We demonstrate that spin $p\text{−}n$ junctions—magnetic semiconductor homojunctions with spin-splitting-induced band offsets—fundamentally redefine carrier transport via spin-dependent recombination probabilities $r_{\sigma {\sigma }^{\prime }}$. By integrating this mechanism into the Shockley model, we predict a 100 times enhancement in magnetoresistance (MR) sensitivity under small forward bias in the case of large intrinsic spin splitting ($0.1\mathrm{eV}$), where exponential modulation of recombination lifetimes ${\tau }_{\sigma {\sigma }^{\prime }}$ by magnetic fields amplifies resistance changes. The MR in our model exhibits a strong dependence on the intrinsic spin-splitting energy, showing a marked enhancement with increasing splitting magnitude. Angular momentum conservation enables magnetically tunable circularly polarized luminescence: Exclusive conduction band or valence band splitting in both neutral regions achieves near-half polarization ($|P_I|\approx 50\%$), while global splitting degrades emission coherence. Furthermore, we propose a “spin Zener filter” exploiting $\sim 1\text{eV}$ valence band splitting in (Ga, Mn)As, where spin-dependent barrier heights generate near 100% spin-polarized tunneling currents within a voltage-selective window. Our extended model incorporating $p$-orbital effects reveals that while the orbit minimally affects the current magnitude, it dramatically alters the circular polarization degree of the luminescence. This significant difference arises primarily from the selection rules $\mathrm{\Delta }m=\pm 1$ imposed by the $p$-orbital character. These results establish spin $p\text{−}n$ junctions as a universal design paradigm for magnetically amplified electronics, polarization-programmable optoelectronics, and voltage-gated spin injection without ferromagnetic contacts.

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• Magnetotransport
• Spin filtering
• Spin optoelectronics
• Junctions
• Methods in transport