n‐n+‐nepitaxial triple layers were grown by liquid phase epitaxy to fabricate lo‐hi‐lo GaAs IMPATT devices. The best diode gave 35.6% cw efficiency with 2.9‐W output power at 10.4 GHz. From a group of 151 devices, 51% had better than 22% efficiency with an average output power of 2.8 W at 10 GHz. Operating voltages varied between 40 and 55 V. A theoretical efficiency of 46% was calculated for narrow spike structures using a quasistatic computer simulation.