Suppression of Gunn oscillatios in thin-film Gunn diodes by a two-dimensional effect is described theoretically and experimentally. Experiments show that Gunn oscillations do not occur in diodes whose nt product is less than 1.1 × 1011 (cm - 2), where t is the film thickness. This stabilized sample is useful for amplifiers. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.