Resonant tunnelling diodes (RTDs) are emerging as some of the more promising electron devices in the field of communications. The rapid progress of molecular beam epitaxy (MBE) growth techniques during the last decade has resulted in RTDs that exhibit remarkable peak-to-valley ratios, opening the door to a variety of useful device applications. To study the applicability of low-power EHF oscillators for personal communications and global-positioning system applications, we fabricated RTDs using AlAs/GaAs/AlAs double- barrier quantum wells and a MESFET fabrication process. The dc and rf characteristics of the RTDs, which showed a high degree of bistability, were obtained by on-wafer probing using an HP8510 network analyzer with a cascade probe station. The devices were then used in a number of simple hybrid oscillator circuits on alumina substrates. Focusing on the oscillation frequency of 37 GHz, we report on the room-temperature short-, medium-, and long-term stability of the oscillators while monitoring thermal drift and dc bias control.