A device simulator that simulates device performance in the cyclic bias steady state was del eloped. and it was applied to GaAs hetero-junction FET (HJFET) pulse pattern effect. Although there is a large time-constant difference between the pulse signals and deep trap reactions, the simulator searches the cyclic bias steady states at about 30 iterations. A non-linear shift in the drain current level with the mark ratio was confirmed. which has been estimated from the rate equation of electron capture and emission based on Shockley-Read-Hall statistics fur deep traps.