# Increase in doping concentrations resulting in a decrease in mobility?

• theBEAST
In summary, the conversation discusses the relationship between doping concentrations and mobility. The initial statement suggests that increasing doping concentrations results in a decrease in mobility. However, there is a discrepancy as the person thought that an increase in doping concentrations would lead to an increase in mobility due to a higher concentration of carriers available for conduction. The expert then explains that while the conductivity does increase with doping, the mobility actually decreases due to free carriers colliding with impurities more frequently. The conversation ends with the person expressing their appreciation for the explanation.
theBEAST

## Homework Statement

So in one of our practice problems it states that as we increase the doping concentrations, the result will be a decrease in mobility.

However, I thought that when you increase doping concentrations, there will be a higher concentration of carriers available for conduction and thus the conductivity increases. So this should result in an increase in mobility? Also, when they say doping concentrations, it can either be p or n type dopants right?

theBEAST said:

## Homework Statement

So in one of our practice problems it states that as we increase the doping concentrations, the result will be a decrease in mobility.

However, I thought that when you increase doping concentrations, there will be a higher concentration of carriers available for conduction and thus the conductivity increases. So this should result in an increase in mobility? Also, when they say doping concentrations, it can either be p or n type dopants right?

The conductivity increases, but the mobility decreases. The conductivity is proportional to the concentration of free carriers, about equal to the doping concentration; and also proportional to the mobility. But the mobility decreases with the doping, as the free carriers collide with the impurities more frequently, so the drift velocity decreases.

ehild

ehild said:
The conductivity increases, but the mobility decreases. The conductivity is proportional to the concentration of free carriers, about equal to the doping concentration; and also proportional to the mobility. But the mobility decreases with the doping, as the free carriers collide with the impurities more frequently, so the drift velocity decreases.

ehild

Oh wow, that's actually pretty cool, thanks!

You are welcome

ehild

I can explain the relationship between doping concentrations and mobility in semiconductors. Doping is the process of intentionally adding impurities to a semiconductor material in order to alter its electrical properties. When we increase doping concentrations, we are essentially increasing the number of impurities in the material.

Now, the mobility of a material refers to the ease with which charge carriers (electrons or holes) can move through it. In a doped semiconductor, the charge carriers are either electrons or holes, depending on the type of dopant used. When we increase the doping concentrations, we are essentially increasing the number of charge carriers in the material.

However, as the doping concentration increases, the distance between these charge carriers decreases, leading to more collisions between them. These collisions hinder the movement of charge carriers and result in a decrease in mobility. So, while the number of charge carriers increases with higher doping concentrations, the mobility decreases due to increased collisions.

It is also important to note that doping concentrations can refer to both p-type and n-type dopants. In both cases, an increase in doping concentrations will result in a decrease in mobility due to increased collisions between charge carriers.

In summary, while an increase in doping concentrations does lead to an increase in the number of charge carriers available for conduction, it also results in a decrease in the mobility of these charge carriers due to increased collisions. This is why an increase in doping concentrations can result in a decrease in mobility.

## 1. What is doping and how does it affect mobility?

Doping is the process of intentionally introducing impurities into a material, typically a semiconductor, in order to alter its electrical properties. When doping concentrations are increased, it can lead to a decrease in the mobility of charge carriers within the material. This is because the added impurities disrupt the regular crystal lattice structure and impede the movement of charge carriers.

## 2. Why does an increase in doping concentrations result in a decrease in mobility?

The addition of impurities through doping creates more defects and imperfections in the crystal lattice of the material. These defects act as obstacles for charge carriers, making it more difficult for them to move freely through the material. As a result, the mobility of charge carriers decreases.

## 3. How does doping affect the conductivity of a material?

Doping can significantly affect the conductivity of a material. When impurities are added, it can either increase or decrease the number of charge carriers in the material, which in turn affects its conductivity. For example, doping with donor impurities can increase the number of free electrons and therefore increase conductivity, while doping with acceptor impurities can decrease conductivity by reducing the number of free electrons.

## 4. Can doping concentrations be too high?

Yes, doping concentrations can be too high. When doping is done in excess, it can lead to a phenomenon called carrier freeze-out, where all the available charge carriers are bound to the impurity atoms and can no longer contribute to conductivity. This can result in a decrease in mobility and overall decrease in the material's electrical properties.

## 5. How does the type of doping impurity affect mobility?

The type of doping impurity can have a significant impact on the mobility of charge carriers. As mentioned before, donor impurities can increase the number of free electrons and therefore increase mobility, while acceptor impurities can decrease mobility by reducing the number of free electrons. Additionally, different impurities can have different sizes and charge states, which can also affect the material's crystal lattice and hence its mobility.

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