Modeling
of carrier transport in semiconductors

In parallel
with the activity on numerical analysis, M. R. carried out
investigations on the
physics and modeling of carrier transport in semiconductors, starting
with studies
on majority- and minority-carrier lifetimes [12], measurements of
surface-states [61] and doping profile [14] by the G-V and C-V
technique. In particular, M. R. coauthored a number of papers proposing
an analytical
model for MOSFETs including drift and diffusion currents [13,64]. The
model was then extended to the case of amorphous silicon [120,122]. Of
particular relevance is the extension of the hydrodynamic model to the
full-band
case, along with a number of related investigations whose aim was an
accurate
modeling of the model's parameters, in essence, the relaxation times
for
average momentum, average energy, average energy flux of the carriers
[5,7,11,24,118,134],
and the impact-ionization coefficients [27,30,83,133]. The anisotropy
of the relaxation
times and of the carrier thermal conductivity has also been
investigated, using
higher order approaches like the Monte Carlo
method
or the correlation function method [47,55,101,102,140]. An interesting
theoretical
result is the derivation of the set of transport equation of classical
Thermodynamics as the limiting case of the hydrodynamic equation; this
is
demonstrated in [56] starting from the microscopic form of the carrier
entropy.
The investigation of the high-field effects has been extended to
include the
models of other relevant phenomena, in particular the Fowler-Nordheim
tunnel, the
hot-carrier injection from silicon into silicon dioxide, the
band-to-band
tunneling, and the trap-assisted tunneling. The models have been
implemented in HFIELDS and validated against experimental data, this
making it possible
to simulate device features that play a major role in the modern VLSI
technology
(e.g., the writing and erasing operation of EEPROM memory cells
[57,113,117,118,142]).

As indicated
above, M. R. had a role in several Projects funded by the European
Union, in
which the Department of Electronics (DEIS) of the University of Bologna
participated. An outline of the activitiesof the most recent Projects
is given
in [36,90,137;32,40,91]. In particular, in the frame of the Project
E-6075 and
in cooperation with the Institute of Integrated Circuit (IIS) of the
ETH Zuerich, some of the expertise gained in the
development of HFIELDS was used in the implementation of the
device-analysis
tool DESSIS [111].