Data retention loss mechanisms in nano-sized localized trapping polysilicon-oxide-nitride-oxide-silicon (SONOS) memory devices were investigated. To study the influence of charge lateral distribution on data retention, we used three channel hot electron injection (CHEI) programming operations with different substrate biases and same erasing operations to obtain three different charge profiles in the nitride layer after successive program/erase cycling. As a result, the mismatch profile of injected electrons and holes leads to serious degradation of data retention for the negative substrate biased CHEI programming. The experimental results suggest that the residual holes accumulation due to the mismatched profile, evidenced by the charge pumping and sub-threshold slope measurement, could be one of main origins of charge loss for nano-sized cycled SONOS cells. When the mismatch of the injected electrons and holes profiles is very serious after cycling, the accumulated holes detrapping by Frenkel-Poole emission and the lateral transport in the nitride layer may be an important data loss mechanism. (C) 2013 Elsevier Ltd. All rights reserved.