Effects Scaling

8/10/2019 Effects Scaling

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SPEEDSPACEPOWER

FUNCTIONALITY


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Full Scaling ( Constant Field Scaling )

Quantity Before Scaling After Scaling

Channel length L L/S

Channel width W W/S

Gate oxide thickness tox tox/S

Junction depth xj xj/S

Power supply voltage Vdd Vdd/S

Threshold voltage Vt Vt/S

Doping Densities Na

Nd

S*Na

S*Nd


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Quantity Before Scaling After ScalingOxideCapacitance

Cox S*Cox

Drain Current Id Id/S

Power Dissipation P P/S2Power Density P/Area P/Area


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Constant Voltage Scaling

Quantity Before Scaling After Scaling

Channel Length L L/S

Channel Width W W/S

Gate oxide thickness tox tox/S

Junction depth xj xj/S

Lateral electric field Ey S*Ey

Supply Voltage Vdd Vdd

Threshold voltage Vt Vt

Doping Densities Na

Nd

S2*Na

S2*Nd


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Quantity Before Scaling After ScalingOxide

CapacitanceCox S*Cox

Drain Current Id S*IdPowerDissipation

P S*P

Power Density P/Area S3* P/Area


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Constant field scaling increases speed, reducessize and reduces power consumption as well.

On the other hand, constant voltage scalingincreases speed, reduces size but increases powerconsumption.

Inspite of this drawback, constant voltage scalingis usually preferred because it is difficult toprovide multiple power supply voltages and tonecessitate complicated level shifter

arrangements required in constant field scaling.

Summary:


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There are ten major short channel effects:

1. Velocity Saturation2. Surface Scattering3. Drain Induced Barrier Lowering (DIBL) &

Subthreshold Conductance

4. Gate Oxide Leakage5. Gate Induced Drain Leakage (GIDL)6. Lower Transconductance7. Stress Induced Leakage Current (SILC)8. Channel Length Modulation9. Impact Ionization10.Hot Electrons


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Variation of drift velocities in Si and GaAswith applied electric field

Mobility: Drift velocity per unit electric field

Velocitysaturation


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At low Ey, the electron drift velocity vd in the channel varieslinearly with the electric field intensity. However, as Eyincreases above 104V/cm, the drift velocity tends to increaseslowly, and approaches a saturation value of vd = 10

7 cm/saround Ey= 105V/cm at 300 K.

As the field increases above 104 V/cm, optical phonons areemitted alongside acoustic phonon.

Due to this, the drift velocity cannot increase above certainlevel and it becomes saturated. So, the current eventually isfound lesser than the anticipated value.


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As the channel length becomes smaller, due to thelateral extension of the depletion layer into the channelregion, the longitudinal electric field component Eyincreases, and the surface mobility becomes field-dependent.

Since the carrier transport in a MOSFET is confinedwithin the narrow inversion layer, and the surfacescattering, that is the collisions suffered by theelectrons that are accelerated toward the interface byEx causes reduction of the mobility and the electrons

move with great difficulty parallel to the interface.

So, the average surface mobility becomes less ascompared to that of the bulk mobility and eventually itaffects the current.


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The equation for source and drain junctionwidths:


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Current flows due to sustained surface inversion.


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The current flow in the channel depends on

creating and sustaining an inversion layer on thesurface.

If the gate bias voltage is not sufficient to

invert the surface (Vgs


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In small-geometry MOSFETs, the potential barrier iscontrolled by both the gate-to-source voltage Vgsand the

drain-to-source voltage Vds.

If the drain voltage is increased, the potential barrier inthe channel decreases, leading to drain-induced barrier

lowering (DIBL).

The reduction of the potential barrier eventually allowselectron flow between the source and the drain, even ifthe gate-to-source voltage is lower than the thresholdvoltage.

The channel current that flows under this conditions (Vgs

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Effects Scaling