We have evidenced a new interfacial trapping phenomenon for hot electron injection in silicon by studying magnetic tunnel transistors (MTTs) with a MgO tunneling barrier emitter and a Cu/Si Shottky barrier collector. Transport measurements on hot electrons indicate that an interfacial charge trapping and a backscattering-induced collector current limitation take place with the MTT spin-valve base both in parallel and antiparallel states when the temperature is lower than 25 K, which results in a rapid decrease of the magnetocurrent ratio from similar to 2000% at 25K to 800% at 17 K. The binding energy of the trapped electron is estimated to be about 1.7 meV, which is also found to increase with the magnetic field. A simple analytic model considering the interfacial electron trapping and releasing is proposed to explain the experimental results.