The sub-threshold electron transport properties of amorphous (a-) germanium telluride (GeTe) phase change material (PCM) ultra-thin films are investigated by using ab initio molecular dynamics, density function theory, and Green’s function simulations. The simulation results reproduce the trends in measured electron transport properties, e.g. current-voltage curve, intra-bandgap donor-like and acceptor-like defect states, and p-type conductivity. The underlying physical mechanism of electron transport in ultra-scaled a-PCM is unraveled. We find that, though the current-voltage curve of the ultra-scaled a-PCM resembles that of the bulk a-PCM, their physical origins are different. Unlike the electron transport in bulk a-PCM, which is governed by the Poole-Frenkel effect, the electron transport in ultra-scaled a-PCM is largely dominated by tunneling transport via intra-bandgap donor-like and acceptor-like defect states.