In this paper the results of an extensive study of the effects of the drift region width on the performance of LHL-GaAs-IMPATT diodes are presented. These results have been obtained using an IMPATT-diode full scale computer simulation program. It is demonstrated that the transition between the regular IMPATT mode and the high-efficiency modes becomes more abrupt as the drift region width is increased. It is also found that when the diode operates according to the high-efficiency modes the efficiency is not a sensitive function of the drift region width. The anomalous behaviour of the admittance of the diode is found to be caused by the increase of the component of the total current.contributing to the power generation mechanism at the expenense of the cold-capacitive current. It is demonstrated that for the diodes whose drift region width is optimized for the regular IMPATT mode the efficiency remains always high and the hysterises in the tuning characteristics are less significant.