# Threshold voltage dependence on Vdb

• opcode0x90
In summary, threshold voltage is the minimum voltage needed for a transistor to switch on and is influenced by the voltage applied to the drain (Vdb). As Vdb increases, the threshold voltage also increases due to the widening of the depletion region and the impact on the electric field between the source and drain. This relationship is not linear but follows a logarithmic pattern, making Vdb a critical factor in circuit design to maintain the desired threshold voltage for optimal performance and reliability.
opcode0x90
Why does the Vth of MOSFETs depends on Vsb and not Vdb even though MOSFETs are symmetrical devices? Is it a convention to choose Vsb over Vdb or am I missing out on something more fundamental?

Because the source is usually kept at the constant. Also after pinch-off the drain is separated from channel charge by a depletion layer.

## 1. What is threshold voltage and how is it related to Vdb?

Threshold voltage is the minimum voltage required for a transistor to switch from the off state to the on state. It is dependent on various factors, including the voltage applied to the drain (Vdb). As Vdb increases, the threshold voltage also increases.

## 2. Why does threshold voltage depend on Vdb?

The threshold voltage depends on Vdb because it affects the depletion region and the strength of the electric field between the source and drain. As Vdb increases, the depletion region widens, making it harder for the transistor to switch on, thus increasing the threshold voltage.

## 3. How does the variation in Vdb affect the threshold voltage?

An increase in Vdb results in an increase in the threshold voltage, while a decrease in Vdb leads to a decrease in the threshold voltage. This is because the depletion region and electric field between the source and drain are affected by the voltage, ultimately affecting the threshold voltage.

## 4. Is the relationship between threshold voltage and Vdb linear?

No, the relationship between threshold voltage and Vdb is not linear. It follows a logarithmic pattern, meaning that a small change in Vdb can result in a significant change in the threshold voltage.

## 5. How does the dependence of threshold voltage on Vdb impact circuit design?

The threshold voltage-Vdb relationship is an essential consideration in circuit design as it affects the performance and reliability of transistors. Designers must carefully select the appropriate Vdb value to ensure that the threshold voltage remains within the desired range for optimal circuit operation.

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