This article describes how to create a new semiconductor material model in the material database. For details on the Semiconductor material input parameters, see the Semiconductor material model page.
While the electrical material database contains models for many common semiconductors, it may be necessary to add new semiconductor models for different systems. This section will describe how to set the parameters necessary for a minimal semiconductor model. A new semiconductor can be added to the material database by opening the material database and choosing the "Semiconductor" option from the "Add" button menu. The newly defined semiconductor can be named and a color can be chosen to represent the material in the layout.
When a new semiconductor material is created, the first tab in the material properties displays the fundamental electrical properties of the semiconductor. These include the basic properties that define the electronic behaviour of a semiconductor, including the relative dielectric permittivity, effective mass, and band gap. Both the effective mass and band gap can be visualized directly in the material database.
The minimum set of fundamental properties required to define a semiconductor include
- relative dielectric permittivity,
- intrinsic work function
The relevant conduction valleys should be enabled.
- Effective mass of the electrons and holes, and
- Band gap energy.
Each of these values may be entered as constants. Note that the relative dielectric permittivity is assumed to be the DC value and is related to the material index as εr = n2. Often times the effective density of states is known, while the effective mass is not: to convert from effective density of states to effective mass, please refer to the section on calculating the effective density of states. To verify the effective mass and band gap values, the intrinsic carrier density ni is automatically calculated at T = 300K,
In addition to specifying the properties described above, temperature dependences may be specified for the effective mass and band gap by clicking on the adjacent buttons "f(T)". This action will bring up a parameter editor which displays the associated formula. A band gap narrowing model can also be specified by choosing the "Slotboom" model from the list of choices. This model will correct the band gap energy with respect to the net dopant concentration. The band gap can be visualized in the adjacent plot to verify its behaviour.
The mobility must be defined for both the electrons and holes, and can include corrections for temperature dependence, impurity (dopant) and free carrier scattering, and velocity saturation.
The minimum definition for a semiconductor material must include a constant lattice scattering mobility for both the electrons and holes. These values can be entered as constants.
The basic mobility may be modified by a temperature dependence, whose model can be accessed by selecting the adjacent "f(T)" button. In addition, corrections to the mobility can be enabled for impurity and free carrier scattering effects using a choice of models. The parameters for the models and their formulas can be accessed by clicking the "..." button next to the model selection.
Note: For a basic correction to the mobility that accounts for doping, choose the "Caughey-Thomas" model. Parameters for this model are available for many common semiconductors.
Note: The Klaassen model for the impurity and free-carrier scattering is only well parameterized for silicon, and is not recommended for other materials.
In addition to corrections to the mobility based on doping density, velocity saturation may be enabled by specifying a saturation velocity for both electrons and holes.
Recombination and Generation
Carrier lifetime information
A basic semiconductor model does not require recombination and generation processes to be enabled. However, a realistic semiconductor model will include some information about the carrier lifetime. To include the carrier lifetime behaviour in the model, enable the "Trap-Assisted (Shockley-Read-Hall) Recombination" model under the "Recombination" tab, and enter the carrier lifetime values for the electrons and holes. If properties of the impurities are known, the offset from the mid-gap energy level can be entered in the "Trap Properties" group. For details on how to determine the carrier lifetimes from trap properties, please consult the description of the Shockley-Read-Hall model in the bulk recombination and generation section of the semiconductor model reference.
NOTE: A basic semiconductor model does not require recombination and generation models for the bulk or surface. A new semiconductor material will be defined without any defined surface interfaces, and in this case, the default behaviour (ideal surfaces) will be used. For more information on defining surface interface properties, please see the section on Boundary Conditions