We identify a new type of electromagnetic oscillator which utilizes the negative differential conductivity of a quantum well tunnel junction to drive surface‐plasmon oscillations on the outside boundaries of the junction. We calculate the complex propagation constant as a function of frequency for the antisymmetric gap‐mode surface plasmon on a state of the art GaAs/AlGaAs double‐barrier tunnel junction. The surface‐plasmon gain coefficient is found to be positive for frequencies up to 5 GHz. Single‐mode oscillation of the surface plasmon at frequencies near 5 GHz is predicted on a tunnel junction with a lateral dimension equal to about 150 &mgr;m.