Abstract-A V t retention distribution tail in a multitimeprogram (MTP) silicon-oxide-nitride-oxide-silicon (SONOS) memory is investigated. We characterize a single-program-chargeloss-induced ΔV t in NOR-type SONOS multilevel cells (MLCs). Our measurement shows the following: 1) A single-charge-lossinduced ΔV t exhibits an exponential distribution in magnitudes, which is attributed to a random-program-charge-induced currentpath percolation effect, and 2) the standard deviation of the exponential distribution depends on the program-charge density and increases with a program V t level in an MLC SONOS. In addition, we measure a V t retention distribution in a 512-Mb MTP SONOS memory and observe a significant V t retention tail. A numerical V t retention distribution model including the percolation effect and a Poisson-distribution-based multiple-charge-loss model is developed. Our model agrees with the measured V t retention distribution in a 512-Mb SONOS well. The observed V t tail is realized mainly due to the percolation effect.Index Terms-Model, percolation, silicon-oxide-nitride-oxidesilicon (SONOS), V t retention distribution.