Trapped charge modulation: a new cause of instability in AlGaAs/InGaAs pseudomorphic HEMT's

A new degradation mechanism of PM-HEMT's subsequent to hot electron stress tests or high temperature storage tests is presented. A noticeable increase in drain-to-source current, I/sub DS/, is observed after the tests. We show that this I/sub DS/ variation is slowly recoverable and is correlated with the presence of deep levels in the device. Stress tests cause a variation of trapped charge. Trapping of holes created by impact-ionization and/or thermally stimulated electron detrapping induce a variation of the net negative trapped charge, leading to a decrease in the threshold voltage, V/sub T/ and a consequent increase in I/sub DS/. The correlation between g/sub m//spl Delta/V/sub T/ and /spl Delta/I/sub DS/ clearly demonstrates that the variation of trapped charge induced by hot electron tests is localized under the gate.