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At high gate voltages, a high electron mobility transistor may have two current conduction channels. One is at the GaAs/ GaAlAs interface and the other is in the n-type GaAlAs layer. Conduction in the latter channel causes degradation of the device transconductance. It is shown in this paper that the threshold voltage for turning on the GaAlAs channel can be controlled by the layer parameters of the HEMT structures. Optimization of a HEMT in an integrated circuit requires control over both the 2-DEG channel and the GaAlAs channel to fit each particular circuit configuration.