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].