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Day 9: September 27, 2010 MOS Transistor Basics
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 27, 2010 MOS Transistor Basics Penn ESE370 Fall DeHon
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Today MOS Transistor Topology Threshold Operating Regions Resistive
Saturation Velocity Saturation Subthreshold Penn ESE370 Fall DeHon
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Last Time Penn ESE370 Fall DeHon
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Refinement Penn ESE370 Fall DeHon
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Body Contact Fourth terminal Also effects fields
Usually common across transistors Penn ESE370 Fall DeHon
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No Field VGS=0, VDS=0 Penn ESE370 Fall DeHon
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Apply VGS>0 Accumulate negative charge Repel Holes + + + + + + + +
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Inversion Surface builds electrons Inverts to n-type
Draws electrons from n+ source Penn ESE370 Fall DeHon
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Threshold Voltage where strong inversion occurs threshold voltage
Around 2ϕF Engineer by controlling doping (NA) Penn ESE370 Fall DeHon
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Resistive Region VGS>VT, VDS small Penn ESE370 Fall DeHon
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Resistive Region VGS>VT, VDS small VGS fixed looks like resistor
Current linear in VDS Penn ESE370 Fall DeHon
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Linear or Resistive Region
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Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (Tox)
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Transistor Strength (W/L)
D Transistor Strength (W/L) Penn ESE370 Fall DeHon
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Transistor Strength (W/L)
D Transistor Strength (W/L) Shape dependence match Resistance intuition Wider = parallel resistors decrease R Longer = series resistors increase R Penn ESE370 Fall DeHon
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Ldrawn vs. Leffective Doping not perfectly straight Spreads under gate
Effective L smaller than draw gate width Penn ESE370 Fall DeHon
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Channel Voltage Voltage varies along channel
If think of channel as resistor Serving as a voltage divider between VS and VD Penn ESE370 Fall DeHon
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Channel Field When voltage gap VG-Vxdrops below VT, drops out of inversion Occurs when: VGS-VDS< VT Channel is “pinched off” Penn ESE370 Fall DeHon
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Pinch Off When voltage drops below VT, drops out of inversion
Occurs when: VGS-VDS< VT Conclusion: current cannot increase with VDS once VDS> VGS-VT current must adjust so that VDS= VGS-VT If current dropped to zero, then would invert and conduct again… Penn ESE370 Fall DeHon
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Saturation In saturation, VDS= VGS-VT Becomes:
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Saturation VDS> VGS-VT Penn ESE370 Fall DeHon
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Saturation Region Penn ESE370 Fall DeHon
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Short Channel Model assumes carrier velocity increases with field
Increases with voltage There is a limit to how fast carriers can move Limited by scattering to 105m/s Encounter when channel short Field = VDS/L Modern processes, L is short enough Penn ESE370 Fall DeHon
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Velocity Saturation Once velocity saturates:
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Velocity Saturation Penn ESE370 Fall DeHon
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Below Threshold Transition from insulating to conducting is non-linear, but not abrupt Current does flow But exponentially dependent on VGS Penn ESE370 Fall DeHon
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Subthreshold Penn ESE370 Fall DeHon
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Subthreshold S D W/L dependence follow from resistor behavior (parallel, series) Not shown explicitly in text λ is a channel width modulation effect Penn ESE370 Fall DeHon
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Subthreshold Slope Exponent in VGS determines how steep the turnoff is
Every S Volts Divide IDS by 10 Penn ESE370 Fall DeHon
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Subthreshold Slope Exponent in VGS determines how steep the turnoff is
Every S Volts Divide IDS by 10 n – depends on electrostatics n=1 S=60mV at Room Temp. (ideal) n=1.5 S=90mV Single gate structure showing S=90-110mV Penn ESE370 Fall DeHon
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IDS vs. VGS Penn ESE370 Fall DeHon
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Admin Text 3.3.2 – highly recommend read HW3 out Andre office hours
Some chance Dental appt. overrun tomorrow’s office hours Penn ESE370 Fall DeHon
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Big Idea 3 Regions of operation for MOSFET Subthreshold Resistive
Saturation Pinch Off Velocity Saturation Short channel Penn ESE370 Fall DeHon
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