Y. Kitaura et al., A MECHANISM OF THRESHOLD VOLTAGE CHANGES FOR WNX GATE GAAS-MESFETS INHIGH-TEMPERATURE STORAGE LIFE TESTS, Microelectronics and reliability, 35(12), 1995, pp. 1515-1520
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.