Effect of device parameters on transconductance

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Trans-conductance is often regarded as an indicator of performance. It is defined as the ratio of change in output current to the change in the gate voltage at a constant drain voltage. Trans-conductance is given by –

Transconductance dependency on gate oxide thickness

The trans-conductance dependency on the gate oxide thickness is given by the following equation.


For the different values of gate oxide thickness for the MOS reference device which was grown earlier, the trans-conductance was calculated to test how it is affected by different gate oxide thickness. It is seen that the trans-conductance decreases with increase in gate oxide thickness. The trend seems to follow a linear decrease, the trans-conductance decreases as the gate oxide thickness increases.

Transconductance dependency on gate length

The dependency o transconductance on the gate length is characterized by the following equation.
The equation suggests a inverse relationship. It is seen that the tranconductance decreases linearly as gate length increase up to a certain gate length, beyond which it saturates where the decrease is not significant.


Transconductance dependency on drain doping concentration

The trans-conductance dependency on the drain doping concentration is shown in the following graph.
For increasing the heavy drain doping concentration from 1x1013 cm-3 to 5x1018 cm-3, the trans-conductance increased marginally.

Transconductance dependency on channel doping concentration

For a range of values of channel doping concentration i.e. from 5*109 cm-3 to 1*1013 cm-3, the I-V characteristics of the MOS reference device was plotted to calculate the trans-conductance and analyse its trend. It is governed by the equation below.
The charge density ‘Qi’ is dependent on the substrate (channel) doping concentration of the device Na. This doping concentration primarily affects the threshold voltage of the device and the current flow until the MOS reaches the saturation region. Hence, once the device reaches saturation, the effects of this doping concentration is muted. Therefore, the current flowing in saturation and hence the trans-conductance is not affected by the doping concentration. The dependency curve is obtained as below.

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