We report on bipolar NPN phototransistors fabricated at ITC-IRST on thick high-resistivity silicon substrates. The phototransistor emitter is composed of a phosphorus n + implant, the base is a diffused high-energy boron implant, and the collector is the 600-800 mum thick silicon bulk, contacted on the backplane. We have studied the current amplification for two different doping profiles of the emitter, obtaining values of beta ranging from 60 to 3000. For various emitter and base configurations, we measured the static device characteristics and extracted the leakage currents and the base resistance, verifying the fundamental relationship between them and the total base capacitances. The use of such phototransistors to detect ionizing particles is exploited and discussed. (C) 2004 Published by Elsevier B.V. RI Forti, Francesco/H-3035-2011; Neri, Nicola/G-3991-2012; Dalla Betta, Gian-Franco/I-1783-2012