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We have studied the electrical degradation of InAlAs/InGaAs Metamorphic HEMTs. The main effect of the application of a bias for an extended period of time is a severe increase in the drain resistance, R/sub D/, of the device. We have identified two different degradation modes: a reduction in the sheet-electron concentration of the extrinsic drain and an increase of the drain contact resistance. Both mechanisms are found to be directly related to impact-ionization. The metamorphic nature of the substrate does not seem to play a role in the observed degradation.