  A phonon and charge transport simulation based on the Geant4 Monte Carlo
toolkit is presented. The transport code is capable of propagating acoustic
phonons, electrons and holes in cryogenic crystals. Anisotropic phonon
propagation, oblique carrier propagation and phonon emission by accelerated
carriers are all taken into account. The simulation successfully reproduces
theoretical predictions and experimental observations such as phonon caustics,
heat pulse propagation times and mean carrier drift velocities.
  Implementation of the transport code using the Geant4 toolkit ensures
availability to the wider scientific community.
