Over the barrier, hot electron transport across 8 nm thick amorphous Al2O3 layers embedded in metal-oxide-semiconductor (MOS) structures was investigated with ballistic electron emission microscopy (BEEM). The oxide ield dependence of the BEEM threshold voltage V-th, which corresponds to the potential maximum of the barrier, was found to be dominated by image force and charge trapping/detrapping effects. The static barrier height at the W-Al2O3 interface Phi(B)=3.90 +/-0.03 eV and the dynamic dielectric constant epsilon(if)=1.86+/-0.1, which reflects the strong image force lowering of the barrier observed at both interfaces. A band offset between the Al2O3 and Si conduction bands of 2.78+/-0.06 eV was deduced. Electron trap levels at energies overlapping the Si band Sap and of densities in the 10(12) cm(-2) range were deduced to lie in the oxide near the Al2O3-Si interface. Their occupancy is determined by the position of the interface Fermi level. For p-type substrates the traps were empty (filled) for positive (negative) applied bias. Local, electrical stressing increased the interface trap charge for n-type substrates, but had negligible consequences on p-type substrates. The Al2O3 was readily stressed to failure upon injecting sub-nano-Coulomb of charge at electron kinetic energies in the 4-6 eV range.