Influence of high fluence neutron irradiation on forward current of semiconductor detectors

Forward current-voltage (I-V) characteristics of non-irradiated and irradiated p/sup +/-n-n/sup +/ detectors, at neutron fluences (/spl phi/) up to 10/sup 14/ n/cm/sup 2/, were measured and the obtained data were analyzed. The I/sub f/-V/sub f/ characteristics confirmed the existence of a critical fluences (/spl phi//sub c/>5/spl times/10/sup 11/ n/cm/sup 2/), where abrupt changes in the dependence of forward current on voltage appear. We found that at /spl phi/>/spl phi//sub c/ a similar abrupt drop of the rectification ratio (Rec) appears, but a large reverse voltage could, nevertheless, be applied. The series resistivity (/spl rho/), calculated from the I/sub f/-V/sub f/ characteristics, for both non-irradiated (NI) and irradiated detectors, shows that for NI detectors the resistivity is of the same order of magnitude like that of the silicon bulk (1200 /spl Omega/ cm). The resistivity (/spl rho/) for irradiated devices increases (with increasing values of /spl phi/, of up to 10/sup 14/ n/cm/sup 2/), up to about 4/spl times/10/sup 3/ /spl Omega/ cm. As a result of the increase of /spl rho/ with /spl phi/, the initial p/sup +/-n-n/sup +/ diode becomes a p/sup +/-/spl nu/-n/sup +/ device. This study explains the rather good charge collection efficiency in spite of strongly affected physical properties.