  A gate voltage application in a Si-based spin metal-oxide-semiconductor
field-effect transistor (spin MOSFET) modulates spin accumulation voltages,
where both electrical conductivity and drift velocity are modified while
keeping constant electric current. An unprecedented reduction in the spin
accumulation voltages in a Si spin MOSFET under negative gate voltage
applications is observed in a high electric bias current regime. To support our
claim, the electric bias current dependence of the spin accumulation voltage
under the gate voltage applications is investigated in detail and compared to a
spin drift diffusion model including the conductance mismatch effect. We proved
that the drastic decrease of the mobility and spin lifetime in the Si channel
is due to the optical phonon emission at the high electric bias current, which
consequently reduced the spin accumulation voltage.
