The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Large Seminar Hall of Micronova at Helsinki University of Technology (Espoo, Finland) on the 19th of December, 2007, at 12 o'clock noon.
Overview in PDF format (ISBN 978-951-38-7066-9) [5744 KB]
VTT Publications 666, ISSN 1455-0849
Dissertation is also available in print (ISBN 978-951-38-7065-2)
Copyright © 2007 VTT Technical Research Centre of Finland
VTT Publications 666, ISSN 1235-0621
VTT-PUBS-666
TKK-DISS-2411
In this Thesis different aspects of band degree of freedom are explored in 2D electron transport and electron-phonon (e-ph) energy relaxation in 2D and 3D electron systems. Here the bands of interest are the conduction band valleys of many-valley semiconductors and spatial sub-bands of two-dimensional-electron gas in a quantum well.
The experimental studies of electronic transport focus on double-gate SiO2-Si-SiO2 quantum well field-effect-transistors (FETs), which are fabricated utilizing silicon-on-insulator structures and wafer bonding. Double-gate FETs are intensively explored at the moment due to their prospects in microelectronics. The inclusion of a back gate electrode provides means to adjust the electron wave functions and the occupancy of the spatial 2D sub-bands. The contrast between single and two-sub-band transport is studied in low temperature conductivity/mobility and magneto transport. For example, the conductivity shows significant drop at the threshold of the second spatial sub-band due to inter-sub-band coupling and sub-band delocalization effect is observed at symmetric well potential. At room temperature several sub-bands are inevitably populated and the most relevant observed effect is the mobility enhancement towards symmetric quantum well potential. This mobility enhancement is one of the benefits of double-gate FETs in comparison to similar single-gate FETs.
In the studies of e-ph energy relaxation we focus on the case where the phonons cannot directly couple the bands of the electron system. If the e-ph matrix elements depend on the band index then the band degree of freedom plays an important role. We developed a mean field theory, which allows elastic inter and intra-band scattering and also Coulomb interaction. Our model reproduces the long wavelength single-band energy loss rate results found in the literature. In the multi-band regime we find a set of new results, which suggest that the energy loss rate is strongly enhanced if the phonons couple asymmetrically to different bands and the single-band interaction is strongly screened. The effect is tested experimentally in heavily doped n-type Si samples by low temperature heating experiments. We find good agreement between the theory and experiment. Our findings enable a design of a novel electron-phonon heat switch.
This thesis consists of an overview and of the following 7 publications:
Keywords: two-dimensional electron gas, mobility, many-valley systems, electron-phonon interaction, SOI
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© 2007 Helsinki University of Technology