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We investigate the accuracy of the Ehrenfest approximation for a solid modelled as an independent electron system coupled to a many-mode phonon system. We construct a quantum to classical correspondence which we use to set the initial conditions of the Ehrenfest approximation given an initial density matrix. We find that the Ehrenfest approximation is accurate when the temperature of the phonon system is much higher than the temperature of the electron system. It is therefore reasonable to use the Ehrenfest approximation in the simulation of the later stages of radiation damage processes. We trace the failure of the Ehrenfest approximation to the absence of a term accounting for spontaneous phonon emission. The analytical work is validated numerically for the model system of a single oscillator coupled to electrons on a one-dimensional chain.

In this study, high brightness LED package is considered. Steady state heat transfer process analysis is firstly carried out using 3-D finite element method. Temperature distribution and thermal resistance of the package are then determined. The FEM results are evaluated by thermal resistance measurement on the package by T3STer system. In addition design study on the thermal performance of the packaging structure is also performed. The analysis results show that die attachment (solder material) plays the most important role in the thermal resistance of LED package. Thermal resistance of the package is mainly caused by the interfacial thermal resistances. It can be found out that Al 2 O 3 isolation ring increases the thermal resistance of the package and pure Aluminum substrate achieves a better performance in the respect of thermal behavior of packaging designs. Mechanical reliability analysis has also been carried out. Failure of the package occurs in the anodized Al 2 O 3 isolation ring during die bonding process due to the material defects of Al 2 O 3 in the manufacturing process. 3D mechanical FEM is used to figure out the failure mechanism. Design optimization on the packaging structure is also performed to improve the mechanical reliability. A few new structure designs are proposed, analyzed and compared. Design with the best reliability among the candidates is chosen and the corresponding manufacturing processes are proposed.