We will not discuss here 2D TI phases found in $(\text{Bi},\text{Sb})_2\text{Te}_3$ and $(\text{Bi},\text{Sb})_2\text{Se}_3$ thin films, where the top and bottom surface states hybridize.

2DEG in magnetically doped QWs
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2DEG in low-dimensional heterostructures
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Most of the chalcogenide heterostructures hosting 2DEG are based on the II-VI semiconductors, such as ZnSe(ZnTe), CdTe, and HgTe. All of them have a zinc-blende crystal structure with direct band gap at the $\Gamma$ point. ZnSe and ZnTe have the low temperature highest band gap values of 2.82 and 2.40 eV respectively. The band gap of CdTe is equal to 1.6 eV, while HgTe has the so-called inverted band structure with the band gap of -0.3 eV. It is worth to mention that by varying the composition of $\text{Hg}_{1-x}\text{Cd}_{x}\text{Te}$, the band gap also varies from a positive to a negative value through a zero gap for a particular composition. It was found in Ref. [@Galazka1967] that such zero gap energy spectrum in $\text{Hg}_{0.9}\text{Cd}_{0.1}\text{Te}$ has linear dispersion $\epsilon\propto{\bf{k}}$, i.e., Dirac cone. Refs. [@Keldysh1964; @Wolff1964; @Zawadzki1966; @Aronov1967] were the first, which recognized the formal analogy between the dispersion relation $\epsilon({\bf{k}})$ of relativistic electrons in vacuum and electrons in semiconductors, where the parameter corresponding to the light velocity is 