ELIMINATE COLUMNS X.MIN=0 X.MAX=4 Y.MIN=0.0 Y.MAX=3
removes every second vertical grid line within the rectangle bounded by x=0, x=4, y=0 and y=3 microns.
7.4.4. Specifying Regions and Materials
Once the mesh is specified, every part of it must be assigned a material type. This is done with REGION statements. For example:
REGION number =<integer><material_type><position parameters>
Region numbers must start at 1 and are increased for each subsequent region statement. You can have up to 200 different regions in ATLAS. A large number of materials is available. If a composition- dependent material type is defined, the x and y composition fractions can also be specified in the REGION statement.
The position parameters are specified in microns using the X.MIN, X.MAX, Y.MIN, and Y.MAX parameters. If the position parameters of a new statement overlap those of a previous REGION statement, the overlapped area is assigned as the material type of the new region. Make sure that materials are assigned to all mesh points in the structure. If this isn’t done, error messages will appear and ATLAS won’t run successfully.
You can use the MATERIAL statement to specify the material properties of the defined regions. But you must complete the entire mesh and doping definition before any MATERIAL statements can be used.
7.4.5. Specifying Electrodes
Once you have specified the regions and materials, define at least one electrode that contacts a semiconductor material. This is done with the ELECTRODE statement. For example:
ELECTRODE NAME=<electrode name><position_ parameters>
You can specify up to 50 electrodes. The position parameters are specified in microns using the X.MIN, X.MAX, Y.MIN, and Y.MAX parameters. Multiple electrode statements may have the same electrode name. Nodes that are associated with the same electrode name are treated as being electrically connected.
Some shortcuts can be used when defining the location of an electrode. If no y coordinate parameters are specified, the electrode is assumed to be located on the top of the structure. You also can use the RIGHT, LEFT, TOP, and BOTTOM parameters to define the location. For example:
ELECTRODE NAME=SOURCE LEFT LENGTH=0.5
specifies the source electrode starts at the top left corner of the structure and extends to the right for the distance LENGTH.
7.4.6. Specifying Doping
You can specify analytical doping distributions, or have ATLAS read in profiles that come from either process simulation or experiment. You specify the doping using the DOPING statement. For example:
DOPING<distribution_type><dopant_type><position_parameters>
Analytical Doping Profiles
Analytical doping profiles can have uniform or Gaussian forms. The parameters defining the analytical distribution are specified in the DOPING statement. Two examples are shown below with their combined effect shown in Figure 7.5.
DOPING UNIFORM CONCENTRATION=1E16 N.TYPE REGION=1
DOPING GAUSSIAN CONCENTRATION=1E18 CHARACTERISTIC=0.05 P.TYPE \ X.LEFT=0.0 X.RIGHT=1.0 PEAK=0.1
The first DOPING statement specifies a uniform n-type doping density of 10E16 cm-3 in the region that was previously labelled as region #1. The position parameters: X.MIN, X.MAX, Y.MIN, and Y.MAX can be used instead of a region number.
The second DOPING statement specifies a p-type Gaussian profile with a peak concentration of 10E18 cm-3. This statement specifies that the peak doping is located along a line from x = 0 to x = 1 microns. Perpendicular to the peak line, the doping drops off according to a Gaussian distribution with a standard deviation of 0.05 mm. At x<0 or x>1, the doping drops off laterally with a default standard deviation that is 70% of CHARACTERISTIC. This lateral roll-off can be altered with the RATIO.LATERAL parameter. If a Gaussian profile is being added to an area that was already defined with the opposite dopant type, you can use the JUNCTION parameter to specify the position of the junction depth instead of specifying the standard deviation using the CHARACTERISTIC parameter.
Figure 7.5. Parameters of the doping statement for Gaussian Doping Profiles.
7.4.7. Specifying Physical Models
Physical models are specified using the MODELS and IMPACT statements. Parameters for these models appear on many statements including: MODELS, IMPACT, MOBILITY, and MATERIAL. The physical models can be grouped into five classes: mobility, recombination, carrier statistics, impact ionization, and tunneling. Chapter 3: “Physics”, Section 3.6: “Physical Models” contains details for each model.
All models with the exception of impact ionization are specified on the MODELS statement. Impact ionization is specified on the IMPACT statement. For example, the statement:
MODELS CONMOB FLDMOB SRH FERMIDIRAC
IMPACT SELB
specifies that the standard concentration dependent mobility, parallel field mobility, Shockley-Read- Hall recombination with fixed carrier lifetimes, Fermi Dirac statistics and Selberherr impact ionization models should be used.
ATLAS also provides an easy method for selecting the correct models for various technologies. The MOS, BIP, PROGRAM, and ERASE parameters for the MODELS statement configure a basic set of mobility, recombination, carrier statistics, and tunneling models. The MOS and BIP parameters enable the models for MOSFET and bipolar devices, while PROGRAM and ERASE enable the models for programming and erasing programmable devices. For example, the statement:
MODELS MOS PRINT
enables the CVT, SRH, and FERMIDIRAC models, while the statement:
MODELS BIPOLAR PRINT
enables the CONMOB, FLDMOB, CONSRH, AUGER, and BGN.
Note: The PRINT parameter lists to the run time output the models and parameters, which will be used during the simulation. This allows you to verify models and material parameters. We highly recommend that you include the PRINT parameter in the MODEL statement.