ALTHOUGH gallium arsenide is usually prepared with low resistivities ( 106 ohm-cm.). This latter material can be produced by zone refining1, by the addition of impurities such as oxygen1, or under growth conditions where the main shallow donor, silicon, is reduced below roughly one part per million1. To explain the occurrence of high-resistivity gallium arsenide, Welker2 suggested that the donors and acceptors are built into the lattice in pairs. More recently, Allen3 has termed this material ‘semi-insulating’, and claims that the existence of n-type high-resistivity material requires that there be a very high degree of compensation between the shallow donor and a deep acceptor. He also states that because of the frequency of occurrence of high resistivities, some mechanism of automatic compensation must be operative. Later, to explain gradual transitions in resistivity along floating-zone refined ingots, Gooch et al.4 have shown that a wide range of resistivities can still occur even on assuming a high degree of automatic compensation. However, according to their model, the degree of automatic compensation must vary along the ingot, yet cannot vary too much otherwise p-type material is predicted, which is not observed.