1 / 12

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.

becky
Download Presentation

Lecture #38

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related