Due to the reduced size of quantum devices, defects play a dominant role in their electrical behaviour. We theoretically show that defects can change the electrical behaviour of a single barrier or a quantum well. For the barrier and the well, we have performed a quantum calculation of a defect assisted tunneling effect based on the WKB theory to describe the wave functions and on the Oppenheimer approach for the determination of the tunneling probability. The main effects are : appearance of a negative differential resistance (NDR) in the currentvoltage characteristics (I-V) of the single barrier and an apparent lowering of the band offset of the quantum well. In the latter case, we have experimentally measured this effect on an AlInAs-GaInAs-AlInAs well, using admittance spectroscopy. The apparent lowering is associated with a native defect located in the barrier, a defect studied using the DLTS technique.