Evidence for ballistic electron transport exceeding 160 mu m in an undoped GaAs/AlxGa1-xAs field-effect transistor

We report measurements of GaAs/AlxGa1-xAs undoped field-effect transistors in which two-dimensional electron gases (2DEG's) of exceptional quality and versatility an induced without modulation doping. Electron mobilities at T = 4.2 K and density 3 x 10(11) cm(-2) exceed 4 x 10(6) cm(-2) V-1 s(-1). At lower temperatures, there is an unusually large drop in scattering, such that the mobility becomes too high to measure in 100-mu m samples. Below T = 2.5 K, clear signatures of ballistic travel over path lengths in excess of 160 mu m are observed in magnetic-focusing experiments. Multiple reflections at the edges of the 2DEG indicate a high degree of specularity.