We have investigated surface properties: chemical, electronic, and structural ones of semiconductor crystals including silicon (Si), germanium (Ge), and III-V compound semiconductors (e.g. GaAs, GaN, InAs, InP) by means of electron diffraction (LEED, RHEED), scanning tunneling microscopy and spectroscopy (STM/STS), and photoelectron spectroscopy (including synchrotron sources). One goal has been to develop the connection or collaboration between the surface science and the semiconductor technology disciplines. More recently we have focused on the question how to decrease surface-related electrical losses and malfunctions in various semiconductor-based devices like capacitors, sensors, solar cells, and transistors. We have tried to understand reasons for formation of defect levels in device surfaces and to find controlled methods to decrease the amount of defect levels.