Abstract: A planar frequency tripler comprised of two semiconductor diode structures connected back-to-back by an n.sup.+ doped layer (N.sup.+) of semiconductor material utilizes an n doped semiconductor material for a drift region (N) over the back contact layer in order to overcome a space charge limitation in the drift region. A barrier layer (B) is grown over the drift region, after a sheet of n-type doping (N.sub.sheet) which forms a positive charge over the drift region, N, to internally bias the diode structure. Two metal contacts are deposited over the barrier layer, B, with a gap between them. To increase the power output of the diodes of a given size, stacked diodes may be provided by alternating barrier layers and drift region layers, starting with a barrier layer and providing a positive charge sheet at the interface of a barrier on both sides of each drift region layer with n-type .delta. doping. The stacked diodes may be isolated by etching or ion implantation to the back contact layer N.sup.