Impacts of ultrathin biaxial strained silicon on ultimate drive velocity, effective gate capacitance and threshold voltage
The biaxial strained silicon has attracted much interest in manufacturing industry due to the potential enhancement in carrier-transport property. The biaxial strain lifts the degeneracy of silicon valence and conduction bands, leading to a reduction in intervalley scattering and lower effective mas...
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| Format: | Thesis |
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2010
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| Online Access: | http://eprints.utm.my/id/eprint/16454/ http://libraryopac.utm.my/client/en_AU/main/search/results?qu=Impacts+of+ultrathin+biaxial+strained+silicon+on+ultimate+drive+velocity%2C+effective+gate+capacitance+and+threshold+voltage&te= |
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| Summary: | The biaxial strained silicon has attracted much interest in manufacturing industry due to the potential enhancement in carrier-transport property. The biaxial strain lifts the degeneracy of silicon valence and conduction bands, leading to a reduction in intervalley scattering and lower effective mass of conduction and valence band states, and hence delivers better mobility and velocity performance in comparison with bulk silicon. In this research, a development of the lightly doped and heavily doped ultrathin strained silicon ultimate drift velocity model with the inclusion of quantum confinement effect is conducted. For nondegenerate regime, the study shows that the ultimate drift velocity is its thermal velocity which is only dependent on temperature. However, in the degenerate regime the ultimate drift velocity is its Fermi velocity which only depends on the carrier concentration. At room temperature, result shows that the ultimate drift velocity of strained silicon is always higher than unstrained silicon as much as 0.5x10 m/s. The single channel and dual channel biaxial strained silicon gate capacitance are also developed. These gate capacitances take into account the quantum capacitance. Result shows that for the single channel device, the relative capacitance is lower by as much as 0.07 compared to dual channel device when the oxide thickness is 2.5 nm and transverse electric field is 60 MV 'm. This means that the quantum capacitance is more dominant in the single channel compared to the dual channel biaxial strained silicon MOSFET. This is due to the dual channel biaxial strained silicon having a thicker dielectric which causes the lower effect of quantum capacitance. The quantum capacitance is more dominant in thinner gate oxide and lower strain. Lastly, an analytical threshold voltage modeling has been reported. This model takes into account the effect of quantum confinement effect on the flatband voltage, built-in voltage and strained silicon work function. Other than that, this model also includes the quantum mechanical effect on the oxide thickness. Other important parameters such as different lateral doping profile in the depletion region, germanium mole fraction of the silicon germanium (SiGe) layer, thickness of strained silicon and silicon germanium, channel length, source, drain and substrate bias are also included. Comparison of this model with the thick strained silicon threshold voltage simulation results was also carried out. Results show that the threshold voltage for ultrathin strained silicon channel is always higher by as much as 0.04 V compared to the thick strained silicon MOSFET. This is because the ultrathin strained silicon has the higher faltband voltage, wider bandgap, lower Fermi energy, and higher barrier between the source and the drain compared to thick strained silicon. |
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