Impact of Uniaxial Strain on the Electrical Characteristics of CMOS and Esaki Tunnel Diode

Process-induced uniaxial strain is widely used in CMOS transistor fabrication in order to increase carrier mobility or current drive because of strain-induced band structure changes. Although a significant body of research exists on uniaxially-strained MOSFETs, most previous effort has focused on carrier mobility, how to augment it with strain and how to understand the strain-induced change. In this study, measurements and analysis of the effects of uniaxial-stress-induced changes in MOSFET mobility are extended to explore stress dependent changes in mobility degradation factor, on- and off-state gate tunneling current, impact ionization rate, C-V characteristics, and ring oscillator performance (standby and dynamic power dissipation and speed). In addition to this exploration of uniaxial-strain-induced changes in MOSFETs, Esaki diodes offer a new transport laboratory, i.e. interband tunneling, for understanding the effects of uniaxial strain. Very few papers have been published on the influence of stress on the current-voltage characteristics of Esaki tunnel diode and the strain effects are poorly understood. In this thesis, the impact of uniaxial strain on the peak tunneling current density of Esaki tunnel diodes is theoretically calculated, considering strain-induced changes in the bandgap, electron repopulation among different valleys (only for multi-valley conduction band minimum) and the reduced mass along the tunneling directions. Esaki tunnel diodes, made of four different materials, i.e. Si, Ge, GaAs and InAs, are studied with tunneling current flowing along three different directions (<100>, <110> and <111>) and with uniaxial stress applied either parallel or perpendicular to the tunneling current direction. The theory is compared to the available experimental results and a good agreement between the calculation and experimental data is obtained. From the theoretical calculation, it is found that uniaxial stress can be used to improve peak tunneling current density of Esaki tunnel diodes and the optimum directions for current and uniaxial stress are identified to achieve the largest increase in peak tunneling current for a given magnitude of uniaxial stress.