The appearance of an S-shaped negative di€erential resistance (NDR) in GaInP/AlGaInP quantum well (QW) lasers has been observed in both single and multi-QW devices at temperatures between 100 and 200 K. Light±current (L±I) and current±voltage (I±V) relationships have been measured in detail at temperatures from 100 to 250 K, using three QW samples with di€erent values of cladding layer thickness (1.0, 0.5 and 0.3 lm). The dependence of the S-shaped NDR characteristics on cladding thickness and laser output intensity shows that the high resistivity p-cladding layer is the cause of NDR. It is because of the impact ionisation of impurities in the p-cladding layer that an S-shaped NDR is produced. An impurity ionisation energy of $62 meV in p-type (Al 0:7 Ga 0:3) 0:52 In 0:48 P is obtained from the I±V curve, which is consistent with that obtained from a hydrogenic model and accepted values.