Comparison of MOS devices with different gate materials - Polysilicon & Metal

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Below is an example of a MOSFET with a metal (aluminium) gate.

Threshold Voltage on Gate Oxide Thickness

An increase in the threshold voltage is seen with increase in oxide thickness. However, the metal gate device seems to have a higher threshold voltage than the polysilicon gate device. The difference is more apparent as the oxide thickness increases.

Threshold Voltage on Gate Length

Increasing the gate length increases threshold voltage. A metal gate device seems to offer a higher threshold voltage than its polysilicon counterpart.

Threshold Voltage on heavy Drain Doping Concentration

The choice of material for the gate is in not to the source/drain doping concentrations. In both the devices, the threshold voltage remained constant throughout. However, metal gate device tends to offer a lower threshold voltage.


Threshold Voltage dependency on Channel Doping Concentration

The channel doping concentration has an effect on the threshold voltage. For both devices, the threshold voltage increases as the channel doping concentration increases. As expected, the metal gate device offers a lower threshold voltage because of the presence more carriers.


Transconductance dependency on Gate Oxide Thickness

Both the metal gate device and polysilicon gate device showed that the transconductance decreases as the gate oxide thickness increases. However, the metal gate device had a higher trans-conductance than the polysilicon gate device.

Transconductance dependency on Gate Length

As expected by theoretical analysis, the transconductance decreased as the gate length increased for both the type of devices. The metal gate device offers a marginally higher trans-conductance than polysilicon gate device.

Transconductance dependency on Heavy Drain Doping Concentration

Increasing the heavy drain doping concentration, the transconductance increases marginally in both the devices. The metal gate device has a higher trans-conductance though.

Transconductance dependency on Channel Doping Concentration

Altering the channel doping concentration has no effect on the transconductance of either device. However, the metal gate device offers a higher transconductance.

Output Resistance dependency on Gate Length

The metal gate device has a higher output resistance than the polysilicon gate device. In fact, the output resistance difference is greater as the gate length increases.

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