SUMMARYIn this paper a new lumped-distributed small-signal and noise model is presented for a class of transit-time semiconductor devices including the IMPATT, BARITT, Transferred Electron and Space Charge Limited (SCL) diodes. The assumption of uniform carrier velocity is made and diffusion is neglected. The diode-model is composed of lumped elements and dispersionless transmission lines, hence it is very suitable for computer-aided circuit analysis. Broad agreement has been found between the experimental behaviour of devices and that of their models.