Bermel ECE 305 F16

ECE 305: Fall 2016

MOSFET Wrap-Up

Professor Peter BermelElectrical and Computer Engineering

Purdue University, West Lafayette, IN USApbermel@purdue.edu

Pierret, Semiconductor Device Fundamentals (SDF)

Chapter 18 (pp. 645-658)

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Outline

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1. Review Questions

2. Device Variability

3. Improved Mobility

4. Conclusion

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Review Questions

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1) Why is the small signal conductance and diffusion capacitance absent

for MOS capacitors?

2) What is the expression for inversion capacitance? Why isn’t there

inversion capacitance in a diode?

3) What is the difference between flatband voltage vs. threshold voltage?

4) When would you use deep depletion formula vs. small signal formula?

5) Explain why there is a difference between low frequency response vs.

high-frequency response for a MOS-C, but there is no such distinction

for MOSFET.

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Variability in Vth at Low Doping

4

2 2B Tth F F

ox ox

AQ qN WV

C C

IBM Journal of Res. And Tech. 2003.11/16/2016

Variability in Threshold Voltage

Bermel ECE 305 F16

5

2 2B Tth F F

ox ox

AQ qN WV

C C

If every transistor has different Vth and therefore different current, circuit design becomes difficult

2

,( )2

o

D G T idealI VL

VZ C

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Vth control by Metal Work-function

6

cs

Fm

Vacuum level

EV

EF

EC

qVbi

( ) i o G TQ C V V

BT s

o

FBVQ

VC

High-k/metal gate MOSFET

bi g p M

FB

qV E

qV

c F

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Tunneling Current

Bermel ECE 305 F16

7

2

( ) ( )GqViT i G th

A

nJ Q V e T E

N

EC

EV

EF

EG

T

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Ge for PMOS, Si for NMOS

Bermel ECE 305 F16 8

Gatep-MOS

Active Area

n-MOS

[1’10]

[001]

90°

(110)-Plane

Gate

135°A

B

p-MOS

[1’10]

[100]

n-MOS

(001)-Plane

B

’

Strained-Si

Pillar

Takagi, TED 52, p.367, 2005

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Outline

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1. Review Questions

2. Device Variability

3. Improved Mobility

4. Conclusion

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Basics of Strain ..

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Compressive

biaxial strain

Enhances mobility in the channel …

Larger lattice

Smaller lattice

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Biaxial Strain to Enhance Mobility

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11

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Uniaxial Compressive Strain to Enhance Mobility

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Biaxial Strain to Enhance Mobility

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Adapted from Chang et. al, IEDM 2005.

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New Channel Materials for improved mobility

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Putting it all together

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MOSFET Simulation Tool:

https://nanohub.org/tools/mosfetsat

Comparing Ge with Si

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Conclusion

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1) The basic behavior of MOSFETs can be captured by

band diagrams, transfer & output characteristics

2) There are a variety of failure modes that can degrade

performance over time

3) Short channels and variability are serious concerns for

MOSFET scaling, but strained lattices can help address

this issue, resulting in effective MOSFET channel

lengths < 15 nm

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