S2Cl2 diluted in the non-conductive CCl4 Solvent has been successfully applied to the passivation of GaAs field-effect transistors. In such a solution, in situ measurements of current-voltage (I-V) characteristics of the devices are accessible while the passivation is in process. By comparing the I-V data measured from the devices upon passivation in various concentrated S2Cl2 solutions, it is found that the volume ratio of 10(-5) (S2Cl2:CCl4) represents the optimal concentration where dipping of a device for 450 s or so will result in a 28% rise in breakdown voltage while the transconductance reduces only by 10%. (C) 2004 Elsevier B.V. All rights reserved.