We have addressed the problem of threshold voltage (V/sub TH/) variation in flash memory cells after heavy-ion irradiation by using specially designed array structures and test instruments. After irradiation, low V/sub TH/ tails appear in V/sub TH/ distributions, growing with ion linear energy transfer (LET) and fluence. In particular, high LET ions, such as iodine used in this paper, can produce a bit flip. Since the existing models cannot account for large charge losses from the floating gate, we propose a new mechanism, based on the excess of positive charge produced by a single ion, temporarily lowering the tunnel oxide barrier (positive charge assisted leakage current) and enhancing the tunneling current. This mechanism fully explains the experimental data we present.