fet basic- some pyq theories

Question 1

Theory: Explain the reasons behind the higher input impedance of a FET. [Year: 2018]

Answer:

• The gate-to-source $p\text{-}n$ junction is operated strictly under reverse-bias conditions ($V_{GS}<0\text{V}$ for $n$-channel devices).

• This reverse voltage forces the charge-free depletion region at the junction to widen significantly.

• The widened depletion layer establishes a broad physical potential barrier that opposes carrier movement.

• Current crossing the reverse-biased junction is restricted to the ultra-low minority carrier reverse saturation current ($I_{GSS}$).

• The resulting steady-state gate leakage current ($I_G$) is pushed down to the nanoampere ($\text{nA}$) or picoampere ($\text{pA}$) range.

• By Ohm’s law ($Z_i=\frac{\Delta V_{GS}}{\Delta I_G}$), dividing the gate voltage by a near-zero leakage current yields an ultra-high input impedance ($10^8\Omega$ to $10^{12}\Omega$).

Question 2

Theory: Why is the input impedance of a MOSFET so high? Explain in brief. [Year: 2021, 2019]

Answer:

• The metallic or polysilicon gate electrode is physically separated from the semiconductor substrate by a thin layer of Silicon Dioxide (${\text{SiO}}_2$).

• The ${\text{SiO}}_2$ layer functions as a highly effective electrical insulator (dielectric barrier).

• This physical insulation prevents the gate from making direct contact with the $p$-type or $n$-type regions, eliminating junction leakage paths.

• The dielectric barrier blocks the steady-state migration of charge carriers under DC operating conditions.

• Because carrier migration is blocked, the active DC gate leakage current ($I_G$) is reduced effectively to zero amperes.

• Control over the conduction channel is managed purely via an electrostatic field across the dielectric layer, operating like a parallel-plate capacitor.

• This field-effect mechanism draws zero static direct current from the signal source, creating an ultra-high input impedance ($10^{10}\Omega$ to $10^{15}\Omega$).

Theory: Explain the construction and operation of a depletion-type MOSFET using necessary diagrams. [Year: 2016]

1. Construction

A Depletion-type MOSFET (D-MOSFET) is structurally built upon a primary semiconductor substrate with a built-in channel established during manufacturing:

• Substrate: A base block of $p$-type material (for an $n$-channel device).

• Source & Drain: Two heavily doped $n^{+}$ regions diffused into the substrate.

• Physical Channel: A narrow, structurally implanted $n$-type channel physically connecting the source and drain regions directly.

• Insulation: A thin dielectric layer of Silicon Dioxide (${\text{SiO}}_2$) grown over the channel surface.

• Gate Terminal: A metallic/polysilicon plate placed on top of the ${\text{SiO}}_2$ layer, ensuring complete electrical isolation from the channel ($I_G\approx 0$).

2. Operation

The D-MOSFET can operate in two distinct modes depending on the polarity of the gate-source voltage ($V_{GS}$):

A. Depletion Mode ($V_{GS}<0\text{V}$)

• Mechanism: A negative potential applied to the gate terminal repels free electrons out of the $n$-channel into the $p$-substrate and attracts holes from the substrate into the channel.

• Result: Recombination depletes the channel of free electron carriers, narrowing its effective width, increasing channel resistance, and decreasing the drain current ($I_D$).

• Limit: At $V_{GS}=V_P$ (pinch-off voltage), the channel is fully depleted, and $I_D$ drops to $0\text{A}$.

B. Enhancement Mode ($V_{GS}>0\text{V}$)

• Mechanism: A positive potential applied to the gate terminal attracts additional minority carrier electrons from the $p$-substrate directly into the $n$-channel.

• Result: This accumulation enhances the total free-carrier concentration in the channel, reducing its resistance and driving the drain current $I_D$ beyond its zero-bias value ($I_{DSS}$).

Question 2

Theory: Construct an n-channel enhancement-type MOSFET and explain its basic operation. [Year: 2022]

1. Construction

An Enhancement-type MOSFET (E-MOSFET) differs fundamentally because it lacks an structurally pre-built channel:

• Substrate: A continuous base block of $p$-type semiconductor material.

• Source & Drain Well: Two isolated, heavily doped $n^{+}$ regions embedded inside the substrate.

• Absence of Channel: No physical $n$-type conduction path connects the source and drain during manufacturing; they are structurally isolated by the $p$-type substrate block.

• Gate Insulation: A layer of insulating Silicon Dioxide (${\text{SiO}}_2$) deposited directly across the substrate region between the source and drain.

• Gate Lead: A metallic layer resting above the ${\text{SiO}}_2$ insulating barrier.

2. Basic Operation

The device relies entirely on electrostatic induction to create a conductive inversion layer:

Step 1: Status at $V_{GS}=0\text{V}$

• When $V_{GS}=0\text{V}$, the two $n^{+}$ regions remain isolated by the back-to-back $p\text{-}n$ junctions of the substrate. Applying a drain-to-source voltage ($V_{DS}$) produces no current flow ($I_D=0\text{A}$). The device is normally OFF.

Step 2: Channel Induction ($V_{GS}>0\text{V}$)

• Carrier Repulsion: Applying a positive voltage to the gate terminal ($V_{GS}>0\text{V}$) drives positive holes within the $p$-substrate downward, away from the ${\text{SiO}}_2$ interface.

• Inversion Layer Formation: Simultaneously, the positive gate field pulls minority free electrons from the substrate upward, accumulating them directly beneath the ${\text{SiO}}_2$ layer.

• Threshold Voltage ($V_{GS(\text{Th})}$): Once $V_{GS}$ exceeds the specific threshold voltage limit ($V_{GS}>V_{GS(\text{Th})}$), the local concentration of electrons becomes so dense that it forms an induced $n$-type conduction path linking the source to the drain.

Step 3: Conduction ($V_{DS}>0\text{V}$)

• With the induced $n$-channel open, applying a positive drain voltage ($V_{DS}$) causes free electrons to flow from source to drain, establishing a continuous drain current ($I_D$). Increasing $V_{GS}$ further enhances the channel depth, increasing $I_D$.

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