The charge retention characteristics of SONOS (silicon-oxide-nitride-oxide- silicon) non-volatile memory cells at elevated temperatures were investigated. Assuming thermal excitation to be the dominant charge loss mechanism, the trap energy distribution in the nitride was determined. We present an improved model which includes the influence of subsequent tunneling of the charge carriers through the bottom oxide after being thermally emitted into the conduction band of the silicon nitride. The trap energy distribution was evaluated from samples with different bottom oxide thicknesses. Changing the oxide thickness changes the tunneling probability and in turn this reveals different parts of the trap energy distribution. Using the improved model, it was found that the calculations based on the different parts of the energy range fit together to consistently describe a full trap energy distribution.