The fabrication and characterisation of GaAlAs buried-heterostructure laser diodes having low threshold currents (10 mA), high uniformity and planar surface structure, and grown exclusively by metalorganic chemical vapour deposition (MOCVD) are described. Single-longitudinal and transverse-mode operation of these devices has been observed. In addition, considerable suppression of relaxation resonance effects has been observed in the high-frequency modulation of these devices.