Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The gate of a MOSFET is insulated from body of FET by deposition of a very thin fragile layer of \(SiO_2\) over the substrate. Reason (R): The device is therefore called an insulated gate field-effect transistor (IGFET). In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

MOSFET is also called: \[ \text{IGFET}=\text{Insulated Gate Field Effect Transistor} \] because the gate is insulated from the channel by a thin: \[ SiO_2 \] layer. This gives MOSFETs extremely high input impedance and negligible gate current.

Answer 1

The Correct Option is A

Concept: MOSFET stands for: \[ \text{Metal Oxide Semiconductor Field Effect Transistor} \] One of the most important structural features of a MOSFET is the presence of an insulating oxide layer between the gate terminal and the semiconductor substrate. This oxide layer is generally: \[ SiO_2 \quad (\text{Silicon Dioxide}) \] Because the gate is electrically insulated from the channel region, the MOSFET is also known as: \[ \text{IGFET} = \text{Insulated Gate Field Effect Transistor} \] The insulation provides:
• Extremely high input impedance
• Very small gate current
• Excellent voltage control of channel conductivity Thus, the naming of IGFET directly originates from the insulated gate structure.

Step 1: Analyze Assertion (A). Assertion states that: The gate of a MOSFET is insulated from the body by a thin layer of \(SiO_2\). This statement is absolutely correct. In MOSFET fabrication:
• A very thin oxide layer of silicon dioxide is grown over the semiconductor substrate.
• The metallic or polysilicon gate is placed above this oxide layer.
• The oxide acts as an insulator and prevents direct electrical conduction between gate and channel. Hence, Assertion (A) is true.

Step 2: Analyze Reason (R). Reason states that: The device is therefore called an insulated gate field-effect transistor (IGFET). This statement is also correct. Since the gate terminal is insulated from the semiconductor channel by the oxide layer: \[ SiO_2 \] the MOSFET receives the alternate name: \[ \text{IGFET} \] Thus, Reason (R) is also true.

Step 3: Determine whether Reason correctly explains Assertion. The reason directly explains the assertion because:
• Assertion explains the physical structure: \[ \text{Gate insulated by } SiO_2 \]
• Reason explains the consequence of that structure: \[ \text{Hence called IGFET} \] Therefore:
• Both Assertion and Reason are correct.
• Reason correctly explains Assertion.

Step 4: Write the final answer. Hence, the correct option is: \[ \boxed{(A)\ \text{Both (A) and (R) are correct and (R) is the correct explanation of (A)}} \]