Silicon charged particle detectors, made by a newly developed process, have been studied. Passivated ion-implanted junctions are fabricated in high resistivity FZ n-type Silicon, using planar technology. For many charged particle detector applications the sensitive volume of the diode has to be high : active areas of several cm2 and thicknesses of 100 to 500 micron are wanted. Diodes with a high breakdown voltage (100 to 500 volts) are required so that the full wafer thickness can be depleted. The diode leakage current has to be small (1-5 nA/cm2/100 μ ) for low noise, high resolution performance. An important parameter characterizing the performance of semiconductor detectors is the dead layer thickness caused by the shallow p junction. Various measurement and calculation techniques are compared and show that window thicknesses in the range of 50 nm can be readily obtained. Alpha-particle spectra are given to prove the superiority of these detectors over conventional Au-Si-Surface Barrier Detectors.