Pump-probe experiments are used to characterize Fe-doped InGaAs/InP multiple quantum wells for ultrafast saturable absorption applications. Sample photoresponse time is well controlled by iron doping from the nanosecond range to a value as short as 0.45 ps for an iron concentration of 6×1018 cm−3. Furthermore, the switching amplitude, that is, the differential transmission ratio (DTR), of samples having increasing Fe doping (null to 6×1018 cm−3) presents two decreasing regimes at moderate optical excitation levels. The first one is explained well by the degree of quality of the material, whereas the second one is interpreted as an inhibition of exciton formation at high doping levels. This interpretation is highlighted by an analysis of the temporal evolution of the DTR. The influence of the doping localization (in the well, in the barrier, or in the whole structure) is studied as well. Finally, an attractive way to maintain a constant value of the DTR with an ultrafast response time (2.6 ps) is proposed and demonstrated. © 2003 American Institute of Physics.