The surface potential of the cleaved surface of a multilayer laser has been investigated by Kelvin Probe Force Microscopy (KPFM) in air and in non-contact mode. The sample is a laser structure emitting at 2.36 mu m, grown by molecular beam epitaxy on GaSb(100) substrates, cleaved along the (110) surface. The structures comprise abrupt GaAlSbAs/GaSb heterojunctions. Using modified commercial equipment, we simultaneously obtain topography and Kelvin potential. Topography gives the localisation of the different layers of the structure, as the native oxide thickness is higher on Al-rich compounds than on GaSb. When applying forward or reverse DC bias to the device, shifts in surface potential are observed. They appeared to be directly correlated to the bulk voltage of the semiconductors. The measurements allow direct determination of the voltage-drops over the heterojunctions and in the active zone of the device, with these drops controlling the emitting efficiency of the laser.