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IntroduetionTrap-to-band direct tunneling (T-B) [I] and thermal excitation (TE) at elevated temperatures [2] are believed to be the major electron detrapping mechanisms in polysilicon-oxide-nitrideoxide-silicon (SONOS) nonvolatile semiconductor memory (NVSM) devices. Understanding their individual and joint contributions to retention loss of SONOS devices can help researchers to cope with the challenge of scaling the programming voltage, while maintaining IO-year data retention time at elevated temperatures. In this paper, for the first time, we present a SONOS retention model that incorporates both T-B and TE detrapping mechanisms. First, the influences of gate dielectric thickness, temperature and trap energy on the electron decay are discussed, based on calculations of detrapping time constants. Next, an analytical SONOS retention model is presented, considering an arbitrary trap energy distribution in the silicon nitride. Finally, the model is verified with a good agreement between measured and simulated SONOS retention characteristics at temperatures from 22°C to 225°C. T-B and TE Detrapping ProcessesFig. 1 illustrates the cross section of a SONOS device with a triple (tunneling oxide, charge storage nitride and blocking oxide) dielectric gate stack. Fig. 2 shows a bandgap diagram of a . . . . .~ ~ SONOS .~ ~~~~~ device in the excess electron state, illustrating T-B and TE electron detrapping processes.Time constants rT.B [3] and rrz [4] characterize the electron decay rates in the T-B and TE processes, respectively. A relatively small rT.B for electrons trapped near the tunneling oxide, as shown in Fig. 3, suggests that T-B dominates detrapping of these *near' traps. Also, a relatively small rTE for electrons trapped in 'shallow' traps, as shown in Fig. 4. suggests that TE plays a key role in detrapping of these 'shallow' traps. Moreover, by comparing with rTE, one finds that neither T-B nor TE can be ignored in modeling electron loss in SONOS devices at temperatures from O°C to 125'C. SONOS Retention ModelAn analytical model on SONOS retention loss is presented. considering the presence of both T-B and TE electron loss processes. We start by assuming the trap densify g(h) is uniform in space, and distributed in energy. At retention time I, the threshold voltage shift AVTH due to trapped electrons is given by where some of the symbols are illustrated in Fig. 2. We show the threshold voltage decay rate (at temperature 7 ) is given hy where D(x') and M(x') are functions of the ONO dielectric thickness. the depth & and distancex' of the emptied nitride traps. h' is a function oft, while x' is a function of I and trap energy. Fig. 5 shows a calculated threshold voltage decay rate of a SONOS transistor in the excess electron state at room temperature as a function of retention time. Under the given conditions, the threshold voltage decay rate is relatively constant, and declines after IO' seconds, while TE shows a comparable influence as T-B on the long-term (f > IO6 s) charge retention. Fig. 6 shows the measu...