  Misfit layer compounds are heterostructures composed of rocksalt units
stacked with few layers transition metal dichalcogenides. They host Ising
superconductivity, charge density waves and good thermoelectricity. The design
of misfits emergent properties is, however, hindered by the lack of a global
understanding of the electronic transfer among the constituents. Here, by
performing first principles calculations, we unveil the mechanism controlling
the charge transfer and demonstrate that rocksalt units are always donor and
dichalcogenides acceptors. We show that misfits behave as a periodic
arrangement of ultra-tunable field effect transistors where a charging as large
as 6\times10^{14} e^-cm^{-2} can be reached and controlled efficiently by the
La-Pb alloying in the rocksalt. Finally, we identify a strategy to design
emergent superconductivity and demonstrate its applicability in
(LaSe)_{1.27}(SnSe_2)_2. Our work paves the way to the design synthesis of
misfit compounds with tailored physical properties.
