INF3410 — Fall 2016

INF3410 — Fall 2016

Excerpt of Sedra/Smith Chapter 5: CMOS Field EffectTransistors (FETs)

Content

CMOS FET Large Signal Models (book 5.1-5.2)

MOSFET circuits at DC (book 5.3)

Further Model Refinements (book 5.4, will be discussedlater)

Excerpt of Sedra/Smith Chapter 5: CMOS Field EffectTransistors (FETs) 2

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Device Concept


Device Cross Section


Short Sidetrack: PN-junction

(from book: Carusone, Johns, Martin)

Cj =Cj0

r

1 + VRΦ0

(1.17)

Cj0 =

√

√

√qKSϵ0

2Φ0

NAND

NA +ND(1.18)

Φ0 = UT ln

�

NAND

n2i

�

(1.6)


nFET/NMOS and pFET/PMOS cross section


Above Threshold Regions of Operation NFET


Above Threshold Regions of Operation PFET

’Above Threshold’ is also called ’Strong Inversion’


nMOSFET Device Symbols


pMOSFET Device Symbols


The EKV model

iD = iF − iR

iF(R) = IS ln�

1 + evG−Vtn−nvS(D)

nVT

�2


Regions of operation dependent on vGS:strong- vs weak inversion

weak inversion = subthreshold vs. strong inversion =above thresholdThese are dependent on vGS ≥ Vtn for strong inversionand VGS < Vtn for weak inversion. The transitionbetween the two is refered to as moderate inversion.


Regions of operation dependent on vDS:triode- vs active region

INDEPENDENT of weak- and strong inversion thetransistor can operate in either:Triode region = ’linear’ region vs. saturation = activeregionThese are dependent on vDS ≥ Vsat for active region andvDS < Vsat for triode region, where the definition of Vsatis different dependent on if the transistor is in weak(Vsat ≈ 4VT)or strong inversion (Vsat = VOV).


Regions of operation summary

So there are 4 differnt combinations possible:weak inversion, triode region OR weak inversion, activeregion OR strong inversion, triode region OR stronginversion, active region!


strong inversion, active region

iD =1

2nkn (vOV − nvS)2 (1 + λvDS)

Different name in the EKV model: β := kn and 1 ≤ n ≤ 2and often n ≈ 1 and is neglected


weak inversion, active region

iD = ISevG−Vtn−nvS

nVT (1 + λvDS) (16.13)

Where IS = 2nknV2T


strong inversion, triode region

EKV:iD = knvDS

�

vOV −n

2(vD + vS)

�

Sedra & Smith:

iD = knvDS

�

vOV −1

2vDS

�

Which is the same for vS = 0 and n = 1For vDS << VOV (1st order Taylor expansion aroundvDS = 0):

iD = knvOVvDS⇒ gDS = knvOVExcerpt of Sedra/Smith Chapter 5: CMOS Field EffectTransistors (FETs) 18

weak inversion, triode region

EKV:iD = e

vOVnVT

�

e−vSVT − e

−vDVT

�

For vS = 0:

iD = evOVnVT

�

1− e−vDVT

�

For vDS << VOV (1st order Taylor expansion aroundvDS = 0):

iD = evOVnVT

VD

VT⇒ gDS = e

vOVnVT

1

VT


Illustration ID vs VGS


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Diode Connected Transistor

In strong inversion:

i =1

2kn(v − Vtn)2


Simple Current Mirror

When will Iout = Iin, Iout 6= Iin, Iout ≈ Iin, Iout ≈ xIin?Excerpt of Sedra/Smith Chapter 5: CMOS Field EffectTransistors (FETs) 23

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INF3410 — Fall 2016

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