A MECHANISM OF THRESHOLD VOLTAGE CHANGES FOR WNX GATE GAAS-MESFETS INHIGH-TEMPERATURE STORAGE LIFE TESTS

Reliability of 0.8 mu m WNx gate GaAs MESFETs with a self-aligned ligh tly doped drain structure has been investigated by means of high tempe rature storage life tests at 250, 275 and 300 degrees C. The observed reduction in threshold voltage followed by drain current increase was just reverse in contrast to those for 'gate sinking' effect reported o n several Au-based gates. The correlation of the threshold voltage red uction 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 submitt ing samples of gate lengths 0.7, 1.0 and 1.5 mu m to high temperature storage life tests. The dependence of threshold voltage changes on gat e orientation relative to the crystal axis was also evaluated with 1.0 mu m gate MESFETs to investigate the model in more detail. MESFETs pa rallel to [001] axis showed minimum absolute threshold voltage changes , while those parallel to piezoelectrically active [011] and [0(1) ove r bar1$] axes showed decreasing and increasing threshold voltage chang es, respectively. From these results, the threshold voltage changes we re tentatively ascribed to the relief of the stress caused by poly-imi de die bonding process for packaging MESFET chips. In other words, WNx gate GaAs MESFET chips themselves were concluded to show no appreciab le degradation up to 1000 hr storage life tests at 250 and 275 degrees C, except for ohmic contact degradation at 300 degrees C.