Vlsi Lecture02

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Vlsi Lecture02

  1. 1. Design and Implementation of VLSI Systems (EN1600) lecture02
  2. 2. Impact of doping on silicon resistivity silicon 4.995×1022 atoms in cm3 Resistivity 3.2 × 105 Ωcm dope with dope with phosphorous boron → or arsenic → p-type n-type1 atom in billion ⇒ 88.6 Ωcm 1 atom in billion ⇒ 266.14 Ωcm1 atom in million ⇒ 0.114 Ωcm 1 atom in million ⇒ 0.344 Ωcm1 atom in thousand ⇒ 0.00174 Ωcm 1 atom in thousand ⇒ 0.00233 Ωcm ⇒ Electrons are more mobile/faster than holes
  3. 3. What happens if we sandwich p & n types? A Al p n B One-dimensional representation  In equilibrium, the drift and diffusion components of current are balanced; therefore the net current flowing across the junction is zero.
  4. 4. PN-junction regions of operation A forward biasIn reverse bias, the width decreases the potentialof the depletion region drop across theincreases. The diode acts junction. As a result,as voltage-controlled the magnitude of the electric field decreasescapacitor. and the width of the depletion region narrows.
  5. 5. nMOS and pMOS transistors Each transistor consists of a stack of a conducting gate, an insulating layer of silicon dioxide and a semiconductor substrate (body or bulk) nMOS transistor pMOS transistor Source Gate Drain Source Gate Drain Polysilicon Polysilicon SiO2 SiO2 polysilicon gate W n+ n+ p+ p+ tox p bulk Si L SiO2 gate oxide n bulk Si n+ n+ (good insulator, εox = 3.9) p-type bodyBody is typically grounded Body is typically at supply voltage
  6. 6. nMOS transistor Source Gate Drain Polysilicon SiO2 n+ n+ p bulk Sig=0: When the gate is at a low voltage (VGS < VTN):  p-type body is at low voltage  source and drain-junctions diodes are OFF  transistor is OFF, no current flowsg=1: When the gate is at a high voltage (VGS ≥ VTN):  negative charge attracted to body  inverts a channel under gate to n-type  transistor ON, current flows, transistor can be viewed as a resistor
  7. 7. nMOS pass ‘0’ more strongly than ‘1’ Source Gate Drain Polysilicon SiO2 n+ n+ p bulk Si• Why does ‘1’ pass degraded?
  8. 8. pMOS transistor Source Gate Drain Polysilicon SiO2 p+ p+ n bulk Si g=0: When the gate is at a low voltage (VGS < VTP):  positive charge attracted to body  inverts a channel under gate to p-type  transistor ON, current flows g=1: When the gate is at a high voltage (VGS ≥ VTP):  negative charge attracted to body  source and drain junctions are OFF  transistor OFF, no current flows
  9. 9. pMOS pass ‘1’ more strongly than ‘0’ Source Gate Drain Polysilicon SiO2 p+ p+ n bulk Si • Why does ‘0’ pass degraded?
  10. 10. An nMOS and pMOS make up an inverter pMOS + nMOS = CMOS
  11. 11. More CMOS gates B A B F = AB 0What is this gate function? What’s wrong about this design?
  12. 12. 3-input NANDs What are the advantages of CMOS circuit style?
  13. 13. Series-Parallel Combinations
  14. 14. What are the transistor schematics of theNOR gate?
  15. 15. AOI
  16. 16. Transmission gate

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