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 × 10 15 n cm - 2 was measured using a 90Sr 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.