A number of anomalous electronic properties of gapless semiconductors (GS) doped with transition metals have been detected during the last decade. The 3d transition elements form resonant states, i.e. states superimposed on the conduction band continuum. Just these iron states with mixed valency and are the reason for all the observed anomalous properties of the GS HgSe:Fe.

The objective of this article is to review the experimental and theoretical results in this field, laying special emphasis on the successes achieved in 1991 - 95. It has been shown that the anomalies of a number of electronic features, in particular of the electron mobility, are caused by the peculiarities of electron scattering. While the change of mobility is informative, it is inadequate to draw comprehensive conclusions on the dominant electron scattering mechanism and, hence, the complicated behaviour of the transport properties. A much more sensitive indicator of the nature of electron scattering are the thermomagnetic effects (TME). Thermoelectric (TE) and TME investigations carried out to date made it possible to answer unequivocally the principal question, which consists of two parts. The first part concerns the formation in the GS HgSe:Fe of impurity ordering created by the iron atoms. The impurity concentration ranges have been established, where the gas-like approximation is valid for the charged donor system and where the liquid-like approximation must be used. The second part of the question concerns the degree of this ordering, which determines the electron scattering and, hence, the transport anomalies. Attention is focused on an entirely new mechanism of inelastic scattering of electrons by neutral (in the HgSe lattice) centres that is important for TE and TME. The new theory, which allows quantitative interpretation of all the observed peculiarities of the electron mobility, TME and TE in the framework of the short-range-order model, is expounded here. In particular, it is shown that the balance of electrons, transferred from within the correlation sphere outside, permits one to calculate the concentration dependence of the packing factor for the ions and, hence, the scattering rate.

It should be mentioned that studying heavily doped semiconductors with deep resonant states is a promising new field and, in addition, it constitutes a method of achieving high electron mobilities, exceeding those in semiconductors with shallow hydrogen-like impurities.