The Bell Labs work on the transistor emerged from war-time efforts to produce extremely pure germanium "crystal" mixer diodes, used in radar units as a frequency mixer element in microwave radar receivers. UK researchers had produced models using a tungsten filament on a germanium disk, but these were difficult to manufacture and not particularly robust. Bell's version was a single-crystal design that was both smaller and completely solid. A parallel project on germanium diodes at Purdue University succeeded in producing the good-quality germanium semiconducting crystals that were used at Bell Labs. Early tube-based circuits did not switch fast enough for this role, leading the Bell team to use solid state diodes instead.