Trapped charge modulation: a new cause of instability inAlGaAs/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_DS, is observed after the tests. We show that this I_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_T and a consequent increase in I_DS. The correlation between g _mΔV_T and ΔI_DS clearly demonstrates that the variation of trapped charge induced by hot electron tests is localized under the gate