Theoretical and experimental investigations have been carried out for V-band (50-75 GHz) silicon double drift flat profile (DD) and double low high low (DLHL) IMPATT diodes. The theoretical designs have been used for the experimental realisation of the diodes for CW operation. The epitaxial layers were grown by silicon molecular beam epitaxy which enabled the realisation of the complex DLHL profile at millimetre-wave frequencies in the silicon material system for the first time. The maximum obtained conversion efficiency is 14.3%. A comparison of theoretical and experimental results for both types of diodes shows general agreement and the superiority of the DLHL structure. © 1991, The Institution of Electrical Engineers. All rights reserved.