MOSFET Design Doubt: Explaining Positive Voltage

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SUMMARY

The discussion clarifies the operation of NMOS and PMOS transistors in relation to their substrate and source connections. It establishes that when the drain is at a positive voltage relative to the source, the device functions as an NMOS transistor, unless operated in inverse mode, which is uncommon. The connection of the substrate to the source does not inherently cut off the transistor; instead, it creates a non-conducting path due to the back-to-back pn junctions when no gate voltage is applied, placing the enhancement-mode MOSFET in cut-off.

PREREQUISITES
  • Understanding of NMOS and PMOS transistor operation
  • Knowledge of MOSFET structure and terminology
  • Familiarity with enhancement-mode MOSFET characteristics
  • Basic concepts of pn junctions and their behavior in circuits
NEXT STEPS
  • Study NMOS and PMOS transistor characteristics in detail
  • Learn about enhancement-mode MOSFET operation and applications
  • Research the effects of substrate connections in MOSFET design
  • Explore the role of gate voltage in MOSFET switching behavior
USEFUL FOR

Electrical engineers, semiconductor designers, and students studying MOSFET technology will benefit from this discussion, particularly those focused on transistor design and operation in electronic circuits.

shawrix
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Can someone explain this sentence in MOSFET design..

The Since the drain will be at a positive voltage relative to the source the two pn junctions can be effectively cut off simply connecting the substrate terminal to source terminal.
 
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As far as I know the substrate and source are usually connected, and does not cut off the transistor in any way.
 
Is it NMOS or PMOS?
 
shawrix said:
Can someone explain this sentence in MOSFET design..

The Since the drain will be at a positive voltage relative to the source the two pn junctions can be effectively cut off simply connecting the substrate terminal to source terminal.

Since the drain is stated to be more + than the source, the device is an NMOS unless the device is operated in the inverse mode, which is almost never the case.

As stated in a previous post, the souce & substrate are usually but not always connected. That connection does not cut off the device.
 
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shawrix said:
Can someone explain this sentence in MOSFET design..

The Since the drain will be at a positive voltage relative to the source the two pn junctions can be effectively cut off simply connecting the substrate terminal to source terminal.

... with the substrate connected to the source, the two back-to-back pn junctions cause the drain-source path to be a non-conducting path. Under these conditions (with the absence of gate voltage) the enhancement-mode MOSFET is in cut-off.
 

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