In this tutorial we will show an example of a 3D simulation of a Si nanowire FET (NW-FET) device, with an all-around gate geometry.

In order to execute correctly the example you should have the following files in the working directory:
Si_NWFET.tib : input file for TiberCAD
Si_NWFET.msh : mesh file produced by GMSH from the script Si_NWFET.geo

Let's give now a look to the input file; for further details you can refer to the program reference manual.

 

Device structure

First we define the device structure: it is composed by four TiberCAD Regions: source, drain and channel. made of Silicon, and oxide (SiO₂).
The channel region (undoped) is constituted by a Si nanowire with a diameter of 20 nm and a length of 60 nm; source and drain regions are both heavily doped n-type 5x10¹⁹ cm^-3 ; finally, oxide is the 1.6 nm - thick gate dielectric.
In this all-around geometry, Gate contact surrounds completely the Si channel, to maximize gate control of channel current and thus the device performances.

 

 

Drift-diffusion simulation

Now, we define the TiberCAD simulation models which will be used in this example.
We are going to calculate Poisson and Drift-diffusion (model driftdiffusion) for all the device (physical_regions = all in options).
A field dependent model for mobility is applied.

Then, we have to specify the the three contacts of our NWFE.

The contacts (boundary regions for this model) have to be associated each to the corresponding region defined in the mesher program (here GMSH) as a Boundary region, in this 3D simulation a Surface (<b>Physical Surface</b> in GMSH)). This is made simply with the syntax:

and so on for the other contacts.
The gate contact is defined as a schottky contact, with a barrier value depending on the gate metal workfunction ( barrier_height).
The gate voltage is expressed by the notation @Vg[0.0]. This means that the gate voltage will be given the value of the variable Vg, specified in one of the sweep block (see below). [0.0] means that the default voltage value is 0.0 V.
Source and drain contacts are defined as ohmic; while source is fixed to 0 (voltage = 0.0), drain voltage is expressed, as for the gate, by the value of the sweep variable Vd ( voltage = @Vd[0.5]).

Next, we define the parameters for the solver of the driftdiffusion model.
coupling = electrons specifies that only one type of carrier is considered, that is electrons, ksp_type = bcgsl defines the solver type, nonlinear_solver = tiber specifies the non-linear solver.

• As a first step, we are going to calculate Id/Vd drain current characteristic.

 

we define two nested sweeps: the external sweep (sweep_gate) calculates the drain current for a series of gate voltages, from -0.5 to 2.0 V; for each calculation, a sweep on drain voltages is performed (simulation = sweep_drain), defined by sweep_drain. In sweep_drain, driftdiffusion is calculated (simulation = driftdiffusion) for a series of drain voltages from 0 to 3.5 V.

 

Finally, in Simulation section, we define the 3D (dimension = 3) simulation to be performed, specified by
solve = sweep_gate: this means that we are going to perform a calculation of Poisson and drift-diffusion
with a double sweep on gate and drain voltage, to calculate Id/Vd drain current characteristics.

Now we can run TiberCAD....

tibercad Si_NWFET.tib

 

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Output

After the execution, the output directory contain the simulation results, as defined in
output_format and plot.
Here we have the IV drain characteristics for each Vg bias between -0.5 V and 1.65 V .

 

The Id/Vg transfer characteristic can be easily calculated as in Tutorial 04

 

 

The subthreshold S parameter, given by

 

 

in this case results to be ~60 mV/dec, very close to ideal limit.

Below you can se the electron density distribution in the device with a bias of Vg = 1.6 V and Vd = 3V.
The 3D channel inversion in the nanowire is clearly visible.

 

The next one is again a picture of the electron density, this time in a section parallel to the z-axiz (channel direction), showing partial channel pinch-off at a saturation drain voltage Vd ~3V.

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Attachment	Size
Si_NWFET.geo	1.96 KB
Si_NWFET.msh	1.3 MB
Si_NWFET.tib	2.29 KB