Ballistic spin field-effect transistors: Multichannel effects

Abstract

We study a ballistic spin field-effect transistor (SFET) with special attention to the issue of multichannel effects. The conductance modulation of the SFET as a function of the Rashba spin-orbit coupling strength is numerically examined for the number of channels, ranging from a few to close to 100. Even with the ideal spin injector and collector, the conductance modulation ratio, defined as the ratio between the maximum and minimum conductances, decays rapidly and approaches 1 with the increase of the channel number. It turns out that the decay is considerably faster when the Rashba spin-orbit coupling is larger. Effects of the electronic coherence are also examined in the multichannel regime, and it is found that the coherent Fabry-Perot-like interference in the multichannel regime gives rise to a nested peak structure. For a nonideal spin injector and/or collector structure, which consists of a conventional metallic ferromagnet-thin insulator two-dimensional electron-gas heterostructure, the Rashba-coupling-induced conductance modulation is strongly affected by large resonance peaks that arise from the electron confinement effect of the insulators. Finally, scattering effects are briefly addressed and it is found that in the weakly diffusive regime, the positions of the resonance peaks fluctuate, making the conductance modulation signal sample dependent.