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, IDS, is observed after the tests. We show that this IDS 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, VT and a consequent increase in IDS. The correlation between DVT and DIDS clearly demonstrates that the variation of trapped charge induced by hot electron tests is localized under the gate.