A review is given of the requirements on MBE-grown layers as far as processing is concerned. Aspects that are considered are: defect density, particulates, background doping and metallic contamination. The stability of the grown layers against thermal anneals is considered. It is shown that normal thermal diffusion in HBT structures is not important, other effects, like transient diffusion following ion implantation, have drastic effects on the grown profiles. As an example the processing of mesa-isolated heterojunction bipolar transistors is treated. It is shown that all-Si transistors can be grown with ideal Gummel plots. The Gummel plots of SiGe HBTs show small non-idealities. The current gain enhancement of the HBTs with respect to the all-Si transistors is shown to be as large as 200 times. Due to transient diffusion, parasitic barriers are formed, that have a detrimental effect on the AC and DC performance.