CGC? • CGB=0
Capacitance in Strong Inversion • But channel isn’t a terminal • Split evenly with source and drain
Capacitance in Strong Inversion • Add in Overlap capacitance
Capacitance Subthreshold • Need to refine model • What showed on Day 9 not quite right • Channel doesn’t start depleted • Starts with substrate doping
Capacitance Depletion • What happens to capacitance here? • Capacitor plate distance?
Capacitance Depletion • Capacitance becomes Gate-Body • Capacitance drops
Capacitance vs VGS • G CGC CGCS=CGCD CGCB
Saturation Capacitance? • Source end of channel in inversion • Destination end of channel close at threshold • Capacitance shifts to source • Total capacitance reduced
Saturation Capacitance CGC CGCS CGCD VDS/(VGS-VT)
Contact Capacitance • n+ contacts are formed by doping = diffusion • Depletion under contact • Contact-Body capacitance • Depletion around perimeter of contact • Also contact-Body capacitance
Contact/Diffusion Capacitance • Cj – diffusion depletion • Cjsw – sidewall capacitance • LS – length of diffusion LS
Capacitance Roundup • CGS=CGCS+CO • CGD=CGCD+CO • CGB=CGCB • CSB=Cdiff • CDB=Cdiff
Step Response? Rsmall Rlarge
Impact of CGD • What does CGD do to the switching response here?
Threshold • Describe VT as a constant • Induce enough electron collection to invert channel
VDS impact • In practice, VDS impacts state of channel
VDS impact • Increasing VDS, already depletes portions of channel
VDS impact • Increasing VDS, already depletes portions of channel • Need less charge, less voltage to invert
In a Gate? • What does it impact most? • Which device, which state/operation?
In a Gate • VDS largest for off device • Easier to turn on
In a Gate • VDS largest for off device • Easier to turn on • Leak more
In a Gate • VDS largest for off device • Easier to turn on • Leak more
Admin • HW3 due Friday
Ideas • Capacitance • To every terminal • Voltage dependent • Threshold • Voltage dependent • Generally do manual analysis without CGC VT CGCS VDS CGCB