It is shown that the anomalously deep and nondegenerate ground state of Ge donors in GaP is better described as a deep state attached to higher X-minima of the conduction band than as a shallow one attached to the absolute minima. This is because of a very strong intervalley interaction between the higher minima at X for point-charge impurities located at the cation site.
 

Journal of the Physical Society of Japan, 49 169-72, 1980.