Impact ionization (II) has a strong influence on the characteristic of nanoscale transistors. In fact, when the transistors are down-scaled drastically, the electric field increases significantly, which leads to a strong effect of hot carriers in these devices. To understand these II activities, it is necessary to know the II rate of the applied material. Together with relaxed Si, uniaxially strained Si and biaxially strained SiGe are becoming popular applied materials to enhance the performance of MOSFETs and fast HBTs (heterojunction bipolar transistors), respectively. This dissertation focuses on developing a model for calculating the II rate for relaxed and strained materials. The model follows the constant-matrix-element approach and has exact energy and momentum conservation during the calculation of the six-dimensional integral in k-space. II coefficients and quantum yield for relaxed Si, strained Si and strained SiGe are calculated through Full-band Monte Carlo (FB-MC) simulations. A good agreement between simulation and experimental data for relaxed Si is obtained. Besides the negative effects of II on nanoscale devices such as noise performance degradation or breakdown voltage reduction, it also has positive effects, e.g. in the case of Impact-Ionization MOSFET (IMOS). This device bases on a controlled avalanche breakdown to overcome the subthreshold limit of 60 mV/dec in conventional MOSFETs. However, the device requires a too high operating voltage and suffers from serious damage by hot carriers to the oxide layer. The vertical IMOS, which works on a dynamic reduction of the threshold voltage due to the floating body charged by the II process, was shown to have a better performance than the lateral IMOS. Hence, this dissertation uses numerical device simulators to investigate and optimize this device. The developed II model which is integrated into an existing MC simulator has been shown to be able to model and simulate the vertical IMOS transistors made of relaxed Si, strained Si or strained SiGe. The Hydrodynamic model is combined with FB-MC in order to investigate I-V characteristics of the vertical IMOS. For vertical relaxed-Si-IMOS, it is proven that a good agreement between simulations and experimental measurements of the device's characteristics can be obtained by using this approach. The simulation results show that the performance of the vertical IMOS can be enhanced by a layer of strained SiGe placed between the drain and the channel. Finally the noise performance of these devices is also investigated.    «

Impact ionization (II) has a strong influence on the characteristic of nanoscale transistors. In fact, when the transistors are down-scaled drastically, the electric field increases significantly, which leads to a strong effect of hot carriers in these devices. To understand these II activities, it is necessary to know the II rate of the applied material. Together with relaxed Si, uniaxially strained Si and biaxially strained SiGe are becoming popular applied materials to enhance the performance...    »