Positive and negative frequency dispersion of output conductance in GaAs MESFETs used in optically controlled microwave amplifiers

GaAs MESFETs, which form the basis of photonic MMIC's and OEIC's, have drawn considerable attention in recent years. However, their optical, microwave and electrical characteristics are largely dependent on internal parameters. In this context, we measure the frequency dependence of output conductance in GaAs MESFETs which may be an important concern for device design and reliability. Measurements were carried out, at room temperature, in the frequency range [10 Hz -10 MHz] under several drain-source polarization, Vds, ranging from 100 mV to 2 V and gate-source potentials, Vgs, from 100 mV to 200 mV. The novelty in the obtained gd(f) results lies in the co-existence, for the same device, of three distinct behaviours: a constant dependence observed for weak V ds < 0.7 V, followed by an increase or positive dispersion for 0.7 V < V ds < 1.5 V and then a negative dispersion for Vds > 1.5 V. This behaviour could be attributed to the effects of deep level traps in the depletion region (that could be nanometric) and to surface and interface recombination.