The challenges for thin film epitaxial techniques for the realisation of HBT device structures are: providing a high contrast between selective epitaxial growth (SEG) and non-selective epitaxial growth (NSEG) on patterned substrates; and fine control of the Ge concentration profile in the base region and the doping levels for all layers. A Gas Source MBE system has been modified for operation either in the conventional GSMBE mode or an Ultra Low Pressure CVD mode, namely the 'by-pass mode'. This allows use of two growth modes to achieve the specified device structures. In particular, it has been demonstrated that a high As doping level (1.1 x 10(18) cm(-3)) at a relatively low growth temperature and a rapid conversion to the non-selective mode, can be obtained using the by-pass mode. Non-graded and graded HBT test structures have been grown and characterised by SIMS and X-ray diffraction in order to calibrate the doping levels, the Ge concentration in the base region and the thickness of the layers.