In this work the effects of controlled rolling parameters and adding of Niobium have been studied. In this order two steel grades with and without Niobium are planed and after steelmaking and continuous casting, rolling process are done. Then, laboratory investigations such as microstructure, mechanical properties and grain size analysis were performed Tensile and Charpy impact tests specimens were machined out of the central part of the rolled billets. The microstructure of the specimens was examined for each experimental condition using optical microscopy. The results indicate that increasing the reheating temperature above the dissolution temperature of Nb (C, N) improved the impact energy values. By increasing the cooling rate from 0.5 to 1.5 ºC/s both tensile strength and impact toughness were improved. High elongation percent was also observed on samples reheated at higher temperature and/or cooled with the higher cooling rates. The obtained mechanical properties were related to the characteristics of microstructural components including acicular ferrite, retained austenite, pearlite and ferrite.

Ensemble Monte Carlo simulations have been carried out to investigate the effects of upper valleys on the characteristics of wurtzite GaN MESFETs. Electronic states within the conduction band valleys at the Γ1, U, M, Γ3 and K are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The following scattering mechanisims, i.e, impurity, polar optical phonon, acoustic phonon, alloy and piezoelectric are inculded in the calculation. Ionized imurity scattering has been treated beyound the Born approximation using the phase-shift analysis. The simulation results show that on the drain side of the gate region, hot electrons attained enough energy to be scattered into the upper satellite conduction valleys. Approximately %20 of the electrons occupy the higher valleys (mainly U and M valley). The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including upper valleys show much closer agreement with the available experimantal data.

A Monte Carlo simulation has been used to model steady-state electron transport in SiC and GaN field effect transistor. The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible. Simulations of SiC MESFETs of lengths 2, 2.6 and 3.2 µm have been carried out and compared these results with those on GaN MESFETs of the same dimensions. The direct current IV characteristics of the two materials were found to be similar, though the GaN characteristics were on the whole superior, reaching their operating point at higher drain voltages and possessing higher gains. However, oscillations in the drain current caused by changes in drain voltage in the GaN devices were not present to the same degree in the SiC devices. This difference is caused partially by the onset of the negative differential regime in SiC at a higher electric field than in GaN but the primary cause is the longer ballistic transport times in SiC. This suggests that GaN MESFETs may prove to have superior frequency response characteristics than SiC MESFETs.

Temperature and doping dependencies of electron mobility in InAs, AlAs and AlGaAs structures have been calculated using an ensemble Monte Carlo simulation. Electronic states within the conduction band valleys at the Γ, L and X are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The simulation shows that intervalley electron transfer plays a dominant role in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. Our simulation results have also shown that the electron velocity in InAs and AlAs is less sensitive to temperature than in other III-V semiconductors like GaAs and AlGaAs. So InAs and AlAs devices are expected be more tolerant to self-heating and high ambient temperature device modeling. Our steady-state velocity-field characteristics are in fair agreement with other recent calculations.