A semiconductor device (10) comprising a bipolar transistor and a field 5 effect transistor within a semiconductor body (1) comprising a projecting mesa (5) within which are at least a portion of a collector region (22c and 22d) and a base region (33c) of the bipolar transistor. The bipolar transistor is provided with an insulating cavity (92b) provided in the collector region (22c and 22d). The insulating cavity (92b) may be provided by providing a layer (33a) in the collector region (22c), creating an access path, for example by selectively etching polysilicon towards monocrystalline, and removing a portion of the layer (33a) to provide the cavity using the access path. The layer (33a) provided in the collector region may be of SiGe:C. By blocking diffusion from the base region the insulating cavity (92b) provides a reduction in the base collector capacitance and can be described as defining the base contact.