Bipolar transistors are mainly used in BiCMOS technologies where circuit operation at very high frequencies is required. Silicon germanium heterojunction bipolar transistors (HBTs) have been produced with cut-off frequencies above 200GHz. Such transistors are ideal for use in radio frequency circuits in mobile communications products. Another important application is optical communications, where SiGe HBTs have been used to produce circuits that operate at 40Gbit/s. In all these applications, the rf performance can be significantly improved if the HBTs are produced on Silicon-On-Insulator substrates. HBTs on SOI is a new research field and only two other groups around the world have reported HBT on insulator devices. At Southampton, a novel growth process has been developed that combines selective growth of the Si collector with non-selective growth of the SiGe base and the Si cap. This process has the advantage of simplicity, since it halves the number of epitaxy steps needed to produce an HBT and eliminates the requirement for LOCOS or shallow trench isolation. SiGe HBTs have been successfully produced on SOI substrates using this combined selective/non-selective growth process. The inclusion of carbon in the SiGe base is currently being investigated, since it reduces boron transient enhanced diffusion and hence allows thinner bases to be produced.