We have developed a new scanning probe microscopy-based technique, scanning differential spreading resistance microscopy (SDSRM), which enables the determination of free carrier distribution inside operating electronic and optoelectronic devices. The results of our SDSRM study of multiquantum-well (MQW) buried heterostructure (BH) lasers under zero and forward biases are reported. Individual QW-barrier layers can be resolved in high-resolution SDSRM. The SDSRM results show different internal carrier distribution within the MQW active region in BH lasers with and without biases and provide direct experimental evidence of electron overbarrier leakage. Our results demonstrate the utility of SDSRM to delineate quantitatively the transverse cross-sectional structure of complex two-dimensional devices such as MQW BH lasers under operating conditions, in which traditional probing such as secondary ion mass spectroscopy, scanning spreading resistance microscopy, and electron beam-induced current microscopy can either apply only to devices under zero bias or provide only qualitative pictures.