GaAs n** plus -i- delta (p** plus )-i-n** plus bulk barrier transistors with ultra thin base (about 100 A) were prepared by molecular beam epitaxy. The current transport over the potential barrier is dominated by thermionic emission. It is a voltage-controlled device in which the barrier and current can be directly controlled by the applied base bias. The current-voltage characteristics are similar to those of triodes or static induction transistors. The devices possess the capability of high frequency response (f//T greater than equivalent to 30 GHz), high power and low noise. The barrier height can also be modulated by the light intensity.