In order to understand fundamental noise mechanisms in high
speed devices, we present data on the temperature dependence of noise at microwave frequencies. For a GaAs Metal-Semiconductor Field Effect Transistor (MESFET) and a High-Electron Mobility Transistor (HEMT) we determine that the noise decreases exponentially with decreasing temperature. The activation energies for the noise in the MESFET and the HEMT are unique and are current and frequency independent. In contrast to the FETs, the temperature dependence of noise in the Hetrojunction Bipolar Transistor (HBT) is shown to be weakly temperature dependent.