This thesis deals with the study of shallow PN junction formation by dopant diffusion from Spin-On Glass (SOG) for future deep sub-micron BiCMOS technology. With the advantages of no transient enhanced diffusion and no metal contamination, diffusion from highly doped SOG (also called spin-on dopant - SOD) is a good technology for shallow junction formation. In this thesis, diffusion of impurities from SOD into Si and polysilicon on silicon structure has been studied. This shallow junction formation technique using SOD has been applied in realisation of two important devices, i.e. high frequency bipolar transistor and deep sub-micron elevated source/drain MOSFET.