Why MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor, and the "oxide" in the name is the key: a very thin layer of silicon dioxide insulates the gate electrode from the channel between source and drain. The gate therefore controls channel current purely by the electric field it produces, with essentially no gate current flowing. That is why MOSFETs have extremely high input impedance and why they are easily damaged by static discharge puncturing that thin insulating layer.
Watch out The choice about a back-biased (reverse-biased) junction gate describes a JFET, where the gate really is a PN junction rather than an insulated electrode.
The O in MOSFET is the oxide insulator under the gate: gate is insulated, not junction-coupled.