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The electron mobility in junctionless (JL) Ge nanowire nFETs with the $\langle 110\rangle $ channel direction is calculated and compared with experimental data. The calculation of the electron mobility is based on the Kubo–Greenwood formula. The wave functions are obtained by solving the self-consistent Poisson–Schrodinger equation. Phonon scattering, surface roughness scattering, and Coulomb scattering (CS) by interface charges and channel charges are considered. Carrier screening on Coulomb charges is calculated using Lindhard’s screening theory. At low electron densities, the JL channels have lower electron mobility than inversion-mode (IM) channels due to CS from channel charges. At high electron densities, CS in JL channels is effectively reduced by screening, resulting in mobility comparable with the IM channels. The L4 electrons have higher mobility than $\text{L}4^\prime $ electrons. Applying uniaxial tensile strain along the channel can lower the energy of L4 valleys to enhance the channel mobility.

The dependence of effective mass on Sn content, pseudomorphic strain, cap layer thickness, and substrate orientation is theoretically studied for Ge-cap/Ge1-x Sn x /Ge substrate p-channel MOSFETs through six-band k·p method. The Luttinger-like and deformation potential parameters were obtained from the fit of Ge1-x Sn x band structures by the empirical pseudopotential method. The valence band alignment is theoretically determined by the empirical pseudopotential method and the model-solid theory. The high Sn content and thin Ge-cap layer both result the small effective mass at zone center along channel direction of the first subband.

Pseudomorphic Ge 0.91 Sn 0.09 on Ge on Si with strong photoluminescence and low defect density is used for p-MOSFET channels. The mobility of Ge 0.91 Sn 0.09 Quantum Well p-MOSFETs are higher than control Ge p-MOSFETs due to hole population in the GeSn wells. The 7.5% mobility enhancement on channel direction is observed using external transverse uniaxial tensile strain (∼0.11%). The highest [Sn] of 9% in the channels grown by CVD, Pt SB S/D, high I on /I off ratio, and strain-enhanced mobility are obtained in this work.

A Ge/GeSn/Ge single quantum well with 5% Sn content was grown by chemical vapor deposition on a Si substrate using Ge2H6 and SnCl4 precursors. External biaxial tensile strain was mechanically applied to the Ge/Ge0.95Sn0.05/Ge quantum well for the photoluminescence measurement. Note that the Ge0.95Sn0.05 layer is still under compressive strain due to the large internal compressive strain of the GeSn, although the external bending produces the tensile strain. The direct emission of tensily strained Ge buffer shifts toward lower energy, while the direct emission of pseudomorphic Ge0.95Sn0.05 quantum well does not have significant shift. The strain-induced energy changes of heavy holes and light holes in the Γ valley are extracted by fitting the photoluminescence spectra. Based on the nonlocal empirical pseudopotential method and the model-solid theory, a type-I band alignment is used for the fitting of photoluminescence spectra. The experimentally extracted band edge shifts from the photoluminescence measurement have good agreement with theoretical calculation.

The indirect-direct transition of Ge 1−x Sn x at Sn content 6.5% is simulated by nonlocal empirical pseudopotential method. The non-parabolicity band of Γ valley is considered for confined mass, density of state, and conductivity mass for the electron ballistic current calculation of Ge 1−x Sn x FinFETs. The integration of alloying Sn content and applying external stress enhances the ballistic current due to carriers transferring from indirect L valleys with small injection velocity to direct Γ valley and other indirect L valleys with high injection velocity.

Type-I, type-II, reverse type-I, and reverse type-II band alignments are found theoretically in strained Ge1−x Sn x (0 ≤ x ≤ 0.3) grown on relaxed Ge1−y Sn y substrates (0 ≤ y ≤ 0.3) using the model-solid theory. The prerequisite bandgaps, and energy difference between the top valence band edge and the average valence band position of GeSn are obtained by the nonlocal empirical pseudopotential method. For the indirect-gap (L valleys) Ge1−x Sn x on relaxed Ge1−y Sn y , the band alignments are type-I and reverse type-I under biaxial compressive strain (x > y) and biaxial tensile strain (x < y), respectively. For the direct-gap (Γ valley) Ge1−x Sn x on relaxed Ge1−y Sn y , the biaxial compressive strain yields type-I and type-II alignment, while the biaxial tensile strain yields reverse type-I and reverse type-II alignments.

The band structure of Ge in the lonsdaleite phase is calculated using first principles. Lonsdaleite Ge has a direct band gap at the Γ point. For the conduction band, the Γ valley is anisotropic with the low transverse effective mass on the hexagonal plane and the large longitudinal effective mass along the c axis. For the valence band, both heavy-hole and light-hole effective masses are anisotropic at the Γ point. The in-plane electron effective mass also becomes anisotropic under uniaxial tensile strain. The strain response of the heavy-hole mass is opposite to the light hole.

The electron mobility of n-channel metal-oxide-semiconductor field-effect transistors using Ge/GeO2/Al2O3 gate stack on (001) Ge substrates is analyzed theoretically and experimentally. Phonon scattering, Coulomb scattering, and interface roughness scattering are taken into account. The Ge peak mobility exceeding Si universal in our device by a factor of 1.3 is due to the reduction of Coulomb scattering of the interface states. As compared to Si, the faster roll-off of the Ge mobility at the effective field larger than 0.3 MV/cm is due to larger interface roughness scattering.