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ESE 568: Mixed Signal Circuit Design and Modeling

Lec 2: September 4th, 2019 MOS Models: Devices and Large Signal

Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

Lecture Outline

!  MOSFET (Large Signal) "  Physical Device "  Device Models

"  Examples

"  2nd order effects

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MOSFET

Metal Oxide Semiconductor Field Effect Transistor: Device and Models

Penn ESE 568 Fall 2019 - Khanna 3

The Operational Basis of a FET

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Metal

Oxide (Insulator)

P-type Semiconductor

The Operational Basis of a FET

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The Operational Basis of a FET

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The Operational Basis of a FET

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MOSFET Physical Structure

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nMOS IV Characteristics

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nMOS IV Characteristics

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nMOS IV Characteristics

11 Only valid for vds near 0 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F.

Najmabadi, UCSD)

nMOS IV Characteristics

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nMOS IV Characteristics

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nMOS IV Characteristics

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Transistor IV curves

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Channel Length Modulation

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Channel-length Modulation

Channel Length Modulation

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Channel-length Modulation

Body Effect

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The Operational Basis of a FET

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Not exactly Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

The Operational Basis of a FET

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Not exactly

Weak Inversion Strong Inversion

Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

Weak Inversion (or Cut-off or Subthreshold)

!  Transition from insulating to conducting is non-linear, but not abrupt

!  Current does flow "  But exponentially dependent on VGS

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Weak Inversion (or Cut-off or Subthreshold)

If VGS <Vth,

IDS = ISWL

!

"#

$

%&e

VGS−VthnkT /q

!

"#

$

%&

1− e−VDSkT /q!

"#

$

%&!

"

##

$

%

&&

1+λVDS( )

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!  Current is from the parasitic NPN BJT transistor when gate is unbiased and there is no conducting channel

Penn ESE 568 Fall 2019 – Khanna

MOS IV Characteristic Equations

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Simplified for hand analysis

Not exactly, subT current flows

[1+λ(vDS−VOV)]

[1+λ(vDS−VOV)]

Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

pMOS Device

24 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

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MOS IV Characteristic Equations

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Simplified for hand analysis

Not exactly, subT current flows

[1+λ(vDS−VOV)]

[1+λ(vSD−VOV)]

Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

nMOS Exercise

!  http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

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Example: Operating Regions

!  Assume VTHn = -VTHp = 0.5V, µnCox = µpCox = 50µA/V2, λ = 0, and M1 and M2 have the same value of W and L (are same size)

!  Determine operating region for M1 and M2 assuming: "  Vbias = 1.2 "  Vbias = 0.2 "  Vbias = 0.65

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2nd Order Effects

!  Mobility Degradation with Normal Field "  Vertical field "  Triode and saturation region

!  Velocity Saturation "  Lateral field "  Saturation region

!  Short Channel Effects 

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Mobility Degradation with Normal Field

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!  Usually modeled empirically !  Affects both saturation and triode regions

"  Strong inversion only

(VDS small)

Mobility Degradation with Normal Field

!  High gate-to-source voltage

!  θ= mobility modulation factor (empirical)

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µn (eff ) ≈µn0

1+θ(VGS −VT )

Penn ESE 568 Fall 2019 – Khanna

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Velocity Saturation

!  Affects saturation region in strong inversion !  Once velocity saturates:

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Mobility degradation due to lateral electric field (VDS/Leff)

Penn ESE 568 Fall 2019 – Khanna

Velocity Saturation

!  Long Channel

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Velocity Saturation

!  Long Channel !  Short Channel

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Short Channel Effects – VT Reduction

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n+ n+

pn+ depletion

region

pn+ depletion

region VGS induced depletion

region

S G

D

n+ n+

S G

D

QB0 QB0(sc)

xj

Leff Leff

VT0 (short channel) = VT0 - ΔVT0

Penn ESE 568 Fall 2019 – Khanna

Short Channel Effects – VT Reduction

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n+ n+

pn+ depletion

region

pn+ depletion

region VGS induced depletion

region

S G

D

n+ n+

S G

D

QB0 QB0(sc)

xj

Leff Leff

VT0 (short channel) = VT0 - ΔVT0

Penn ESE 568 Fall 2019 – Khanna

Short Channel Effects - DIBL

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!  Drain Induced Barrier Lowering "  VT Reduction with Drain Bias

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MOS IV Characteristic Equations

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Simplified for hand analysis

Not exactly, subT current flows

[1+λ(vDS−VOV)]

[1+λ(vDS−VOV)]

Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)

Lecture Outline

!  MOSFET has modes of operation "  Subthreshold "  Triode "  Saturation "  Have simplified model for hand analysis

!  Second order effects "  Mobility Degradation with Normal Field "  Short Channel Effects

"  Velocity Saturation "  VT reduction "  DIBL

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Admin

!  HW 1 due Sunday at midnight in Canvas "  Allow time for submitting "  Make sure your submission is the correct file

"  Check!

!  Diagnostic Quiz extended "  Complete by Friday midnight or get a 0

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