  High electron mobility transistors are widely used as microwave amplifiers
owing to their low microwave noise figure. Electronic noise in these devices is
typically modeled by noise sources at the gate and drain. While consensus
exists regarding the origin of the gate noise, that of drain noise is a topic
of debate. Here, we report a theory of drain noise as a type of partition noise
arising from real-space transfer of hot electrons from the channel to the
barrier. The theory accounts for the magnitude and dependencies of the drain
temperature and suggests strategies to realize devices with lower noise figure.
