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The splitting energy δ between the X1-band with an effective mass m 1 * and the X3-band with m 2 *, and the effective mass ratio m 2 */m 1 * for the conduction band of n-GaP were determined as a function of the temperature T by measuring the infrared absorption and fitting theoretical curves of the X1→X3 absorption band. As T rises, δ decreases as 0.349-6.83×10-5 T 2/(T+460) [eV], which can be explained by a faster shift of the upper X3-band than the lower X1-band toward the valence band. This empirical equation gives δ=0.316 eV at 773 K. The m 2 */m 1 * increases, which indicates that the upward-bending of the X1-band has a stronger T-dependence than the X3-band.

Temperature dependences of the threshold energy, δ, between the X1-band with effective mass m 1 * and the X3-band with m 2 *, and the ratio m 2 */m 1 * were obtained for the donor(Te)-doped GaP with a concentration of N D=1.8∼60×1017 cm-3, from the X1–X3 absorption bands measured at various temperatures T and the theoretical curve fittings. δ increases smoothly with decreasing T for 135<T<300K for the lightly-doped sample, whereas δ shows anomalous increase below about 190K for the heavily-doped samples. This anomaly is explained by the electrons falling deep into the band-tail. It can be confirmed by the calculations of electron energy distributions in the parabolic part and band-tail of the conduction band using a density-of-states model. The calculations give the average depth of the band-tail as 11 meV for N D=20.9×1017 cm-3.