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Velocity saturation – Mobility degradation – body effect – CLM

Velocity saturation - Mobility degradation - body effect - CLM

The non-ideal current-voltage effects such as velocity saturation, mobility degradation, body effect, and the channel length modulation CLM are explained.

At the high lateral electric field Elat that is equal to Vds/L, the velocity of the carrier ceases to increase linearly with the field strength is called Velocity saturation.
The velocity saturation results in lower Ids that which is expected at High Vds.

Mobility Degradation

At high vertical field strengths Evert that is equal to Vgs/tox, the carriers scatter in the oxide SiO2 interface and the process gets slower is called Mobility degradation.

Mobility degradation also reduces the current Ids that are expected at high Vgs.

Channel length modulation CLM

The saturation current Isat of the non-ideal transistor increases with Vds, which is mainly caused by CLM in which higher Vds increase the size of the depletion region surrounding the drain terminal.

The CLM shortens the effective channel length Leff.
Leff = L – Ld
Increase in Vds, Decreases the Leff.
And the current equation gets multiplied by the factor (1+λVds).

Body effect

The threshold voltage Vth indicates the minimum voltage required to invert the channel.
The Vth is primarily determined by the oxide thickness tox and the channel doping levels.
An increase in the Vsb source to body voltage increases the Vth through the effect called body effect.
Vth = Vt0 + ϒ (√(Φs + Vsb) – √Φs)
ϒ = (√(2 q εsi Na) / Cox) body effect coefficient.

Short channel and Narrow channel effect

Short channel and Narrow channel effect

In short channel effect for smaller channel length L the threshold voltage Vth starts reducing.
In narrow channel effect for smaller channel length L the threshold voltage Vth starts increasing.

Subthreshold leakage

Short channel and Narrow channel effect
When Vgs is less than Vth, the device current Id drops off exponentially rather than becoming directly zero this is mainly because of subthreshold conduction. The subthreshold conduction is mainly due to Weak inversion when Vgs is less than Vth.
The subthreshold leakage current increases significantly more with the increase in Vds that is because of the DIBL (Drain induced barrier lowering) effect.
The subthreshold leakage is dependent on the process we use and the technology we consider for our design.

Sometimes the subthreshold leakage can be used as an advantage in low power VLSI circuit design. also in applications such as DRAMs, it is advantageous because DRAMS depends on the storage of charge on the capacitor.

Junction leakage

The source S and drain D diffusion regions are typically in a reverse-biased fashion.
When there is transient or voltage fluctuation, which caused the substrate voltage to increase, The diodes tend to conduct and experiences the junction leakage current into the substrate.
The reverse-biased diode conduction current can be expressed as
Id = Is (e^(Vd/Vt) – 1)

Here, diode saturation current Is depends on the doping levels and area and perimeter of the diffusion region.

Temperature dependence

The parameters such as mobility (µ), the saturation voltage Vsat, the threshold voltage Vth, and on current Ion all these decrease with an increase in temperature.
The subthreshold leakage, band to band tunneling BTBT, and Idsat at low Vdd all these increase with an increase in temperature.
In general, the overall system performance is improved by cooling the device.

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