- #1
johndoe
- 41
- 0
What is stack effect in cmos design and how would it affect the threshold voltages?
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.
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?
berkeman said:What is the context of your question? What is the application?
The stack effect, also known as the short channel effect, is a phenomenon in CMOS design where the threshold voltage of a transistor decreases as the channel length gets shorter. This can cause performance and reliability issues in integrated circuits.
The stack effect causes the threshold voltage to decrease due to the increased electric field in the channel region as the channel length decreases. This leads to a decrease in the transistor's switching threshold, making it more susceptible to noise and other disturbances.
The stack effect can lead to various consequences, such as increased leakage current, reduced noise margins, and reduced reliability and lifespan of the integrated circuit. It can also affect the performance and speed of the circuit.
There are several techniques that can be used to mitigate the stack effect in CMOS design. These include using thicker gate oxides, implementing channel doping gradients, introducing lightly-doped drain (LDD) structures, and using different gate materials.
Current research efforts are focused on developing new materials, such as high-k dielectrics, to replace traditional gate oxides and improve transistor performance. Other approaches include using non-planar transistor architectures, such as FinFETs, to reduce the effects of the stack effect.