In this video we are going to learn about different boundary conditions available in
the CHARGE solver.
The Boundary condition objects allow the user to define various electrical and thermal boundary
conditions at the simulation boundaries and interfaces.
All the available boundary conditions can be found in the “boundary conditions”
section under the “CHARGE” tab.
Once added, they appear inside the “boundary conditions” group under the CHARGE solver
in the objects tree.
Here we only discuss the electrical boundary conditions available for the CHARGE solver.
The thermal boundary conditions are only available in coupled heat and charge simulation mode.
For information about thermal boundary conditions, you can refer to the HEAT solver online course
(HT100) or the related link below.
In the property editor window of every boundary condition object, the Geometry tab defines
the location where the boundary condition object gets applied by using reference geometries.
This can be at the interface between two materials, at the surface of a geometric solid, at a
simulation boundary, or at the intersection of a solid and the simulation boundary.
To get familiar with the concept and different types of reference geometries, please visit
the “Reference Geometries” unit of this course.
The rest of the settings for different boundary condition types can differ due to the difference
in their definition and behavior.
An electrical boundary condition is the most commonly used boundary condition in CHARGE
simulations and can be used to define a bias voltage at a simulation boundary.
It is most frequently used to define electrical contacts for a semiconductor device.
The BC mode can be steady-state or transient keeping in mind that a transient definition
is only valid in the case of a transient simulation.
In the steady-state mode, the voltage can remain constant by choosing the sweep type
to be single or can be swept linearly over a range of values by selecting the “range”
In addition, it is possible to define a table of custom voltage values to sweep over when
the sweep type is set to “value”.
The “force ohmic” option, when enabled, will make sure that the metal-semiconductor
junction formed at the contact is of ohmic type not Schottky to avoid the blockage of
current through the contact resulting from a Schottky metal-semiconductor junction.
Unless you have the intention to form a Schottky contact, this option should always remain
enabled to allow an electrical contact to function properly.
There is also an option to apply an AC small signal to the electrical contact which will
be covered in “small signal AC simulations” subsection of this course.
Series and shunt resistances can also be defined for an electrical boundary condition to model
non-idealities such as voltage drop or current leakage at the contacts resulting from these
When a transient simulation is being performed, choosing the transient bc mode allows the
user to define a table of voltage versus time which can be used to define a time-varying
voltage at the corresponding boundary.
This will be covered in the “transient simulations” subsection of this course.
The recombination processes in a semiconductor can be divided into two major categories.
Bulk recombinations which were discussed in the materials section of this course and surface
The surface recombination process concerns the charge behavior at the interface between
a semiconductor and a conductor or a semiconductor and an insulator where Electrons and holes
interact with trap states at the surface and recombine.
The effectiveness of surface recombination is described by surface recombination velocity,
chosen to reflect the non-ideal nature of the material surface.
A higher surface recombination velocity means more recombination at the surface.
In addition, surface recombination velocity may be temperature dependent and a model can
be used to represent this in a surface recombination boundary condition.
When applying surface recombination at the interface between a conductor and a semiconductor,
due to numerical stability reasons, the surface recombination should not be applied to the
This is accomplished by un-checking the "Apply to majority carriers" option, which is the
default state for a newly added boundary condition.
This assumption is appropriate at a conductor interface where the doping concentration is
The relative change in the carrier densities due to recombination may be large for the
minority carriers, but will typically be negligible for the majority carriers.
Therefore, the physical behavior of the interface is correctly modeled by applying the surface
recombination model to the minority carriers only, while fixing the majority carriers at
their equilibrium density.
Unlike conductors, when specifying the surface recombination properties for a semiconductor-insulator
interface, it is correct to ensure that the "Apply to majority carriers" option is checked.
This will ensure that the density of both the majority and minority carriers at the
insulator interface are adjusted according to the surface recombination model.
Finally, the energy level of the trap states in the surface recombination model can be
specified by setting its energy offset from the mid-gap energy level of the semiconductor
Deepest trap levels which result in highest recombination rate will have a mid-gap energy
level or equivalently zero offset.