Microwave mixing and noise measurement techniques have been used to characterize energy relaxation and noise phenomena for the two-dimensional electron gas (2DEG) medium in single AlGaAs/GaAs modulation-doped quantum wells. Mixing experiments at 94 GHz yield the energy relaxation time directly, in good agreement with optical methods previously reported. The noise output power at low microwave frequencies (1.5–3.5 GHz), is shown to have one term due to Nyquist noise at the electron temperature, and a second frequency-dependent term caused by fluctuations in the electron temperature.