Charge-collection efficiency of heavily irradiated silicon diodes operated with an increased free-carrier concentration and under forward bias

The charge-collection efficiency of Si pad diodes irradiated with neutrons up to $8 \times 10^{15} \ \rm{n} \ cm^{-2}$ was measured using a $^{90}$Sr source at temperatures from -180 to -30°C. The measurements were made with diodes under forward and reverse bias. Under reverse bias the diodes were continuously illuminated with light of a short penetration depth, which modified the occupation probability of the defects in the detector bulk. It was found that forward bias gives the highest charge-collection efficiency at any given voltage. Calculations of the charge-collection efficiencies with a simple model using two effective deep defects were compared with measurements.