SOME semiconductors have comparatively low resistivity, and this is due to their containing electrically active impurities with energy-levels near a band edge. Others have very high resistivity and this is due to their containing a predominance of impurities with levels far from the band edges, that is, far compared with the thermal energy kT. These latter materials are often referred to as insulators, but since they can carry electronic currents we prefer to introduce the term `semi-insulator' to describe them. A material such as cadmium sulphide, which by suitable choice of impurity content can be made a semiconductor or a semi-insulator, is of particular interest and much work has been done on cadmium sulphide in the past few years. Gallium arsenide is another substance with the same property, but has the advantage that both n- and p-type material may be readily prepared.