Phonon Limited Electron Mobility in Germanium FinFETs: Fin Direction Dependence

We investigate the phonon limited electron mobility in germanium (Ge) fin field-effect transistors (FinFETs) with fin rotating within (001), (110), and (111)-oriented wafers. The coupled Schrödinger–Poisson equations are solved self-consistently to calculate the electronic structures for the two-dimensional electron gas, and Fermi's golden rule is used to calculate the phonon scattering rate. It is concluded that the intra-valley acoustic phonon scattering is the dominant mechanism limiting the electron mobility in Ge FinFETs. The phonon limited electron motilities are influenced by wafer orientation, channel direction, fin thickness W_\rm fin, and inversion charge density N_\rm inv. With the fixed W_\rm fin, fin directions of \langle 110\rangle, \langle 1\bar12\rangle and \langle \bar110\rangle within (001), (110), and (111)-oriented wafers provide the maximum values of electron mobility. The optimized W_\rm fin for mobility is also dependent on wafer orientation and channel direction. As N_\rm inv increases, phonon limited mobility degrades, which is attributed to electron repopulation from a higher mobility valley to a lower mobility valley as N_\rm inv increases.