Abstract. The operation of gallium arsenide schottky barrier field effect transistor is greatly affected by several anomalies such as frequency dispersion of output impedance, z ds ; this low frequency behaviour is related to the presence of capture centres at the channel/substrate interface. In this context, we investigate the influence of such defects via a circuit consisting of a capacitance in series with a resistance placed in parallel to the output of a transistor, between drain and source. Then, using PSPICE program we made some simulations at several bias of the device and different values of elements characterising traps. It was found that the dispersion increases when the value of the drain bias increases. It was also shown that the dispersion increases with increasing trap resistance. Whereas, the trap capacitance greatly affects the initial and final saturation of the output impedance IntroductionDespite the great evolution of modern technology in order to achieve high performance devices, surface and interface phenomena still play undesirable effects [1][2][3]. In particular, for gallium arsenide metal semiconductor field effect transistor, GaAs MESFET, the conducting channel, limited by two depleted zones, is highly perturbed by uncontrollable defects and capture centres. Thus, there always exist some persistent limitations such as low-frequency anomalies which affect the performance of broadband systems [4][5][6]. In fact, it was noted that the output impedance of GaAs MESFET at high frequencies can be substantially different than its dc value [7,8]. This behavior, although of relatively minor importance in microwave and digital circuits, can profoundly affect the performance of analogical integrated circuits.The origin of frequency dispersion phenomenon is still a subject of some controversy. Although it usually attributed [9-11] to the influence of deep traps localized at the interface between active layer and semi-insulating substrate, it was also reported that its origin could be due to short channel effect and/or to the diminution of the square resistance of n + layer beneath ohmic contacts. Other interpretations were related to device structure with buried channel or p-well structure [12,13]. Consequently, the presence of localized surface states was modeled by an equivalent circuit consisting of a surface state capacitance, C ss , and a resistance, R ss , [14].In this work, to put into evidence the dispersive phenomenon, we first observe experimental frequency dependence of output impedance, Z ds (f) in GaAs MESFETs. Then, we use SPICE program to simulate Z ds (f) at different bias values via an equivalent circuit in low frequency that takes into