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Lecture #38

OUTLINE The MOSFET: Bulk-charge theory Body effect parameter Channel length modulation parameter PMOSFET I-V Small-signal model Reading : Finish Chapter 17, 18.3.4. Lecture #38. Problem with the “Square Law Theory”. Ignores variation in depletion width with distance y.

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Lecture #38

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  1. OUTLINE The MOSFET: Bulk-charge theory Body effect parameter Channel length modulation parameter PMOSFET I-V Small-signal model Reading: Finish Chapter 17, 18.3.4 Lecture #38 EE130 Lecture 38, Slide 1

  2. Problem with the “Square Law Theory” • Ignores variation in depletion width with distance y EE130 Lecture 38, Slide 2

  3. Modified (Bulk-Charge) Model • linear region: • saturation region: EE130 Lecture 38, Slide 3

  4. 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 Lecture 38, Slide 4

  5. 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 minimizegso that IDsat will be the same for all transistors in a circuit EE130 Lecture 38, Slide 5

  6. MOSFET VT Measurement • VT can be determined by plotting IDSvs.VGS, using a low value of VDS IDS VGS EE130 Lecture 38, Slide 6

  7. Channel Length Modulation Parameter, l • Recall that as VDS is increased above VDsat, the width DL of the depletion region between the pinch-off point and the drain increases, i.e. the inversion layer length decreases. EE130 Lecture 38, Slide 7

  8. P-Channel MOSFET • The PMOSFET turns on when VGS < VTp • Holes flow from SOURCE to DRAIN  DRAIN is biased at a lower potential than the SOURCE • In CMOS technology, the threshold voltages are usually symmetric: VTp = -VTn VG • VDS < 0 • IDS < 0 • |IDS| increases with • |VGS - VTp| • |VDS| (linear region) VS VD GATE IDS P+ P+ N VB EE130 Lecture 38, Slide 8

  9. PMOSFET I-V • Linear region: • Saturation region: m = 1 + (3Toxe/WT) is the bulk-charge factor EE130 Lecture 38, Slide 9

  10. Small Signal Model • Conductance parameters: EE130 Lecture 38, Slide 10

  11. Inclusion of Additional Parasitics EE130 Lecture 38, Slide 11

  12. Cutoff Frequency • fmax is the frequency where the MOSFET is no longer amplifying the input signal • Obtained by considering the small-signal model with the output terminals short-circuited, and finding the frequency where |iout / iin| = 1 • Increased MOSFET operating frequencies are achieved by decreasing the channel length EE130 Lecture 38, Slide 12

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