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# Early Voltage in MOSFETs - PowerPoint PPT Presentation

Early Voltage in MOSFETs. Due to channel length modulation:. Saturation Voltage. V pinchoff = V DS,sat = V GS – V TH Separates resistive from saturation region The drain current is given by Solving for V DS,sat : Good to solve for quiescent voltage-current.

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Presentation Transcript

• Due to channel length modulation:

Chapter 3 Slides

• Vpinchoff = VDS,sat = VGS – VTH

• Separates resistive from saturation region

• The drain current is given by

• Solving for VDS,sat:

• Good to solve for quiescent voltage-current.

Chapter 3 Slides

Ex: Find VDS,sat for an NFET

Chapter 3 Slides

Cox

Gate oxidecapacitance

inversionlayer

Depletion cap,function of x

Cdep

Variations in VTHAcross Channel

• We assume VTH is constant across channel

THIS IS NOT TRUE!

• Depletion region is thick at S and thin at D.

Chapter 3 Slides

• PFETs typically have a shape factor 3 or 4 times larger than NFETs

• Body effect can be eliminated in PFETs by tying the n-well to VDD

• Need 6m spacing between n-wells to isolate.

• Dr. Engel always does this on input devices, not always elsewhere.

Chapter 3 Slides

• What really happens if VGS < VTN?

• In digital design, IDS = 0.

• We call it “weak inversion” or W.I.

• IDS is primarily due to Idrift in strong inversion and Idiffusion in weak inversion.

Chapter 3 Slides

• IDS = Idrift + Idiffusion

• If VGS > VTN the channel has been inverted.

• To be more precise, we can say the channel has been “strongly inverted” (S.I.) due to an abundance of carriers in the channel.

• Inversion is independent of whether the FET is in the linear or saturation region.

Chapter 3 Slides

Weak Inversion Idiffusion

• Drain is more reverse biased than source:

• To find Idiff, compute gradient

• Because no carriers are lost as they travel from S to D, current is the same for all x and gradient is not a function of x.

• Note: This is not really true due to recombination, but its close!

Chapter 3 Slides

Chapter 3 Slides

Chapter 3 Slides

• S must be big for device to be useful.

• If VDS = 100mV, can be neglected.

• For W.I. vDS,Sat 100mV

• Looks like a BJT

Chapter 3 Slides

• Let

• Shape factor as a function of  :

Lets you chose shape to match inversion mode.

Chapter 3 Slides

Chapter 3 Slides

Chapter 3 Slides

Question: How many digits are significant?

Chapter 3 Slides

• Suppose the previous circuit is the input device of an amplifier.

• Small-signal model holds as long as the deviations are small

Chapter 3 Slides

• Taking a Taylor expansion of one variable:

Chapter 3 Slides

Chapter 3 Slides

Chapter 3 Slides

Chapter 3 Slides

Ex: Find gm and rO