The input impedance of a MOSFET is of the order of _______

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Examoogle · Accepted Answer

Several M$\Omega$

Explanation: The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) exhibits an extremely high input impedance, typically ranging from $10^{10} \, \Omega$ to $10^{15} \, \Omega$. This is primarily because the gate terminal is physically isolated from the channel by a thin layer of silicon dioxide ($SiO_2$) dielectric.

Working Principle: In a MOSFET, the gate is capacitively coupled to the semiconductor channel through a highly insulating layer of $SiO_2$. Since this dielectric material has very low conductivity, virtually no DC current flows into the gate terminal regardless of the gate voltage applied, leading to an extremely high input resistance.

Key Formulas: $Z_{in} = \frac{V_{GS}}{I_G}$ — The definition of input impedance where $I_G \approx 0$ in an ideal MOSFET.

Key Points: The $SiO_2$ layer acts as a nearly perfect insulator.; Higher impedance reduces loading effects on preceding stages.; MOSFETs are voltage-controlled devices, whereas BJTs are current-controlled.; Static discharge can easily damage the gate oxide due to high impedance.

Advantages: Extremely high input impedance minimizing circuit loading; Low power consumption in static states; Scalability for high-density VLSI circuits

Disadvantages: Susceptible to electrostatic discharge (ESD) damage; Parasitic gate capacitance limits switching speed at very high frequencies

Applications: CMOS logic gates in microprocessors; High-impedance buffers and amplifiers; Power switching circuits

Additional Info: Because the gate leakage current is negligible, MOSFETs are often ideal for applications where signal source impedance is high.. Option A ($\Omega$) represents a low-impedance device like a diode. Options B and C represent low to moderate impedance ranges not typical for a MOSFET gate.

Answer: D is correct — The input impedance of a MOSFET is typically several M$\Omega$ (or often G$\Omega$ to T$\Omega$ in modern devices) due to the insulated gate structure.

Exam Tip: Always remember that while a BJT has a finite base current (low/medium input impedance), the MOSFET gate draws effectively zero DC current, making it the preferred choice for high-impedance signal interfacing.

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