Studies of the doping profile characteristics of molecular‐beam epitaxy (MBE) of GaAs on GaAs substrates are reported. Highly resistive regions at the substrate—epitaxial‐layer interface, and within the epitaxial layer, may occur if the growth is interrupted. It is shown that almost any arbitrary voltage dependence of the capacitance of such structures can be achieved by varying the dopant deposition rate during epitaxy. To illustrate the versatility of this growth technique, voltage‐variable capacitors have been prepared with Schottky barriers on MBE GaAs layers with precisely controlled doping profiles. Low‐frequency measurements (up to 100 MHz) demonstrate that capacitance variations in excess of a factor of 10 have been achieved by varying the applied bias voltageVfrom 0.3 V (with no significant forward conduction) to −1.0 V. The feasibility of frequency tuning diodes withC−1/2[sine wave]&jgr;−V, where &jgr; is the effective barrier height, is shown. At ‐3 V bias, cutoff frequencies >40 GHz have been measured. With an improved diode design, cutoff frequencies in excess of 200 GHz are expected at similar bias voltages.