  We have fabricated ambipolar transistors on chemically prepared
hydrogen-terminated Si(111) surfaces, in which a two-dimensional electron
system (2DES) or a two-dimensional hole system (2DHS) can be populated in the
same conduction channel by changing the gate voltage of a global gate applied
through a vacuum gap. Depending on the gate bias, ion implanted n$^+$ and p$^+$
regions function either as Ohmic contacts or as in-plane gates, which laterally
confine the carriers induced by the global gate. On one device, electron and
hole densities of up to $7.8\times10^{11}$ cm$^{-2}$ and $7.6\times10^{11}$
cm$^{-2}$ respectively are obtained. The peak electron mobility is
$1.76\times10^5$ cm$^{2}$/Vs, and the peak hole mobility is $9.1\times10^3$
cm$^{2}$/Vs at 300 mK; the ratio of about 20 is mainly due to the very
different valley degeneracies (6:1) of electrons and holes on the Si(111)
surface. On another device, the peak electron mobility of $2.2\times10^5$
cm$^{2}$/Vs is reached at 300 mK. These devices are hexagonal in order to
investigate the underlying symmetry of the 2DESs, which have a sixfold valley
degeneracy at zero magnetic field. Three magnetoresistance measurements with
threefold rotational symmetry are used to determine the symmetry of the 2DESs
at different magnetic field. At filling factor $1 < \nu < 2$, the observed
anisotropy can be explained by a single valley pair occupancy of composite
fermions (CFs). Qualitatively the CFs preserve the valley anisotropy, in
addition to the twofold valley degeneracy. At magnetic field up to 35 T, the
2/3 fractional quantum Hall state is observed with a well developed hall
plateau; at $\nu<2/3$, the three magnetoresistances show a large anisotropy
(50:1). We also show that device degradation is not a serious issue for our
measurements, if the device is kept in vacuum or a nitrogen gas environment and
its time in air is minimized.
