The equations solved by the Semiconductor interface are highly nonlinear and are consequently difficult to solve in the absence of good initial conditions. The default initial conditions are good approximate guesses for the carrier concentration in the absence of applied voltages or currents. For stationary models when currents flow, it is often necessary to ramp up the currents or voltages slowly to the desired operating point within the solver. To achieve this, ensure that the currents and voltages in the model are controlled by parameters and then use the Auxiliary sweep settings in the
Study Extensions section of the
Stationary study step. Then set up the voltage and current parameters so that these are swept up from zero. Note that you select the parameter on which the continuation solver should be used in the
Run continuation for list. When performing multiple sweeps it is desirable to set the
Reuse solution for previous step setting to
Auto. Note also that the solution from one study can be used as the starting point for a second study by altering the settings in the
Values of Dependent Variables section of the
Stationary study step. To do this, select the
Initial values of variables solved for check box, select
Solution for the
Method, and then select the appropriate solution from the
Study list.
In addition to ramping up currents and voltages, it is also possible to gradually introduce nonlinear contributions to the equation system using the continuation solver. This is particularly useful when problems are highly nonlinear. When this approach is appropriate a Continuation Settings section is present in some of the semiconductor feature settings. By default
No continuation is selected, which means that the equation contribution is as specified. For
User defined an input for the
Continuation parameter appears and this should be set to a parameter with values between 0 and 1 to determine the scaling of the equation contribution. Adding the name of a parameter to this setting and ramping it from 0 to 1 in the continuation solver will gradually introduce the contribution into the equation system. By choosing
Use interface continuation parameter the continuation parameter is linked to the value of the interface-level continuation parameter specified in the
Continuation Settings section of the Semiconductor interface’s
Settings window. This enables several features to be ramped up simultaneously. Finally it is possible to ramp up the dopant concentration, starting from a small fraction of that specified (or even zero) and gradually increasing the doping. By default the
Continuation Settings for the doping features link to an interface-level doping continuation parameter that can be set up in the
Continuation Settings section of the Semiconductor interface’s
Settings window. In turn this can be linked to the interface level continuation parameter or specified separately. For the finite volume method the doping continuation parameter should be started at a small value (for example,
10−6) rather than zero, as the continuation solver does not handle the transition from no doping to finite doping when this numerical method is used. The doping continuation can also be disabled or defined within each individual feature.
The Semiconductor Equilibrium study step can often provide a good initial condition for subsequent ramping up of applied voltages.
In many cases the time-dependent solver performs better than the continuation solver for nonlinear stationary problems. By changing the Equation setting in the Semiconductor interface
Settings window from the default (
Study controlled) to
Stationary, you can force the physics interface to use the stationary equation form even though the time dependent solver is used. Then parameters can be defined as functions of time, so that the applied currents and voltage, as well as nonlinear equation contributions and the doping, can be gradually turned on.
Using the correct solver tolerance for a problem is also important. The nonlinear solver tolerances are set up differently in 1D, 2D, and 3D to achieve a balance between solution speed and accuracy. For minority carrier devices it might be necessary to tighten the solver tolerances to ensure that the solution is converged to within sufficient accuracy. For a Stationary study the solver tolerance is adjusted in the Stationary Solver node of the
Solver Configurations branch (if this branch is not visible click the study and from the
Study toolbar, click
Show Default Solver (
)). The
Relative tolerance setting determines the fractional accuracy that the carrier concentrations are solved to. The Time-Dependent Solver has both relative and absolute tolerance settings. The relative tolerance setting is available on the
Time Dependent study step in the
Study Settings section. The absolute tolerance is set in the
Time-Dependent Solver node on the
Solver Configurations branch.
When modeling nonisothermal semiconductor devices, the solver settings must be adjusted so that the temperature degree of freedom is in a separate segregated group from the carrier densities and the potential. To do this, click the study and from the Study toolbar, click
Show Default Solver (
) (this step is not necessary if the
Solver Configurations node is already visible in the model tree). Then expand the Solver Configurations node until you can see either the
Stationary Solver 1 or the
Time-Dependent Solver 1 node (depending on whether the study is stationary or time-dependent). Right-click this node and select
Segregated. Expand the resulting
Segregated 1 node and select the
Segregated Step node. Then in the
General settings for this node, delete the
Temperature (mod1.T) variable from the list. Next right-click the
Segregated 1 node and select
Segregated Step. In the settings for the resulting
Segregated Step 1 node, add
Temperature (mod1.T) to the list. Note that the node numbers given above can be altered if multiple studies or study steps are present in the model.