The interaction between device technology and the control of defects has been strong since the first transistor. The rapid and unparalled development of this technology can be monitored by changes in device size and process complexity, or reciprocally, in defect density. In the future the emphasis on computation functionality will shift to imaging and learning. In addition high volume manufacturing will cast an emphasis on capital cost, yield, effluent control and safety. These foci require the development of new sensitive defect assessment tools, the modelling of complex defect systems and the understanding of defect properties on the atomic scale. We shall illustrate this interplay between technology and materials science by topics in ULSI circuits, silicon photovoltaics and silicon microphotonics. For ULSI the control of both bulk and surface defects is decisive. The physical chemistry at the silicon surface plays a key role in metal contamination. We shall discuss the use of RF-PCD for in-line monitoring and investigation of reaction kinetics. Silicon for low cost, photovoltaic applications confronts problems involving complex defect systems, but comparatively simple processes. The goal is to optimize growth and cell processing. The objective of silicon microphotonics is to establish an IC compatible process technology for integration of optical interconnection with silicon electronics. By building the first silicon-based LED operating at λ = 1.54 μm at room temperature it has been shown that erbium doping is a viable approach. Codoping with F or O largely enhances the light output. We will show a process simulator for the Si:Er:F system as an example for ligand field engineering.