We studied the mechanism of Ga autodoping in HgCdTe layers grown on GaAs by metalorganic vapor phase epitaxy (MOVPE). We determined growth conditions suitable for reducing the carrier concentration in unintentionally doped HgCdTe layers. We detected Ga atoms in HgCdTe layers rather than in CdTe buffer layers. Ga atoms did not diffuse into HgCdTe layers from GaAs substrates through CdTe buffer layers, but were incorporated from the vapor phase during HgCdTe growth. We assumed that the GaAs back surface reacted with Hg and metalorganic precursors during HgCdTe growth and this reaction produced volatile organic-gallium which acted as the Ga autodoping source. To confirm this, we coated the GaAs back surface with Si3N4 and obtained HgCdTe layers with carrier concentrations below 1015 cm−3. We also clarified the role of Hg in the incorporation mechanism. Because the concentration of both carriers and Ga atoms increased with Hg partial pressure, Hg behaved as a catalyst for the organic-gallium production. Using layers with low Ga concentrations, we fabricated photodiodes whose performance was as good as those fabricated using liquid phase epitaxial (LPE) grown HgCdTeCdZnTe layers, because the tunneling current due to the residual donor concentration was suppressed.