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Day 10: September 26, 2012 MOS Transistor Basics

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Presentation on theme: "Day 10: September 26, 2012 MOS Transistor Basics"— Presentation transcript:

1 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 Fall DeHon

2 Today MOS Transistor Topology Threshold Operating Regions Resistive
Saturation Velocity Saturation Subthreshold Penn ESE370 Fall DeHon

3 Last Time Penn ESE370 Fall DeHon

4 Refinement Depletion region  excess carriers depleted
Penn ESE370 Fall DeHon

5 Body Contact Fourth terminal Also effects fields
Usually common across transistors Penn ESE370 Fall DeHon

6 No Field VGS=0, VDS=0 Penn ESE370 Fall DeHon

7 Apply VGS>0 Accumulate negative charge Repel holes (fill holes)
Penn ESE370 Fall DeHon

8 Channel Evolution Increasing Vgs
Penn ESE370 Fall DeHon

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

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

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

12 Resistive Region VGS>VT, VDS small Penn ESE370 Fall DeHon

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

14 Linear (Resistive) Region
Penn ESE370 Fall DeHon

15 Linear (Resistive) Region
Blue curve marks transition from Linear to Saturation Penn ESE370 Fall DeHon

16 Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (Tox)
Penn ESE370 Fall DeHon

17 Preclass Ids for identical transistors in parallel?
Penn ESE370 Fall DeHon

18 Preclass Ids for identical transistors in series? (Vds small)
Penn ESE370 Fall DeHon

19 Transistor Strength (W/L)
D Transistor Strength (W/L) Penn ESE370 Fall DeHon

20 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

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

22 Channel Voltage Voltage varies along channel
Think of channel as resistor Penn ESE370 Fall DeHon

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

24 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 Fall DeHon

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

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

27 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 Fall DeHon

28 Preclass 3 What is Vm? Penn ESE370 Fall DeHon

29 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 Fall DeHon

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

31 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 Fall DeHon

32 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

33 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 Fall DeHon

34 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 Fall DeHon

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

36 Saturation VDS> VGS-VT Penn ESE370 Fall DeHon

37 Saturation Region Blue curve marks transition from Linear
to Saturation Penn ESE370 Fall DeHon

38 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 Fall DeHon

39 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 Fall DeHon

40 Velocity Saturation Once velocity saturates:
Penn ESE370 Fall DeHon

41 Velocity Saturation Penn ESE370 Fall DeHon

42 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

43 Subthreshold Penn ESE370 Fall DeHon

44 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

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

46 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 Fall DeHon

47 IDS vs. VGS Penn ESE370 Fall DeHon

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

49 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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