The electron flow in large-area, space-charge-limited diodes is susceptible to transit time instability. The instability spatially and temporally modulates the electron beam and results in a wide, time-dependent, electron energy distribution and high-power rf emission. The electromagnetic waves that power the instability are guided and amplified in the anode–cathode (AK) region which acts as a parallel-plate transmission line with negative conductance. We investigated how to mitigate this instability in large-area rectangular electron diodes. We found that by slotting the rectangular cathode, along both its axes, with parallel grooves of predetermined depth and periodicity, the AK region was modified into a slow wave structure. The unstable electromagnetic waves are slowed down and attenuated to such an extent that the instability is eliminated and stable electron flow is achieved.