Lecture03_MOS_Models_2up

Lecture03_MOS_Models_2up - EECS240 Spring 2008 Lecture 3:...

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EECS240 – Spring 2008 Lecture 3: MOS Models for Design Elad Alon Dept. of EECS EECS240 Lecture 3 2 Why Modeling? Analog circuits more sensitive to detailed transistor behavior Precise currents, voltages, etc. matter Digital circuits have much larger “margin of error” Models allow us to reason about circuits Provide window into the physical device and process “Experiments” with SPICE much easier to do
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EECS240 Lecture 3 3 Levels of Abstraction Best abstraction depends on questions you want to answer Digital functionality: MOSFET is a switch Digital performance: MOSFET is a current source and a switch Analog characteristics: MOSFET described by BSIM with 400 parameters? MOSFET described by measurement results? EECS240 Lecture 3 4 Why not Square Law? Square law model most widely known: But, totally inadequate for “short-channel” behavior Also doesn’t capture moderate inversion (i.e., in between sub-threshold and strong inversion) () 2 , 1 2 Dsa t n ox GS th W IC V V L µ =⋅ ⋅ ⋅ ⋅
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EECS240 Lecture 3 5 Square Law Model Assumptions Charge density determined only by vertical field Drift velocity set only by lateral field Neglect diffusion currents (“magic” V th ) Constant mobility And many more… EECS240 Lecture 3 6 A Real Transistor Ultra-thin Gate Dielectric Direct Tunneling Current Quantum Effects Pocket Implant Reverse short channel effect Slower output resistance scaling with L Gate Electrode Gate Depletion Quantum Effect Short Channel Effects Velocity Saturation and Overshoot Source-end Velocity Limit S/D Engineering S/D resistances S/D leakage Retrograde Doping Body effect
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EECS240 Lecture 3 7 Now What?
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Lecture03_MOS_Models_2up - EECS240 Spring 2008 Lecture 3:...

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