For a thermally grown silicon dioxide layer, in a MOSFET, the leakage current between the gate and channel is:

Concept:

The gate rests on a thin oxide layer over the source-drain channel.

The oxide layer, effectively a dielectric, acts as a capacitor and isolates the gate and rest of the substrate body, source, drain, and channel.

Hence no electrical connection exists between the gate and the rest of the parts, because of that extremely small leakage current of its gate capacitor exist which means the input impedance of a MOSFET is very high.

Additional Information

MOSFET:

A Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has 4 Gate, Drain, Source, and Substrate (Body) terminal terminals. But in many practical circuits, MOSFET is used by connecting three terminals Gate, Drain, and Source while connecting the Substrate (Body) terminal to the source.

drain, source, the gate is the terminal for MOSFET.