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ECE 875: Electronic Devices

ECE 875: Electronic Devices. Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu. Lecture 37, 11 Apr 14. Chp 06: MOSFETs Aspects of realistic MOSFET operation (n-channel p-substrate) Comment on 2D mobility m Use of field oxide in CMOS

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ECE 875: Electronic Devices

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  1. ECE 875:Electronic Devices Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu

  2. Lecture 37, 11 Apr 14 • Chp 06: MOSFETs • Aspects of realistic MOSFET operation (n-channel p-substrate) • Comment on 2D mobility m • Use of field oxide in CMOS • Short channel effects on ON operation: • high E (y) => velocity saturation => lower IDS • micron-scale = worst • nano-scale = not so bad • scaling • Good test for future ON/OFF operation: sub-threshold (not fully ON) swing VM Ayres, ECE875, S14

  3. Problem: if you reduce just L while keeping all other fabrication and operating parameters the same: performance degrades: RHS: VM Ayres, ECE875, S14

  4. Diagnose the problem: assuming that Z is still effectively the same, that leaves Qn(y) and <vel> as possible causes: Lec 36: we noted that <vel> has a saturation effect at high E Also: expect higher E (y) when same VDS is applied across a shorter length. Check this possibility out

  5. Previous steps leading to eq’n (23) for ID: VM Ayres, ECE875, S14

  6. No restrictions on E (y): can go all the way up to pinch-VDsat / L: Result: VM Ayres, ECE875, S14

  7. Repeat with new <vel>: much more complicated mathematically: VM Ayres, ECE875, S14

  8. Simpler mathematics:for n-channel in Si: using green line or blue line approximation for realistic red line to get simpler v(E ): <vel> E (y) that increases along L VM Ayres, ECE875, S14

  9. Simpler mathematics: Goal: find new VDsat. Then draw ID as in saturation after that point. Up to new lower VDsat continue to use: VM Ayres, ECE875, S14

  10. Simpler mathematics: Also: replace Qn eq’n 20 with something simpler: eq’n (32): VM Ayres, ECE875, S14

  11. Find ID: But goal was: find new VDsat VM Ayres, ECE875, S14

  12. Find ID E (y) = Ecritical where velocity saturation becomes a problem: VM Ayres, ECE875, S14

  13. Dyi (y) Dyi (y = L) = (VatD – VatS) – (VatS – VatS) = VDS = Sze VD VM Ayres, ECE875, S14

  14. VM Ayres, ECE875, S14

  15. Accomplished goal in terms of Ec E c for Si is pretty well known: 1 x 107 cm/s

  16. Compare: With Constant mobility assumption: Pinch- With field dependent mobility assumption in the two-piece linear approximation: Velocity saturation- VM Ayres, ECE875, S14

  17. Pinch-VDsat > Velocity saturation-VDsat Therefore: Pinch-ID > Velocity saturation-ID Pinch-VDsat Velocity saturation-VDsat VM Ayres, ECE875, S14

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