A semi-empirical method was used in a full-peak efficiency calculation procedure; it was based on the measurements of characteristic x-ray lines of pure elemental and compound samples and on the application of the fundamental parameter method (FPM) for the theoretical calculation of the characteristic x-ray intensities. Radioactive standards were applied for the efficiency calibration of different semiconductor detectors [Si(Li), Si-PIN and HpGe]. The efficiency curves were fitted to the characteristic intensities measured by a conventional Si(Li) and two Si-PIN detectors, using I-125 and Fe-55 radioactive sources for x-ray excitation in a ring-shaped geometrical arrangement. Several quantitative applications in x- and gamma-ray spectrometry were carried out with these calibrated detectors: (i) x-ray fluorescence (XRF) analysis of sediment samples [Si(Li)], (ii) determination of specific activity of Cs-137 nuclide in sediment core samples originated from an oxbow area of the river Tisza, using gamma-spectroscopy (HpGe) with low-background conditions, (iii) study of electrosorption phenomena at solid-liquid interfaces by making use of an in situ radiotracer method based on the application of low-energy x-ray emitting radionuclides (e.g. Zn-65), (iv) quantitative XRF analysis of ceramic glaze and (v) electron microprobe analysis of aerosol particles using an instrument equipped with a thin-window Si(Li) detector.