  We describe Simudo, a free Poisson/drift-diffusion steady state device model
for semiconductor and intermediate band materials, including self-consistent
optical absorption and generation. Simudo is the first freely available device
model that can treat intermediate band materials. Simudo uses the finite
element method (FEM) to solve the coupled nonlinear partial differential
equations in two dimensions, which is different from the standard choice of the
finite volume method in essentially all commercial semiconductor device models.
We present the continuous equations that Simudo solves, show the FEM
formulations we have developed, and demonstrate how they allow robust
convergence with double-precision floating point arithmetic. With a benchmark
semiconductor pn-junction device, we show that Simudo has a higher rate of
convergence than Synopsys Sentaurus, converging to high accuracy with a
considerably smaller mesh. Simudo includes many semiconductor phenomena and
parameters and is designed for extensibility by the user to include many
physical processes.
