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Questions and Answers

What does the vertical solid line represent in the NMOS circuit symbol?

  • Drain electrode
  • Channel electrode
  • Source electrode
  • Gate electrode (correct)

What is formed in an n-channel depletion MOSFET to create a channel?

  • P-type impurity atoms
  • No impurity needed
  • N-type impurity atoms (correct)
  • Conductive materials

What occurs in the substrate when gate voltage Vg increases from zero?

  • Current flows to the drain
  • Negative ions are exposed (correct)
  • More holes are created
  • Free electrons are generated

What is the nature of the depletion region in a MOSFET when in the OFF state?

<p>Consists only of immobile negative ions (D)</p> Signup and view all the answers

At what threshold voltage does a conductive channel begin to appear in the MOSFET?

<p>300mV to 500mV (D)</p> Signup and view all the answers

What happens to the channel resistance as the gate voltage Vg increases?

<p>Channel resistance decreases (A)</p> Signup and view all the answers

What describes the state of the MOSFET when VG is less than the threshold voltage (VTH)?

<p>No channel exists and ID equals zero (A)</p> Signup and view all the answers

What effect does keeping VG constant and above VTH have on the MOSFET?

<p>The MOSFET becomes ON with ID &gt; 0 (C)</p> Signup and view all the answers

How does increasing the gate voltage (VG) affect the on-resistance (Ron)?

<p>It decreases the on-resistance. (A)</p> Signup and view all the answers

What is the effect of increasing the channel length on the on-resistance (Ron)?

<p>On-resistance increases as channel length increases. (B)</p> Signup and view all the answers

How does increasing the thickness of the oxide layer (tox) affect the channel capacitance?

<p>It decreases the capacitance. (B)</p> Signup and view all the answers

If the gate-source voltage surpasses the threshold voltage (VTH), what condition can still restrict current flow?

<p>If VD rises significantly higher than VG - VTH. (D)</p> Signup and view all the answers

What is the relationship between charge density (Q) and the voltage difference (VG - VTH)?

<p>Charge density increases with increasing (VG - VTH). (B)</p> Signup and view all the answers

When the channel is pinched off near the drain, what still allows current to flow?

<p>High electric field in the depletion region. (C)</p> Signup and view all the answers

What occurs to the charge density of the channel as one moves from the source to the drain?

<p>Charge density decreases. (D)</p> Signup and view all the answers

What happens to the performance of the device as the channel width (W) increases?

<p>The channel resistance decreases. (D)</p> Signup and view all the answers

What happens to the drain current ID as VDS increases past VGS – VTH?

<p>ID decreases and eventually reaches saturation. (A)</p> Signup and view all the answers

What is the significance of the channel length modulation in MOSFET operation?

<p>It leads to a finite output impedance and drain current increase. (C)</p> Signup and view all the answers

How is the drain current ID described when VDS is much smaller than 2(VGS – VTH)?

<p>It behaves as a linear function of VDS. (B)</p> Signup and view all the answers

What defines the maximum drain current IDmax in terms of VGS and VTH?

<p>IDmax = ½ μn Cox (VGS – VTH)² at VDS = VGS – VTH. (D)</p> Signup and view all the answers

Which statement best describes the equivalent on-resistance Ron of a MOSFET?

<p>Ron is determined by μn Cox W (VGS – VTH). (D)</p> Signup and view all the answers

What characterizes the I-V characteristics of a MOSFET when it reaches the saturation region?

<p>The current remains constant regardless of further increases in VDS. (C)</p> Signup and view all the answers

What occurs to the channel when pinch-off is achieved in a MOSFET?

<p>The channel effectively starts to decrease in length. (D)</p> Signup and view all the answers

In the context of MOSFET characteristics, what does the term 'over drive voltage' refer to?

<p>The difference between VGS and VTH. (C)</p> Signup and view all the answers

Flashcards

Depletion region

A region under the gate oxide in a MOSFET where there are no mobile charge carriers.

N-channel NMOS

A type of MOSFET where the channel is formed by a negative charge carrier (electrons) using an n-type material.

Threshold Voltage (VTH)

The gate voltage at which a conductive channel begins to form in a MOSFET, enabling current flow.

PMOS

A type of MOSFET where the channel is formed by a positive charge carrier (holes) using a p-type material.

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Depletion MOSFET

A type of MOSFET where a channel exists even without a gate voltage, unlike enhancement-mode MOSFETs.

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Channel Resistance

The resistance offered by the conductive channel in a MOSFET, which varies with the gate voltage.

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MOSFET ON

The state of a MOSFET when the gate voltage exceeds the threshold voltage, creating a conductive channel between the source and drain.

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MOSFET OFF

The state of a MOSFET when the gate voltage is below the threshold voltage, blocking current flow between the source and drain.

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On-resistance (Ron)

The resistance of the source-drain path in a MOSFET.

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Channel Length Effect

Increasing channel length increases on-resistance.

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Gate Oxide Thickness (tox)

The thickness of the insulating layer between the gate and the channel.

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Pinch-off Voltage

The voltage at which the channel narrows near the drain.

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Threshold Voltage (VTH)

The minimum gate voltage needed to create a conducting channel.

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Gate-Channel Capacitance

A voltage-controlled capacitance between gate and channel.

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MOSFET Width Effect

Increasing width (W) decreases the channel resistance.

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Channel Charge Density

The number of free electrons per unit length in the channel.

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Drain Current (ID)

The current flowing through the MOSFET's drain.

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Aspect Ratio (MOSFET)

The ratio of width to length (W/L) of the MOSFET channel, affecting the drain current.

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Channel Length Modulation

Drain current slightly increasing with drain voltage in saturation region due to channel length decrease.

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Triode Region

Region of MOSFET operation where MOSFET behaves as a linear resistor.

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Saturation Region

MOSFET operation where drain current becomes constant with increasing drain voltage.

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Overdrive Voltage

(VGS - VTH); Voltage difference that affects current.

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Pinch-off

Point where the channel under the gate closes off, increasing the drain voltage causing a constant current.

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Channel Length Modulation Coefficient

(λ); It measures how much the drain current changes with the drain-source voltage in the saturation region.

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Study Notes

Circuit Symbol and Conventions

  • NMOS: Vertical solid line is the gate, broken line is the channel, broken line indicates enhancement mode.
  • PMOS: Similar structure to NMOS
  • Separation between gate and channel line denotes the oxide, which insulates the gate from the channel.
  • Body and source are typically connected.

Depletion MOSFET

  • Similar construction to NMOS but forms an actual channel by adding n-type impurities to a p-type substrate.
  • Gate can be positive, zero, or negative.

Operation of NMOS

  • Drain and source are grounded, gate voltage is varied.
  • As gate voltage increases from zero, positive gate charge is mirrored by negative charge in the substrate.
  • Formation of a channel results in negative ions and creating a depletion region.
  • Channel forms when gate voltage exceeds a threshold voltage.

MOSFET Operation

  • When gate voltage exceeds a threshold (300mV-500mV), a conductive channel forms between the drain and source.
  • As gate voltage increases, electron density in the channel increases, and channel resistance decreases.
  • The conductive channel acts as a resistor.

Drain Voltage Application

  • Applying drain voltage creates a voltage drop across the channel.
  • Channel resistance is voltage dependent.

Effect of Channel Length and Oxide Layer

  • Longer channels result in higher on-resistance.
  • Thicker oxide (tox) layers reduce capacitance between gate and substrate.
  • Less charge on the gate leads to lower electron density and higher on-resistance.

MOSFET as a Voltage Controlled Resistance

  • If VGS > VTH, the device acts as a voltage controlled resistance.
  • Potential difference between the gate and oxide-silicon interface must exceed VTH.
  • If drain voltage is higher than source voltage, voltage along the channel increases towards the drain.

Pinch-off

  • If drain voltage is high enough (VGS - VTH), channel pinches off near the drain.
  • Gate-substrate potential difference isn't sufficient to attract electrons, and the channel is pinched off.

Channel Characteristics Beyond Pinch-off

  • If VDS increases further than VGS - VTH, operation enters a saturation region.
  • No channel exists between drain and certain source point.

Channel Length Modulation

  • Channel length varies with VDS.
  • As drain voltage increases, effective channel length shrinks, which increases drain current.
  • Channel length modulation phenomenon occurs due to the early effect or channel length modulation coefficient.

MOSFET I-V Characteristics

  • MOSFET I-V characteristics show behaviour similar to saturation and forward regions of a BJT.
  • At low VDS, the MOSFET acts as a linear resistor.
  • As VDS increases, the current saturates or reaches a maximum (IDmax).

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