Highly sensitive magnetic effects on the voltage‐current characteristics in a longp+‐&pgr;‐n+diode biased forwardly are analyzed. The solutions of this analysis are consistent with the following assumptions: The similar treatment for the Righi‐Leduc effect, which is produced by the varied effect of the magnetic field on hot and cold carriers, is also applicable for the case where there is a density gradient in the sample. In addition, since the diffusion current and the effect of surface recombination are small enough to be neglected, the current flows through only the volume‐recombination process. The major results are as follows: (1) Highly sensitive magnetic effect in ``double‐injection regime'' mainly depends on the recombination process for the injected electrons which are minority carrier. This differs from the effect in ``Ohmic regime'' where majority carrier contributes to the current. (2) The threshold voltage, at which the negative resistance occurs, is strongly dependent on a ratio of the recombination densityNRto the majority carrier densityp0. (3) The currentJin the double‐injection regime beyond a minimum voltage follows aJ∝V2+slaw, wheresis a function of the mobility ratiob, and it is given ass≈2(blog2−1).