  Using a metal-oxide-semiconductor field effect transistor (MOSFET) structure
with a high-quality CoFe/n^+Si contact, we systematically study spin injection
and spin accumulation in a nondegenerated Si channel with a doping density of ~
4.5*10^15cm^-3 at room temperature. By applying the gate voltage (V_G) to the
channel, we obtain sufficient bias currents (I_Bias) for creating spin
accumulation in the channel and observe clear spin-accumulation signals even at
room temperature. Whereas the magnitude of the spin signals is enhanced by
increasing I_Bias, it is reduced by increasing V_G interestingly. These
features can be understood within the framework of the conventional spin
diffusion model. As a result, a room-temperature spin injection technique for
the nondegenerated Si channel without using insulating tunnel barriers is
established, which indicates a technological progress for Si-based spintronic
applications with gate electrodes.
