The question asks about the construction of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and how it achieves high input impedance. Let's explore the concept:

MOSFET Structure and High Input Impedance:

• A MOSFET consists of three terminals: Gate, Drain, and Source. Additionally, it has a substrate made of semiconductor material.
• The key feature of a MOSFET that results in high input impedance is the thin oxide layer between the gate and the channel (the path between the source and the drain).
• This oxide layer is typically Silicon Dioxide (SiO₂), which is an excellent insulator.

Explanation:

• The gate electrode is insulated from the channel by this thin oxide layer.
• Because of this insulation, there is virtually no direct path for current to flow into the gate, significantly increasing the input resistance or impedance of the MOSFET.
• This means that no significant current flows into the gate; instead, it controls the channel conductivity by capacitive action.
• High input impedance is advantageous because it reduces the load on the preceding stage in an electronic circuit.

Evaluating Options:

• The gate is insulated from the channel by a thin oxide layer (Correct option): As discussed, this insulation leads to high input impedance due to the absence of a direct DC path for current between the gate and the channel.
• The gate uses a resistive layer to limit current: This is incorrect because a resistive layer would not provide the high impedance structure characteristic of a MOSFET.
• The gate is connected directly to the channel: This would lower input impedance, contrary to MOSFET design.
• The device has a thick substrate for support: While substrate support is necessary, it does not contribute to the high input impedance of a MOSFET.

Therefore, the correct answer is that the gate is insulated from the channel by a thin oxide layer, providing the MOSFET with high input impedance.