A high resistivity crystal of gallium arsenide is doped with at least 0.2 part per million by weight of chromium. The crystal may have a resistivity of at least 108 ohm centimetres at 300 DEG K. It may also contain other electrically active ingredients, the dominant constituent being chromium. The crystal may be grown by pulling from a melt of gallium arsenide, the latter being prepared by raising liquid gallium to the melting point of gallium arsenide and maintaining arsenic vapour in proximity therewith (see Division B1).ALSO:A rod of gallium arsenide containing 0.2-360 ppm of chromium is pulled from a melt formed in situ from gallium and chromium in an alumina crucible (65) and arsenic vaporised from a mass (75) outside the crucible. The crucible is contained in a graphite susceptor (63) which is heated by an induction coil (67). The pulled rod is rotated at 25 rpm. The pull rate is 1.5 in/hr. Pulling is effected in argon at atmospheric pressure.