SHORT CHANNEL EFFECTSBy:

Ashish A Bait

Sagar Paarcha

Nilesh Narkhede

Suraj More

OVERVIEW • Pre-requisite

• What is Short Channel

• What are Short Channel Effects

• DIBL

• Surface scattering

• Hot electron effect

• Impact Ionisation

• Velocity saturation

PRE-REQUISITE • Threshold voltage:- The minimum voltage

across gate and source which allows current flow through the transistor.

• Sub-threshold current:- The current flowing through the transistor before threshold voltage is called sub-threshold current.

• Pinch-off voltage :- It is a voltage after which there is no effect of increase in drain to source voltage on the current flowing through transistor.

WHAT IS SHORT CHANNEL• Channel length ~= depletion width of

source and drain

SHORT CHANNEL EFFECTS• Short channel MOS has good processing

speed, requires low operating potential and increases transistor density on the chip.

• Although the performance degrades with decrease in channel length.

• It faces some serious issues like DIBL, surface scattering, velocity saturation, impact ionisation, hot electron effect.

DRAIN INDUCED BARRIER LOWERING(DIBL)

• Increase in drain voltage reduces the barrier face by electrons or holes in the source allowing them to go from source to drain where gate voltage remain unchanged.

• Gate looses the control of flow of current through MOS and become as good as redundant.

DRAIN INDUCED BARRIER LOWERING(DIBL)

DRAIN INDUCED BARRIER LOWERING(DIBL)

Long Channel Short Channel

SURFACE SCATTERING

SURFACE SCATTERING • In long channel Ex>>Ey,but in short

channel Ex is not negligible.

• Ex and Ey field makes electron to travel in zig-zag path, reducing their mobility.

• As the channel length becomes shorter electric field Ey increases causing surface mobility to become field dependent.

HOT ELECTRON EFFECT

HOT ELECTRON EFFECT• At high velocity carriers drifting near the

drain gains extra energy called hot carriers.

• These hot carriers tunnel through gate oxide thus reducing the total current flow from source to drain.

• Reduces the input impedance.

HOT ELECTRON EFFECT• The technique is used in floating gate

devices to increase the threshold by trapping the electrons in the floating gate

• To reduce hot electron effect:- drain is lightly doped, resulting in less electric field created for carriers.

IMPACT IONISATION

IMPACT IONISATION • Electron travelling to the drain creates

electron-hole pair by impact ionisation.

• Secondary electrons are collected at drain causing current to increase in saturation.

• Secondary holes are collected at substrate causing to increase latch up.

VELOCITY SATURATION

VELOCITY SATURATION• Up to certain point velocity is directly

proportional to electric field.

• But after the point velocity saturates and has no effects of increase in electric field.