Environmental effects on GaAs MESFETs

In many anticipated applications, GaAs FET devices and circuits will be subject to adverse environmental conditions. Up until now no performance related data has been reported either for passivated or unpassivated devices. In the present paper, we report on failure modes resulting from high relative humidity and temperature cycling in the presence of ionic contamination in an environmental chamber. MESFETs with (i) gate floating and (2) with gate biased with respect to the source (source grounded and drain floating), were cycled -10°C to 100°C at 95% relative humidity. Four types of MESFET structures with gold and aluminum contact systems have been tested: unpassivated non-hermetic; passivated non-hermetic; passivated hermetic and unpassivated hermetic. The effects of Na and Cl ions, at levels of 1014cm-2, were investigated. Electrical measurementS before and after exposure included DC characterization. RF characterization will be reported at a later time. Devices which exhibited excessive leakage currents were analyzed using the scanning electron microscope, Auger electron spectroscopy, EBIC and X-ray spectroscopy. The results show that passivated non-hermetic, unpassivated non-hermetic and passivated hermetic exhibit all of the following failure modes: a. Corrosion of the metallization resulting from ionic contamination, moisture and the galvanic couple; b. Electrolytic conduction between electrically biased metallizations, where the rate of transfer of metal ions from one electrode to another, across the surface depends upon the electrolytic current flow. The conductivity of the surface is a function of the amount of moisture on the surface. The transfer of metal shows up as a gate to source leakage current resulting from a metal-film developing in the transfer path. The close spacings between gate-source-drain make this failure mechanism highly probable for GaAs FETs. Gate leakage current as a function of number of cycles and ion contamination has also been determined for all commercial devices tested. MESFETs with aluminum gates showed corrosion phenomena. The Au-Al reactions and aluminum corrosion were also present on passivated non-hermetic MESFETs indicating that present passivation utilized (SiO2) is not a barrier to ions and moisture. MESFETs with Au gates exhibited migrative resistive shorts (MGRS). Interdigitated metallization structures with controlled spacings were utilized to determine kinetics of moisture-ion migration and hence determine activation energies. For SiO2passivation the activation energy was determined to be 1.1 eV.