Solid-state neutron detectors based only on boron-rich semiconductors are of interest for their potential to provide the highest thermal neutron detection efficiencies of any solid-state neutron detectors. A simple physical model, recently shown to generate thermal neutron capture product spectra that agree quantitatively with full-physics GEANT4 simulation, is used to compare the capture product energy spectra and the upper limits to neutron detection efficiency of planar conversion layer, sandwich and all-boron-carbide detectors for the case of normally incident, mono-energetic, thermal neutrons. All-boron-carbide semiconductor detectors are deduced to be greatly superior to all other boron-rich solid-state detector types in their maximal neutron detection efficiencies and potential for avoiding false-positive detector output signals in mixed radiation fields. If boron-carbide semiconductors of optimal quality and thickness in the range 20–50 µm were used in creating such detectors, the normal-incidence thermal neutron detection efficiencies could reach 60% to 90%, respectively, in total and still 48% to 78% using only the peak corresponding to the kinetic energy sum for the nuclei emitted in the most-probable ¹⁰B(n,α)⁷Li capture reaction.