The operation of 1-3 nm thick SOI MOSFETs, in double-gate (DG) mode and single-gate (SG) mode (for either front or back channel), is systematically analyzed. Strong interface coupling and threshold voltage variation, large influence of substrate depletion underneath the buried oxide, absence of drain current transients, degradation in electron mobility are typical effects in these ultra-thin MOSFETs. The comparison of SG and DG configurations demonstrates the superiority of DG-MOSFETs: ideal subthreshold swing and remarkably improved transconductance (consistently higher than twice the value in SG-MOSFETs). The experimental data and the difference between SG and DG modes is explained by combining classical models with quantum calculations. The key effect in ultimately thin DG-MOSFETs is volume inversion, which primarily leads to an improvement in mobility, whereas the total inversion charge is only marginally modified.
Electronic structures and the phonon-limited electron mobility of inversion layers have been studied at 300 K for the thin Si (100) layer of double-gate (DG) silicon-on-insulator (SOI) structures by using a one-dimensional self-consistent calculation and a relaxation time approximation. Both symmetric and asymmetric DG SOI systems have been investigated. The self-consistent calculation presents the electronic structures specific to DG SOI Si inversion layers and the range of the specific electronic structures as functions of Si layer thickness tSi and the vertical effective electric field Eeff. Outside this range, the mobility behavior as a function of Eeff is almost identical to that of bulk Si inversion layers. In this range, however, as tSi decreases, the phonon-limited electron mobility μph increases gradually to a maximum around tSi=10 nm, decreases for tSi=10–5 nm, rises rapidly to another maximum in the vicinity of tSi=3 nm and finally falls. The former gradual increase in the mobility μph results from a reduction of phonon scattering caused by the interaction of upper and lower inversion layers. For tSi of less than approximately 10 nm, the mobility of each subband is reduced by an enhancement of scattering rates due to a confinement effect in general. However, the rapid increase of the fraction of electrons in the lowest energy subband that has a higher mobility than other subbands brings about the latter mobility increase in the vicinity of tSi=3 nm.
The origin of the potential profile in silicon single-electron transistors (SETs) fabricated using pattern-dependent oxidation (PADOX) is investigated by making use of the geometric structure measured by atomic force microscope (AFM), the bandgap reduction due to compressive stress generated during PADOX obtained using the first-principles calculation, and the effective potential method. A probable mechanism for the formation of the potential profile responsible for SET operation is proposed. The width reduction in the silicon wire region in the SET produces a tunnel barrier, while the compressive stress lowers the bottom of the conduction band through the bandgap reduction and forms a potential well corresponding to an island in the tunnel barrier.
SiO 2 /Si/SiO 2 quantum wells fabricated on SIMOX silicon-on-insulator substrates are examined in the quantized Hall regime. An 8 nm quantum well behaves as a single layer of two-dimensional electrons at accessible gate voltages. By using front and back gates, the wave function in the confinement direction can be shifted continuously between two SiO 2 /Si interfaces formed through different processes. We find that this results in a continuous evolution of the valley splitting which is asymmetric with electrical gate bias. Wider quantum wells show bilayer behavior where the valley splitting is different in each layer, demonstrating that its control shown by the 8 nm well arises due to the different properties of the two interfaces. Estimates of the valley splitting are made through Landau level coincidences and activation energies. The coincidence between Landau levels of opposite spin, opposite valley, and like cyclotron indices at ϭ6 shows anticrossing behavior.
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