Alpha particle resolution of a surface barrier detector/preamplifier combination is adversely affected by excessive noise of the detector. The detector noise increases when the substrate is exposed at the site of contamination introduced into the coated wafer surfaces during the manufacture of the detector. Mechanisms by which substrate exposure may occur are presented in this paper. Increase in detector noise by substrate exposure is shown to follow the basic photoconductivity equation. Noise performance of a detector with surface contamination is observed to have three modes of degradation, depending upon the nature of contamination. Of the three modes, the third one caused by an outgassing/insulating type of contamination is sporadic in nature and is difficult to identify. Long-term vacuum cycling tests are evolved to identify this mode of degradation.