Experimental Determination of the Impact Ionization Coefficients in Irradiated Silicon

We present new results on the influence of radiationinduced
damage on the electron Impact Ionization (I.I.) coefficient
, suggesting a small but distinct reduction of at high fluence
with respect to unirradiated silicon. Experiments on thick (1.5 m)
and thin (1 m) epitaxial silicon samples confirm that such a reduction
of is expected even in cases where impact ionization is not
simply a field driven process because of strongly non local transport
conditions.
A consistent increase on the breakdown voltage of a 3D radiation
detector has been evaluated by means of TCAD simulations using
the experimentally extracted I.I. coefficient for irradiated silicon.
These results clarify the impact of radiation damage on some
of the key model parameters for TCAD simulations and allow for
improved accuracy toward predictive breakdown simulations of
silicon particle detectors, e.g., for the ATLAS experiment.