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Lecture 20

Lecture 20. OUTLINE The MOSFET (cont’d) Long-channel I - V characteristics Reading : Pierret 17.2; Hu 6.6. Derivation of NMOSFET I - V. V D > V S Current in the channel flows by drift Channel voltage V C ( y ) varies continuously between the source and the drain

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Lecture 20

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  1. Lecture 20 OUTLINE The MOSFET (cont’d) • Long-channel I-V characteristics Reading: Pierret 17.2; Hu 6.6

  2. Derivation of NMOSFET I-V • VD > VS • Current in the channel flows by drift • Channel voltage VC(y) varies continuously between the source and the drain • Channel inversion charge density W EE130/230A Fall 2013 Lecture 20, Slide 2 R. F. Pierret, Semiconductor Device Fundamentals, Figs. 17.6

  3. 1st-Order Approximation • If we neglect the variation of Qdep with y, then where VT is defined to be the threshold voltage at the source end: The inversion charge density is then EE130/230A Fall 2013 Lecture 20, Slide 3

  4. NMOSFET Current (1st-order approx.) • Consider an incremental length dy of the channel. The voltage drop across this region is in the linear region EE130/230A Fall 2013 Lecture 20, Slide 4

  5. Saturation Current, IDsat (1st-order approximation) C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 6-16 IDS saturates when VD reaches VG-VT  Set VD = VG-VT in the equation for ID EE130/230A Fall 2013 Lecture 20, Slide 5

  6. Problem with “Square Law Theory” • Ignores variation in depletion width with distance y: where EE130/230A Fall 2013 Lecture 20, Slide 6

  7. Modified (Bulk-Charge) I-V Model In linear region: In saturation region: EE130/230A Fall 2013 Lecture 20, Slide 7

  8. MOSFET Threshold Voltage, VT The expression that was previously derived for VTis the gate voltage referenced to the body voltage that is required reach the threshold condition: Usually, the terminal voltages for a MOSFET are all referenced to the source voltage. In this case, and the equations for IDS are EE130/230A Fall 2013 Lecture 20, Slide 8

  9. The Body Effect Note that VTis a function of VSB: where g is the body effect parameter When the source-body pn junction is reverse-biased, |VT| is increased. Usually, we want to minimizeg so that IDsat will be the same for all transistors in a circuit. EE130/230A Fall 2013 Lecture 20, Slide 9

  10. MOSFET VT Measurement • VT can be determined by plotting IDSvs.VGS, using a low value of VDS IDS VGS EE130/230A Fall 2013 Lecture 20, Slide 10

  11. Long-Channel MOSFET I-V Summary • In the ON state (VGS>VT for NMOS; VGS<VT for PMOS), the inversion layer at the semiconductor surface forms a “channel” for current to flow by carrier drift from source to drain In the linear region of operation (VDS < (VGSVT)/m): In the saturation region of operation (VDS > (VGSVT)/m): EE130/230A Fall 2013 Lecture 20, Slide 11

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