A generalised small signal analysis of a DAR Impatt diode has been carried out using recent values of ionisation rates and saturated drift velocities of electrons and holes for Si and GaAs taking both drift and diffusion of charge carriers into account. Results show similar discrete negative conductance frequency bands separated by positive conductance frequency bands for asymmetrical structure as in the ideal case establishing that the harmonically related frequencies can be avoid in Si DAR Impatt diode. However, unlike the ideal case, here the symmetrical DAR Impatt also exhibits finite negative conductance. GaAs DAR Impatt shows similar variations of negative conductance as in Si at high frequencies (in the mm wave range) but at the low frequency side (< 1 GHz) it gives uniform negative conductance unlike Si where the negative conductance comes only at higher frequencies. In our calculations we have considered thin depletion layers (0.8, 1 μm and 2 μm) to show the usefulness of the device in the mm wave range. Further, the DAR Impatt will be able to withstand higher biasing current density without thermal runaway due to space charge cancellation and thus will deliver more power than the conventional Impatts. © 1982.