  We report electron spin resonance (ESR) measurements on a large-area silicon
MOSFET. An ESR signal at g-factor 1.9999(1), and with a linewidth of 0.6 G, is
observed and found to arise from two-dimensional (2D) electrons at the Si/SiO2
interface. The signal and its intensity show a pronounced dependence on applied
gate voltage. At gate voltages below the threshold of the MOSFET, the signal is
from weakly confined, isolated electrons as evidenced by the Curie-like
temperature dependence of its intensity. The situation above threshold appears
more complicated. These large-area MOSFETs provide the capability to
controllably tune from insulating to conducting regimes by adjusting the gate
voltage while monitoring the state of the 2D electron spins spectroscopically.
