Epitaxially grown p(+)-i-n(+) junctions were processed to diodes by mesa etching and contacted by Cr/Au metallization. Rim currents, which often plague mesa devices were reduced by a special design with the contact area separated from the mesa rim, The dependence on the current voltage characteristics on contact area, on temperature and on external load was investigated. Four different regimes were identified where Zener tunnelling, Esaki tunnelling, tunnelling through midgap states and diffusion of minority carriers dominate the current, respectively. Esaki tunnelling leads to negative differential resistance which causes oscillations depending on the external load. The results are interpreted by considering the nominal intrinsic width, the extension of the depletion layer and the abruptness of the doping transition.