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e report the on-wafer characterization of S-parameters and microwave noise temperature (T50) of discrete metamorphic InGaAs high electron mobility transistors (mHEMTs) at 40 and 300 K and over a range of drain-source voltages (VDS). From these data, we extract a small-signal model (SSM) and the drain (output) noise current power spectral density (Sid) at each bias and temperature. This procedure enables Sid to be obtained while accounting for the variation of SSM, noise impedance match, and other parameters under the various conditions. We find that the noise associated with the channel conductance can only account for a portion of the measured output noise. Considering the variation of output noise with physical temperature and bias and prior studies of microwave noise in quantum wells, we hypothesize that a hot electron noise source (NS) based on real-space transfer (RST) of electrons from the channel to the barrier could account for the remaining portion of Sid. We suggest further studies to gain insights into the physical mechanisms. Finally, we calculate that the minimum HEMT noise temperature could be reduced by up to ∼50% and ∼30% at cryogenic temperature and room temperature, respectively, if the hot electron noise were suppressed.