Day 10: September 26, 2012 MOS Transistor Basics

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Presentation transcript:

Day 10: September 26, 2012 MOS Transistor Basics ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 10: September 26, 2012 MOS Transistor Basics Penn ESE370 Fall2012 -- DeHon

Today MOS Transistor Topology Threshold Operating Regions Resistive Saturation Velocity Saturation Subthreshold Penn ESE370 Fall2012 -- DeHon

Last Time Penn ESE370 Fall2012 -- DeHon

Refinement Depletion region  excess carriers depleted Penn ESE370 Fall2012 -- DeHon

Body Contact Fourth terminal Also effects fields Usually common across transistors Penn ESE370 Fall2012 -- DeHon

No Field VGS=0, VDS=0 Penn ESE370 Fall2012 -- DeHon

Apply VGS>0 Accumulate negative charge Repel holes (fill holes) + + + + + + + + - - - - - - - - - Penn ESE370 Fall2012 -- DeHon

Channel Evolution Increasing Vgs Penn ESE370 Fall2012 -- DeHon

Gate Capacitance Changes based on operating region. Happy if you treat as parallel plate Capacitor for HW4. Penn ESE370 Fall2012 -- DeHon

Inversion Surface builds electrons Inverts to n-type Draws electrons from n+ source Penn ESE370 Fall2012 -- DeHon

Threshold Voltage where strong inversion occurs  threshold voltage Around 2ϕF Engineer by controlling doping (NA) Penn ESE370 Fall2012 -- DeHon

Resistive Region VGS>VT, VDS small Penn ESE370 Fall2012 -- DeHon

Resistive Region VGS>VT, VDS small VGS fixed  looks like resistor Current linear in VDS Penn ESE370 Fall2012 -- DeHon

Linear (Resistive) Region Penn ESE370 Fall2012 -- DeHon

Linear (Resistive) Region Blue curve marks transition from Linear to Saturation Penn ESE370 Fall2012 -- DeHon

Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (Tox) Penn ESE370 Fall2012 -- DeHon

Preclass Ids for identical transistors in parallel? Penn ESE370 Fall2012 -- DeHon

Preclass Ids for identical transistors in series? (Vds small) Penn ESE370 Fall2012 -- DeHon

Transistor Strength (W/L) D Transistor Strength (W/L) Penn ESE370 Fall2012 -- DeHon

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 Fall2012 -- DeHon

Ldrawn vs. Leffective Doping not perfectly straight Spreads under gate Effective L smaller than draw gate width Penn ESE370 Fall2012 -- DeHon

Channel Voltage Voltage varies along channel Think of channel as resistor Penn ESE370 Fall2012 -- DeHon

Preclass 2 What is voltage in the middle of a resistive medium? (halfway between terminals) Penn ESE370 Fall2012 -- DeHon

Voltage in Channel Think of channel as resistive medium Length = L Area = Width * Depth(inversion) What is voltage in the middle of the channel? L/2 from S and D ? Penn ESE370 Fall2012 -- DeHon

Channel Voltage Voltage varies along channel If think of channel as resistor Serves as a voltage divider between VS and VD Penn ESE370 Fall2012 -- DeHon

Impact on Inversion What happens when What is Vmiddle-Vs? Vgs=2Vth ? Vds=2Vth? What is Vmiddle-Vs? Penn ESE370 Fall2012 -- DeHon

Channel Field When voltage gap VG-Vxdrops below VTH, drops out of inversion Occurs when: VGS-VDS< VTH What does this mean about conduction? Penn ESE370 Fall2012 -- DeHon

Preclass 3 What is Vm? Penn ESE370 Fall2012 -- DeHon

Channel Field When voltage gap VG-Vxdrops below VT, drops out of inversion Occurs when: VGS-VDS< VT What is voltage at Vmiddle if conduction stops? What does that mean about conduction? Penn ESE370 Fall2012 -- DeHon

Contradiction? Vg-Vx < Vt  cutoff (no current) No current  Vg-Vx=Vgs Vg-Vx=Vgs > Vt  current flows Penn ESE370 Fall2012 -- DeHon

Way out? Vg-Vx < Vt  cutoff (no current) No current  Vg-Vx=Vgs Vg-Vx=Vgs > Vt  current flows Act like Vds at Vgs-Vt Penn ESE370 Fall2012 -- DeHon

Channel Field When voltage gap VG-Vxdrops below VT, drops out of inversion Occurs when: VGS-VDS< VT Channel is “pinched off” Penn ESE370 Fall2012 -- DeHon

Channel Field When voltage gap VG-Vxdrops below VT, drops out of inversion Occurs when: VGS-VDS< VT Channel is “pinched off” Current will flow, but cannot increase any further Penn ESE370 Fall2012 -- DeHon

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 Penn ESE370 Fall2012 -- DeHon

Saturation In saturation, VDS-effecitve=Vx= VGS-VT Becomes: Penn ESE370 Fall2012 -- DeHon

Saturation VDS> VGS-VT Penn ESE370 Fall2012 -- DeHon

Saturation Region Blue curve marks transition from Linear to Saturation Penn ESE370 Fall2012 -- DeHon

Preclass 3 What is electrical field in channel? Velocity: v=F*μ Leff=25nm, VDS=1V Field = VDS/L Velocity: v=F*μ Electron mobility: μn = 500 cm2/V What is electron velocity? Penn ESE370 Fall2012 -- DeHon

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 How relate to preclass 3 velocity? Encounter when channel short Modern processes, L is short enough Penn ESE370 Fall2012 -- DeHon

Velocity Saturation Once velocity saturates: Penn ESE370 Fall2012 -- DeHon

Velocity Saturation Penn ESE370 Fall2012 -- DeHon

Below Threshold Transition from insulating to conducting is non-linear, but not abrupt Current does flow But exponentially dependent on VGS Penn ESE370 Fall2012 -- DeHon

Subthreshold Penn ESE370 Fall2012 -- DeHon

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 Fall2012 -- DeHon

Subthreshold Slope Exponent in VGS determines how steep the turnoff is Every S Volts Divide IDS by 10 Penn ESE370 Fall2012 -- DeHon

Subthreshold Slope Exponent in VGS determines how steep the turnoff is Every S Volts (S not related to source) 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 Fall2012 -- DeHon

IDS vs. VGS Penn ESE370 Fall2012 -- DeHon

Admin Text 3.3.2 – highly recommend read HW3 due Thursday HW4 out Second half on Friday HW3 due Thursday HW4 out Penn ESE370 Fall2012 -- DeHon

Big Idea 3 Regions of operation for MOSFET Subthreshold Resistive Saturation Pinch Off Velocity Saturation Short channel Penn ESE370 Fall2012 -- DeHon