  Mainly due to their outstanding performance the position sensitive silicon
detectors are widely used in the tracking systems of High Energy Physics
experiments such as the ALICE, ATLAS, CMS and LHCb at LHC, the world's largest
particle physics accelerator at CERN, Geneva. The foreseen upgrade of the LHC
to its high luminosity (HL) phase (HL-LHC scheduled for 2023), will enable the
use of maximal physics potential of the facility. After 10 years of operation
the expected fluence will expose the tracking systems at HL-LHC to a radiation
environment that is beyond the capacity of the present system design. Thus, for
the required upgrade of the all-silicon central trackers extensive measurements
and simulation studies for silicon sensors of different designs and materials
with sufficient radiation tolerance have been initiated within the RD50
Collaboration.
  Supplementing measurements, simulations are in vital role for e.g. device
structure optimization or predicting the electric fields and trapping in the
silicon sensors. The main objective of the device simulations in the RD50
Collaboration is to develop an approach to model and predict the performance of
the irradiated silicon detectors using professional software. The first
successfully developed quantitative models for radiation damage, based on two
effective midgap levels, are able to reproduce the experimentally observed
detector characteristics like leakage current, full depletion voltage and
charge collection efficiency (CCE). Recent implementations of additional traps
at the SiO$_2$/Si interface or close to it have expanded the scope of the
experimentally agreeing simulations to such surface properties as the
interstrip resistance and capacitance, and the position dependency of CCE for
strip sensors irradiated up to $\sim$$1.5\times10^{15}$
n$_{\textrm{eq}}\textrm{cm}^{-2}$.
