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This example describes how to simulate the electrical behavior of an n-channel metal oxide semiconductor field effect transistor (MOSFET). In particular, we will calculate gate and drain I-V characteristics as well as leakage current with trap assisted and band to band tunneling.

 

A project file and script file are provided to assist with the application example. The project file contains the material properties, geometry, and simulation region required to run the example.

 

 

NOTE: If you are using an older version of CHARGE (v.4.6.631 or older) then please download the simulation (.ldev) and script (.lsf) files from this archived KB page as the current files may not run with the older CHARGE.

 

Simulation Setup

 

Open the nmos.ldev file in CHARGE. Notice that the MOSFET is specified as follows:

 

A 2 um silicon layer is grown on a thick silicon dioxide (oxide) layer.

The channel length is 1.5 um.

SiO2 is used as the gate oxide.

To define the space-charge regions in the silicon, first, the background doping concentration of the silicon is set to represent a p-type epitaxial layer with a concentration of 1x1015 cm-3. This is accomplished by defining a region of constant doping that encompasses the entire geometry. Next, diffusion doping regions are used to specify the dopant concentrations in the source and drain region and the region underneath the gate.

 

The doping concentration of the source and drain regions can be visualized as shown below. Click on the "mesh" button to mesh the simulation. Once the meshing is done, open and run the doping_profile.lsf script file provided in the associated files section.

 

hmfile_hash_00308ab9

source_profile_zoom65

 

Material Properties

For this example, silicon, aluminum, n-gate silicon and SiO2 (silicon dioxide) are defined. Expand the Material Group in the Objects Tree so you can view the materials used in the simulation. Three types of materials are defined:

 

Conductor (for aluminum in the source, drain and body contacts and n-gate silicon for the gate)

Insulator (for the field oxide, SiO2)

Semiconductor (for the bulk silicon)

 

The conductors are defined with a work function. The insulator is defined with a DC dielectric permittivity value. The semiconductor is defined using the more complex semiconductor models available in the material database. In particular the "Recombination" tab of this material is of interest in this example.

 

For all parts of this example, the bulk recombination models, trap-assisted, radiative and Auger recombination are enabled.

 

Further, trap assisted tunneling can be enabled using the field drop down menu under "Carrier Lifetime Correction Models" section.

when enabled, two models, Hurkx and Schenk are available. For the Hurkx model used here the tunneling mass coefficient mt is 0.25.

 

Similarly, under "Band to Band Tunneling",  two models, Hurkx and Schenk are available.  For the Hurkx model used here, the coefficients are as follows:

 

A = 4e14 cm-3s-1

B = 1.9e7 V/cm

Eta =2.5

 

For descriptions of the recombination processes as well as the tunneling models, please see Lumerical's knowledgebase on Semiconductors.

 

recombination_zoom47

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