An atomic force microscope (AFM) is used to locally deplete the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The depletion is induced by repeated mechanical scribing of the surface layers of the heterostructure using the AFM tip. Measuring the room-temperature resistance across the scribed lines during fabrication provides in situ control of the depletion of the 2DEG. Variation of the room-temperature resistance of such lines tunes their low-temperature characteristics from tunneling up to insulating behavior. Using this technique, an in-plane-gate transistor and a single-electron transistor were fabricated. © 1999 American Institute of Physics.