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rasmita sahoo

why the saturation current increases with decrease in diameter of the CNT

I had done the simulation using your tool “CNTFET Lab” available at nanohub. I have done the simulations for different diameter of the CNT (by changing the chirality) used in the channel. The simulation result shows that the saturation drain current increases with decrease in the diameter of the CNT. In conventional MOSFET the trend is just opposite. So I am in doubt why it is coming like this? Is it correct? If yes what is the cause?

Further I have also checked the same for coaxial CNTFET using the tool “FETToy” which is also available at nanohub. For this case the drain current increases with increase in diameter.

Please clarify this by giving your valuable guidance

Thanking you

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    David A Saenz

    I am agree with Shaahin, the more conduction channels, the more electrons that can flow through the device. That is, in other words, increasing the number of atoms that can conduct electrons, the higher current you can get. The only reason I would think of CNT having the inverse effect is the lack of interaction between CNT opposite walls due to the higher distance separating them.

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    Shaahin G Shirazi

    I checked the results for 4 different (n,0) CNT zigzag channels in coaxial structure. When V(gate)=V(drain)= 5V and V(source)=0V, following results obtained

    For (13,0) drain current is 0.0025A For (17,0) drain current is 0.0035A For (20,0) drain current is 0.0040A For (25,0) drain current is 0.0050A

    Also, when V(gate)=V(drain)= 1V and V(source)=0V the following results obtained

    For (13,0) drain current is 27.471μA For (17,0) drain current is 80.1608μA For (20,0) drain current is 92.977μA For (25,0) drain current is 118.302μA

    Then in this tool, as it can be seen, the saturation current increases by increasing the CNT diameter.

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    Shaahin G Shirazi

    It can be explained by considering the device energy band profile. Saturation current in these devices is thermionic emission (TE) current at high enough gate fields. In TE mechanism the careers flow from source to drain over top of the potential barrier (between source and channel). When the gate field increases, the potential barrier is pulled down and the Fermi level of the source (set as reference) can stay at the top of the conduction band of channel. Then more careers can flow at energy range between the source Fermi level and channel conduction band. There is an inversion relation between CNT diameter and its energy band gap. The source Fermi level is reference while should be stated at the middle band gap of channel (undoped CNT) at zero gate field. Consequently, when the energy band gap decreases (diameter increases) the potential barrier between source and channel decreases. Therefore, more careers can flow via TE mechanism at the same gate field for higher diameter values. In such situation, the threshold voltage also decreases. For this reason you can get higher current by device with higher CNT diameter.

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