Stack Effect in CMOS Design: Threshold Voltage Impact

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The stack effect in CMOS design refers to the impact on threshold voltage (Vt) when transistors are arranged in series. In a configuration with two NMOS transistors, the Vt of the top transistor increases when the bottom transistor is turned on, as the source voltage of the top transistor is elevated due to the gate voltage applied to the bottom transistor. This increase in Vt occurs only during active transitions when the bottom NMOS is conducting; if the bottom NMOS is off, the top NMOS is effectively grounded, and the stack effect does not influence its Vt. The discussion highlights the importance of understanding the operational context, particularly in applications like a two-input NAND gate. Overall, the stack effect is crucial for accurate threshold voltage modeling in CMOS circuits.
johndoe
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What is stack effect in cmos design and how would it affect the threshold voltages?
 
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This sounds somewhat like a HW question.
A "stack" is just two or more transistors in series. So think of a standard CMOS inverter but with two NMOS gates in series on the bottom half. Can you determine how doing that would effect the threshold of each transistor? Pay close attention to Vsb.
 
Last edited:
es1 said:
This sounds somewhat like a HW question.
A "stack" is just two or more transistors in series. So think of a standard CMOS inverter but with two NMOS gates in series on the bottom half. Can you determine how doing that would effect the threshold of each transistor? Pay close attention to Vsb.

Ok with two nmos in series, assuming their body is both grounded, Vt of the top nmos would increase since when a gate voltage is applied to the bot nmos, the source voltage (Vs) of the top nmos will be at a voltage Vgs higher than its body.

But here comes the question, such a 'stack effect' with the increase in Vt of the first nmos would only happen when the bot nmos is turned on right?
 
johndoe said:
Ok with two nmos in series, assuming their body is both grounded, Vt of the top nmos would increase since when a gate voltage is applied to the bot nmos, the source voltage (Vs) of the top nmos will be at a voltage Vgs higher than its body.

But here comes the question, such a 'stack effect' with the increase in Vt of the first nmos would only happen when the bot nmos is turned on right?

What is the context of your question? What is the application?
 
berkeman said:
What is the context of your question? What is the application?

The stack effect on Vt for a two input NAND gate. (2 nmos in series for pull down) If I am understanding correctly, the Vt for the nmos on top would increase only during transitions where the bot nmos is on? cause if the bot nmos is off, the top nmos would also be grounded and stack effect would not have any effect on vt?
 

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