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To study trap models related to the variable retention time (VRT) phenomenon in dynamic random access memory (DRAM), we derived equations to calculate the data retention time tret of DRAM and the activation energy for two trap models, i.e., the metastable and oxide trap models. Measuring the tret of VRT cells for various bias and temperature conditions, the dependence of activation energy differences in tret on bias at high and low retention states was extracted. Furthermore, the dependence of the electric field on bias at high and low retention states was also extracted. Using those parameters, we successfully distinguished the two types of trap models.