A mixed level device and circuit simulation has been performed to analyze DX trap-induced slow transient effects on the performance degradation of AlGaAs/GaAs HEMT circuits. The variation of the output pulsewidth and the hysteretic characteristics of the input-output voltage transfer function in DCFL HEMT inverters have been simulated. In the model, a DX trap rate equation is calculated in the AlGaAs layer. The self-consistent Schrödinger and Poisson equations are solved numerically at each cross section of a device. A two-region Grebene-Ghandhi model is employed to derive the I-V characteristics. Connections between individual HEMT devices in simulated circuits are treated as nodes of circuit equations. In the simulation, all of the device equations and the circuit equations are iteratively solved until a self-consistent solution is achieved. Our simulation confirms that the output pulse broadening and narrowing effects in a string cascaded DCFL HEMT inverters are a consequence of the inverter voltage transfer function shift caused by deep traps.