The appearance of an S-shaped negative differential 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 different values of cladding layer thickness (1.0, 0.5 and 0.3 mum). 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. II 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 similar to 62 meV in p-type (Al0.7Ga0.3)(0.52)In0.48P is obtained from the I-V curve, which is consistent with that obtained from a hydrogenic model and accepted values.