Starting with Boltzmann’s equation in differential form
In addition to the trajectories, which are defined by the geometry of the phase space via the Hamiltonian , it is also required to give a specific description of the collision operator on the right hand side. In the field of semiconductor simulations it is common to describe the collision operator in the form
As such the expression for can also be rendered as as a short form for
represents the trajectory travelled before a scattering event, while the particle continues to travel on a trajectory as indicated in Figure 5.3 afer a scattering event. With the two curves requiring to match at the parameter corresponding to a collision/scattering event in the following manner
Utilizing these settings, Equation 5.42 can be rendered as
In the following the dependence of the curves on the parameter shall be suppressed, where it facilitates readability without adversely affecting clarity.
Here it is desirable to find an integrating factor, so that the left hand side resembles a total derivative. The integrating factor in this case is
The last term deserves special consideration. It is easy to argue that it represents the initial conditions of the sought function from which the system begins to evolve. It however also accommodates boundary conditions as becomes apparent, when considering that the points of the trajectories corresponding to the parameter not necessarily lie within the domain of interest. In this case the trajectory within the domain uses the value at the boundary instead as illustrated in Figure 5.4. Thus the term accommodates both, initial as well as boundary conditions for this integral equation [92].