SHORT CHANNEL EFFECTS IN MOSFETS- VLSI DESIGN

SHORT-CHANNEL EFFECTS
A MOSFET is considered to be short when the channel length ‘L’ is the same order of magnitude as the depletion-layer widths (xdD, xdS). The potential distribution in the channel now depends upon both, transverse field Ex, due to gate bias and also on the longitudinal field Ey, due to drain bias When the Gate channel length <<1 m, short channel effect becomes important .
This leads to many
undesirable effects in MOSFET.
The short-channel effects are attributed to two physical phenomena:
A) The limitation imposed on electron drift characteristics in the channel,
B) The modification of the threshold voltage due to the shortening channel length.
In particular five different short-channel effects can be distinguished:
1. Drain-induced barrier lowering and “Punch through”
2. Surface scattering
3. Velocity saturation
4. Impact ionization
5. Hot electrons

1. SHORT CHANNEL EFFECTS IN MOSFETS NITHIN KALLEPALLY VLSI DESIGN

2. SHORT-CHANNEL EFFECTS A MOSFET is considered to be short when the channel length ‘L’ is the same order of magnitude as the depletion-layer widths ( x dD , x dS ). The potential distribution in the channel now depends upon both, transverse field Ex, due to gate bias and also on the longitudinal field Ey , due to drain bias When the Gate channel length <<1  m, short channel effect becomes important . This leads to many undesirable effects in MOSFET.

3. The short-channel effects are attributed to two physical phenomena: A) The limitation imposed on electron drift characteristics in the channel, B) The modification of the threshold voltage due to the shortening channel length. In particular five different short-channel effects can be distinguished: 1. Drain-induced barrier lowering and “Punch through” 2. Surface scattering 3. Velocity saturation 4. Impact ionization 5. Hot electrons

4. DRAIN INDUCED BARRIER LOWERING Drain induced barrier lowering and Punchthrough The potential barrier, in small-geometry MOSFETs, is controlled by a two-dimensional electric field vector (in other words by both V GS and V DS ).If the drain voltage is increased the potential barrier in the channel decreases(= pn junction band cure with more – ve slop), leading to Drain-Induced Barrier Lowering (DIBL). Under DIBL condition electrons can flow between the source and drain even if VGS < VTH. Because of charge sharing the Threshold Voltage also decreases,Reduction in S and D junction depth xj can reduce the VTH shift as charge shared by drain and source gets lower.

5. Punchthrough Increase in VDS makes depletion width more and more to increase and there is occurrence of punch through once these depletion regions of source and drain touches .It can be decreased by having higher doping levels either in substrate or near source and drain.

6. SURFACE SCATTERING For small-geometry MOSFETs, the electrons mobility in the channel depends on a two-dimensional electric field (Ex, Ey ). The surface scattering occurs when electrons are accelerated toward the surface by the vertical component of the electric field Ex S Causes a reduction in the mobility The average surface mobility is about half as much as that of the bulk mobility G S D Substrate

7. VELOCITY SATURATION The electron velocity is related to the electric field through the mobility: V= μE For higher fields the velocity does not increase with electric field, we have degradation of mobility because of scattering by vertical field. This leads to earlier saturation of current. i.e. ,before VGS-VTH. Net result is reduction in drain current . The velocity saturation reduces the transconductance of short-channel devices in the saturation condition.

8. IMPACT IONIZATION The presence of high longitudinal fields can accelerate electrons that may be able of ionizing Si atoms by impacting against them Normally most of the e- are attracted by the drain, so it is plausible a higher concentration of holes near the source If the holes concentration on the source is able to creates a voltage drop on the source-substrate n-p junction of about 0.6V then e- may be injected from source to substrate e- travel toward the drain, increasing their energy and create new e-h pairs e- may escape the drain fields and affect other devices

9. HOT ELECTRONS The channel Hot Electrons effect is caused by electrons flowing in the channel for large VDS e- arriving at the Si-SiO2 interface with enough kinetic energy >3.1ev to surmount the surface potential barrier are injected into the oxide This may degrade permanently the C-V characteristics of a MOSFETs

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