Arsenic and boron implanted n+pn- reachthrough diodes have been produced in a high-frequency BIFET (bipolar-JFET) process. The I-V characteristics are here measured on structures formed by the top-gate/channel/bottom-gate sandwich of the separated-gate JFET's. A variety of reachthrough voltages have been realized by adjusting the implantation conditoins of the channel. Theoretically the I-V characteristics of the diodes are evaluated by solving the Poisson equation in the 1-D case with asymmetric gates. Special attention is paid to the voltage VFMRT necessary to reach the flatband condition after reachthrough. The lowest values, approaching the junction built-in voltage, are achieved if the doping of the reverse biased gate is several times higher than that of the forward biased gate, a situation which is realized in the present n+pn- devices if VGT>VGB. High impact-ionization currents are measured just after reachthrough for devices with VRT>1 V, which in the JFET's gives undesirable source/drain currents.