  In this work we report on Hall effect, resistivity and thermopower
measurements in n-type indium selenide at room temperature under either
hydrostatic and quasi-hydrostatic pressure. Up to 40 kbar (= 4 GPa), the
decrease of carrier concentration as the pressure increases is explained
through the existence of a subsidiary minimum in the conduction band. This
minimum shifts towards lower energies under pressure, with a pressure
coefficient of about -105 meV/GPa, and its related impurity level traps
electrons as it reaches the band gap and approaches the Fermi level. The
pressure value at which the electron trapping starts is shown to depend on the
electron concentration at ambient pressure and the dimensionality of the
electron gas. At low pressures the electron mobility increases under pressure
for both 3D and 2D electrons, the increase rate being higher for 2D electrons,
which is shown to be coherent with previous scattering mechanisms models. The
phase transition from the semiconductor layered phase to the metallic sodium
cloride phase is observed as a drop in resistivity around 105 kbar, but above
40 kbar a sharp nonreversible increase of the carrier concentration is
observed, which is attributed to the formation of donor defects as precursors
of the phase transition.
