*Vx Vx* min.

Note finally that the characteristics of the system under study can depend on the gate voltage *Vg* (at different values of the charge carrier density *n*2*D*) owing to the dependence of the renormalized Fermi velocity on *n*2*<sup>D</sup>* [101, 102]. In this case, a controlling factor is the filling of minibands with electrons (holes). For the experimental observations, it is convenient to have partially filled either the lower electronic miniband or the upper hole one (in this case, the higher electronic or lower hole minibands are distinguishable). This takes place if *n*2*<sup>D</sup> n*<sup>∗</sup> <sup>2</sup>*<sup>D</sup>* <sup>=</sup> 4/*d*2. This condition can be rewritten in the form of a limitation imposed on the gate voltage [90]

$$|V\_{\mathcal{S}}| \ll 4\pi e n\_{2D}^\* L\_{\mathcal{S}} / \varepsilon\_{s}^\*$$

where *Lg* is the gate thickness and *ε*∗ *<sup>s</sup>* is the effective dc permittivity of the substrate. For the layered substrate structure (see Fig. 19a), we have

$$
\varepsilon\_s^\* = \frac{\varepsilon\_{s1} d\_I + \varepsilon\_{s2} d\_{II}}{d}.
$$
