Functional characterization of a high-gain BJT radiation detector

n-p-n bipolar phototransistors have been designed and fabricated on high-resistivity silicon substrates. A technology featuring a double implant for the emitter allowed us to obtain a typical current gain of about 600. The device has been tested with alpha particles from a Pu-239 source, beta particles from Sr-90, and X-rays from Am-241 using a simple experimental setup, where the detector is directly connected to the oscilloscope. In the case of electrons, pulse heights of 100 mV have been observed, with pulse length of 50 mu s, measured on a load resistor in series to the emitter. The parameters driving the time performance have been measured, obtaining a good agreement with the electrical model of the device. We report on the functional characterization of the device, in particular the time response, the energy calibration, and the electronic noise measurement.