Semiconductor structures and a method of forming semiconductor structures The avalanche breakdown characteristics, such as breakdown voltage and impact ionisation coefficient, of a semiconductor structure can be controlled by controlling the Brillouin-zone-averaged energy bandgap () of the material forming the structure. Consequently, the avalanche breakdown characteristics of a device may be tailored independently of the bandgap E.sub.g. The Brillouin-zone-averaged energy bandgap () may be controlled by controlling the composition of the semiconductor used or by straining its lattice.