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Design and Implementation of VLSI Systems (EN1600) lecture07

Design and Implementation of VLSI Systems (EN1600) lecture07. Sherief Reda Division of Engineering, Brown University Spring 2008. [sources: Weste/Addison Wesley – Rabaey Pearson - Baker Wiley]. Schedule for 4 lectures Ideal (Shockley) Model Non-ideal model Inverter DC characteristics

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Design and Implementation of VLSI Systems (EN1600) lecture07

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  1. Design and Implementation of VLSI Systems (EN1600) lecture07 Sherief Reda Division of Engineering, Brown University Spring 2008 [sources: Weste/Addison Wesley – Rabaey Pearson - Baker Wiley]

  2. Schedule for 4 lectures Ideal (Shockley) Model Non-ideal model Inverter DC characteristics SPICE MOS transistor theory

  3. gate-oxide-body sandwich = capacitor • Operating modes • Accumulation • Depletion • Inversion • The charge accumulated is proportional to the excess gate-channel voltage (Vgc-Vt)

  4. Cut off Vgs < Vt NMOS transistor, 0.25um, Ld = 10um, W/L = 1.5, VDD = 2.5V, VT = 0.4V The MOS transistor has three regions of operation • Linear (resistor): • Vgs > Vt & Vds < VSAT=Vgs-Vt Current prop to Vds • Saturation: • Vgs > Vt and Vds ≥ VSAT=Vgs-Vt Current is independent of Vds

  5. MOS structure looks like parallel plate capacitor while operating in inversion Gate – oxide – channel Qchannel = CV C = εoxWL/tox = CoxWL (where Cox=εox/tox) V = Vgc – Vt = (Vgs – Vds/2) – Vt How to calculate the current value?

  6. Charge is carried by electrons Carrier velocity v proportional to lateral E-field between source and drain v = μE μ called mobility E = Vds/L Time for carrier to cross channel: t = L / v Carrier velocity is a factor in determining the current

  7. Now we know How much charge Qchannel is in the channel How much time t each carrier takes to cross I=Q/t

  8. Can be ignored for small Vds In linear mode (Vgs > Vt & Vds < Vgs-Vt) • For a given Vgs, Ids is proportional (linear) to Vds

  9. In saturation mode (Vgs > Vt and Vds ≥ Vgs-Vt) pinched off • Now drain voltage no longer increases current

  10. Operation modes summary

  11. Transistor capacitance • Gate capacitance: to body + to drain + to source • Diffusion capacitance: source-body and drain-body capacitances

  12. Gate capacitance as a function of Vgs

  13. Csb, Cdb Undesirable, called parasitic capacitance Capacitance depends on area and perimeter Use small diffusion nodes Comparable to Cg Varies with process Channel-stop implant N 1 A Side wall Source W N D Bottom x Side wall j Channel L Substrate N S A Source/Drain diffusion capacitance

  14. Covered ideal (long channel) operation (Shockley model) of transistor Next time: short-channel transistors TA Summary

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