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Questions and Answers
What does the vertical solid line represent in the NMOS circuit symbol?
What does the vertical solid line represent in the NMOS circuit symbol?
What is formed in an n-channel depletion MOSFET to create a channel?
What is formed in an n-channel depletion MOSFET to create a channel?
What occurs in the substrate when gate voltage Vg increases from zero?
What occurs in the substrate when gate voltage Vg increases from zero?
What is the nature of the depletion region in a MOSFET when in the OFF state?
What is the nature of the depletion region in a MOSFET when in the OFF state?
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At what threshold voltage does a conductive channel begin to appear in the MOSFET?
At what threshold voltage does a conductive channel begin to appear in the MOSFET?
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What happens to the channel resistance as the gate voltage Vg increases?
What happens to the channel resistance as the gate voltage Vg increases?
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What describes the state of the MOSFET when VG is less than the threshold voltage (VTH)?
What describes the state of the MOSFET when VG is less than the threshold voltage (VTH)?
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What effect does keeping VG constant and above VTH have on the MOSFET?
What effect does keeping VG constant and above VTH have on the MOSFET?
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How does increasing the gate voltage (VG) affect the on-resistance (Ron)?
How does increasing the gate voltage (VG) affect the on-resistance (Ron)?
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What is the effect of increasing the channel length on the on-resistance (Ron)?
What is the effect of increasing the channel length on the on-resistance (Ron)?
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How does increasing the thickness of the oxide layer (tox) affect the channel capacitance?
How does increasing the thickness of the oxide layer (tox) affect the channel capacitance?
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If the gate-source voltage surpasses the threshold voltage (VTH), what condition can still restrict current flow?
If the gate-source voltage surpasses the threshold voltage (VTH), what condition can still restrict current flow?
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What is the relationship between charge density (Q) and the voltage difference (VG - VTH)?
What is the relationship between charge density (Q) and the voltage difference (VG - VTH)?
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When the channel is pinched off near the drain, what still allows current to flow?
When the channel is pinched off near the drain, what still allows current to flow?
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What occurs to the charge density of the channel as one moves from the source to the drain?
What occurs to the charge density of the channel as one moves from the source to the drain?
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What happens to the performance of the device as the channel width (W) increases?
What happens to the performance of the device as the channel width (W) increases?
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What happens to the drain current ID as VDS increases past VGS – VTH?
What happens to the drain current ID as VDS increases past VGS – VTH?
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What is the significance of the channel length modulation in MOSFET operation?
What is the significance of the channel length modulation in MOSFET operation?
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How is the drain current ID described when VDS is much smaller than 2(VGS – VTH)?
How is the drain current ID described when VDS is much smaller than 2(VGS – VTH)?
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What defines the maximum drain current IDmax in terms of VGS and VTH?
What defines the maximum drain current IDmax in terms of VGS and VTH?
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Which statement best describes the equivalent on-resistance Ron of a MOSFET?
Which statement best describes the equivalent on-resistance Ron of a MOSFET?
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What characterizes the I-V characteristics of a MOSFET when it reaches the saturation region?
What characterizes the I-V characteristics of a MOSFET when it reaches the saturation region?
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What occurs to the channel when pinch-off is achieved in a MOSFET?
What occurs to the channel when pinch-off is achieved in a MOSFET?
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In the context of MOSFET characteristics, what does the term 'over drive voltage' refer to?
In the context of MOSFET characteristics, what does the term 'over drive voltage' refer to?
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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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