Effect of device parameters on threshold voltage

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The threshold voltage, commonly abbreviated as VTH, of a MOSFET is the minimum gate-to-source voltage VGS (th) that is needed to create a conducting path between the source and drain terminals. It is an important scaling factor to maintain power efficiency, and an important device performance metric.

Threshold Voltage dependency on gate oxide thickness

The dependency is governed by the following equation.


The gate oxide was varied from a thickness of 20Å to 313.76Å by varying the diffusion time and temperature. As can be seen, the threshold voltage ‘VTN’ has an almost linear increase with increase in gate oxide thickness. An increase in oxide thickness also results in an increase in the threshold voltage. This is because as the thickness increases, more voltage is required to create the inversion layer in the substrate, just below the gate oxide.

Threshold voltage dependency on gate length

The threshold voltage of the MOSFET depends on the gate length following the equation.
It is seen that the threshold voltage increases linearly up to a certain gate length, beyond which it saturates and the increase is not significant.

Threshol voltage dependency on drain doping concentration

The concentration of heavy Source/Drain doping has negligible influence on the threshold voltage. It is seen that the threshold voltage remains constant even if source-drain doping is varied. This is because increase in source/drain doping mostly influences the magnitude of current in the saturation region (operating region) through the channel of the MOS V-I characteristics rather than the threshold voltage.


Threshold voltage dependency on channel doping concentration

The dependency of the threshold voltage ‘VTH’ on the channel doping concentration maybe given by the equation below.
The dependency is as obtained below.



From the resulting plot, it is seen that the threshold voltage increases as the doping concentration of the channel increases thus conforming to the equation.

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