Ionic gating is a powerful technique to realize field‐effect transistors (FETs) enabling experiments not possible otherwise. So far, ionic gating has relied on the use of top electrolyte gates, which pose experimental constraints and make device fabrication complex. Promising results obtained recently in FETs based on solid‐state electrolytes remain plagued by spurious phenomena of unknown origin, preventing proper transistor operation, and causing limited control and reproducibility. Here, a class of solid‐state electrolytes for gating (Lithium‐ion conducting glass‐ceramics, LICGCs) is explored, the processes responsible for the spurious phenomena and irreproducible behavior are identified, and properly functioning transistors exhibiting high density ambipolar operation with gate capacitance of ≈20 − 50 µF cm−2\[20{\bm{ - }}50\;\mu F c{m^{{\bm{ - }}2}}\] (depending on the polarity of the accumulated charges) are demonstrated. Using 2D semiconducting transition‐metal dichalcogenides, the ability to implement ionic‐gate spectroscopy to determine the semiconducting bandgap, and to accumulate electron densities above 1014 cm−2 are demostrated, resulting in gate‐induced superconductivity in MoS2 multilayers. As LICGCs are implemented in a back‐gate configuration, they leave the surface of the material exposed, enabling the use of surface‐sensitive techniques (such as scanning tunneling microscopy and photoemission spectroscopy) impossible so far in ionic‐gated devices. They also allow double ionic gated devices providing independent control of charge density and electric field.