A mechanism of threshold voltage changes for WN x gate GaAs MESFETs in high temperature storage life tests

Abstract

Reliability of 0.8 μm WN x gate GaAs MESFETs with a self-aligned lightly doped drain structure has been investigated by means of high temperature storage life tests at 250, 275 and 300 °C. The observed reduction in threshold voltage followed by drain current increase was just reverse in contrast to those for ‘gate sinking’ effect reported on several Au-based gates. The correlation of the threshold voltage reduction with Shottky barrier height and other MESFET parameter changes during the tests suggested a model related to the short channel effect for the threshold voltage reduction, which was proved true by submitting samples of gate lengths 0.7, 1.0 and 1.5 μm to high temperature storage life tests. The dependence of threshold voltage changes on gate orientation relative to the crystal axis was also evaluated with 1.0 μm gate MESFETs to investigate the model in more detail. MESFETs parallel to [001] axis showed minimum absolute threshold voltage changes, while those parallel to piezoelectrically active [011] and [0 1 1] axes showed decreasing and increasing threshold voltage changes, respectively. From these results, the threshold voltage changes were tentatively ascribed to the relief of the stress caused by poly-imide die bonding process for packaging MESFET chips. In other words, WN x gate GaAs MESFET chips themselves were concluded to show no appreciable degradation up to 1000 hr storage life tests at 250 and 275 °C, except for ohmic contact degradation at 300 °C.