which method we should follow to calculate total resistance (Rt) for graphene FETs
1. Rt = drain-source voltage/drain current
Rt = Vds/Id with constant gate voltage (Vg) and varying Vds
2. Rt = Vds/Id with contact Vds and varying Vg
to plot Rt vs Vg
Rsd or Rtotal is determined from graphene output conductance which is the slope of Ids-Vds curve at low enough Vgs value and inverting the slope you get Rsd. Low enough Vgs and low enough Vds ensures the channel is uniform and the Rsd will be more accurate as Ids-Vds will be mostly linear. High enough Vgs value makes the channel non-uniform and affects the channel mobility through vertical field. High enough Vds value distorts the overall channel thickness and Rsd then depends on channel potential from source to drain. Therefore, use low Vds values from 20 mV-0.5 V even if you will need to use high Vgs values for every Ids-Vds curve at a fixed Vgs value to get your Rsd-Vg characteristics. In fact Rsd or Rtotal is the sum of Rc= contact resistance+Rch= channel resistance+Ra = source to drain junction extension resistance+Rdiff . Rch is mostly determined from Ids-Vds slope and Rc must be determined from experimental technique. Ra and Rdiff both depend on access to channel hence on Vg and Vds and Rdiff is determined by source drain junction profile type, i.e. ultra shallow junction or gaussian profile. Ideally Rch is mostly determined Ids and Vds values. Therefore your Rsd versus Vg plot determined experimentally may look different including significant non linearity due to Ra and Rdiff coming out of Vg dependent spatial extent of channel close to extension region along the doping profile of the junction close to the interface. I am attaching one paper that you may look for more information.
If my answer is useful for your query, please recommend my answer.
Sincerely,
Dr. Nabil Shovon Ashraf.
Sheet resistance for graphene (Google it ) .
http://iopscience.iop.org/article/10.1088/2053-1583/2/3/034011/pdf
The following attached thesis will give you clues to calculate Rc, Rch, Ra to determine Rtotal. In fact the thesis has a plot of Rtotal versus Vg that you are trying to extract and therefore should look at. Please recommend my answer if this is useful.
Sincerely,
Dr. Nabil Shovon Ashraf.
I think its more complicated (yet simple) than just calculating R =V/I. In case of a graphene device, one needs to calculate components of the total resistance - namely, sheet resistance and contact resistance. An important factor is also the graphene-metal work function. For detailed discussion involving different metal-graphene contacts, please refer to the following paper from our group:
Article Contact resistance study of various metal electrodes with CVD graphene
I hope this helps you.
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