The critical design issues for unipolar silicon quantum-well intersubband lasers an investigated in this paper. Group IV lasing at near-infrared and middle-infrared wavelengths is analyzed for surface-emitting and edge-emitting geometries. Calculations show feasibility of SiGe/Si and SiGeC/Si lasing. Silicon-on-insulator structures are proposed for index-guided lasers, vertical-cavity lasers and micro-cavity lasers. (C) 1998 Academic Press Limited.