# Question about majority and minority concentrations in diodes

• theBEAST
In summary, for a REV biased diode, the concentration of majority carriers (holes) increases as we get closer to the junction. This is because the np product must remain constant, and as electrons are swept across the depletion region, holes increase to compensate for the decrease in electrons. Additionally, during forward bias, the depletion region is reduced as electrons and holes are generated in pairs, leading to a constant hole concentration near the metal contact. In reverse bias, the opposite occurs, with increased hole concentration and decreased electron concentration near the junction. The minority concentrations increase near the junction due to the generation of electron-hole pairs when free electrons collide with immobile atoms.
theBEAST
In my notes package it shows that for a REV biased diode, in the p type side, the concentration of majority carriers (pp0 holes) increases as we get closer to the junction. However, I don't get how this makes sense, since the metal contact between the negative terminal of the source and the p-type region is negatively charge, shouldn't we expect the holes to be attracted to the metal contact? Thus, in the graph shown below, the concentration of pp0 should be larger at the beginning and decrease as it gets closer to the junction?

PS: feel free to ignore some of the things I wrote :P

The np product at that point is given by $n^{2}_{i}e^{F_n-F_p/kT}$.

If none of these quantities could have changed at a given point, then it must be constant. As you see in the diagram the $n_{p0}$ concentration drops to zero near the depletion region as those electrons have been are swept across by the large electric field.

So if the np product must be constant, and n decreases, p has to increase to account for it.

Conceptually, you could also think of holes being quickly swept from the other side and "piling up" at the end of the depletion region.

shallowbay said:
The np product at that point is given by $n^{2}_{i}e^{F_n-F_p/kT}$.

If none of these quantities could have changed at a given point, then it must be constant. As you see in the diagram the $n_{p0}$ concentration drops to zero near the depletion region as those electrons have been are swept across by the large electric field.

So if the np product must be constant, and n decreases, p has to increase to account for it.

Conceptually, you could also think of holes being quickly swept from the other side and "piling up" at the end of the depletion region.

Hmm, that makes a lot of sense when you put it that way. But what about the other way to think of it? The fact that there is a negative charge on the left contact and thus the positive holes should be attracted to it. This should result in an increase in concentration of holes near the metal contact. In other words, there are less holes close to the pn junction and thus the potential barrier is increased?

Also, can anyone explain this then:

Why would the minority concentrations increase near the junction?

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Hmm, that makes a lot of sense when you put it that way. But what about the other way to think of it? The fact that there is a negative charge on the left contact and thus the positive holes should be attracted to it. This should result in an increase in concentration of holes near the metal contact. In other words, there are less holes close to the pn junction and thus the potential barrier is increased?Also, can anyone explain this then:Why would the minority concentrations increase near the junction?
Look first of all, holes can't really do anything even if they are attracted by terminal. Look it the other way, its the electrons that move and generate holes. Now having said that first look at the frwd bias condition.

Now we know that the depletion region is formed of two polarities of atoms negative and positive. The positive atom or the donor exists on the N side and negative atom exists on the P side.
Now as we forwards bias the PN junction the free electrons on the N side will penetrate the depletion region and impart energy to the immobile positive atoms to release an electron, this way a hole will be created together with an electron. As holes don't really move the electron will move towards the P side whereas the hole will be left behind thus reducing the depletion region.

Same will happen on the P side, some of the incoming electrons will collide with the negative immobile atom and this will eject an electron from the atom thus creating a hole and electron simultaneously, thus reducing the junction width.

As we move towards the metal contact in P side the hole concentration will become constant whereas the excess electron concentration will also become constant after recombination as electron hole pairs are created in pairs.

Same applies for the reverse bias as a free electron on P side strikes the immobile negative atom a electron and hole will be generated simultaneously and this accounts for the decreased concentration of electrons and increased concentration of holes as electrons generated will move towards the N side leaving the hole.

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I can understand your confusion about the majority and minority concentrations in diodes. It is important to note that in a diode, the p-type region has an excess of holes and the n-type region has an excess of electrons. This is due to the doping process during the manufacturing of the diode.

When a reverse bias is applied, the majority carriers (holes in the p-type region and electrons in the n-type region) are pushed away from the junction towards the metal contact. This creates a depletion region near the junction where there is a lack of majority carriers. However, the concentration of majority carriers does not decrease to zero, as there are still some carriers present due to thermal generation.

Therefore, the concentration of majority carriers (pp0 holes) will increase as we move closer to the junction because the depletion region becomes narrower. This is because the electric field becomes stronger near the junction, causing the majority carriers to be pushed closer together.

I hope this explanation helps to clarify your doubts. It is important to remember that in a reverse biased diode, the majority carriers are still present, but they are pushed away from the junction. This is what allows the diode to function as a one-way valve for current flow.

## What is a majority concentration in diodes?

A majority concentration in diodes refers to the type of doping material used to create the semiconductor material. In an N-type semiconductor, the majority concentration is made up of negatively charged electrons, while in a P-type semiconductor, the majority concentration is made up of positively charged holes.

## What is a minority concentration in diodes?

A minority concentration in diodes refers to the type of doping material that is present in smaller quantities compared to the majority concentration. In an N-type semiconductor, the minority concentration is made up of positively charged holes, while in a P-type semiconductor, the minority concentration is made up of negatively charged electrons.

## How do majority and minority concentrations affect the behavior of diodes?

The majority and minority concentrations in diodes play a crucial role in determining the flow of current through the device. When a diode is forward biased (positive voltage applied to the P-type material and negative voltage applied to the N-type material), the majority carriers are able to flow freely across the junction, allowing current to pass through. However, when a diode is reverse biased (negative voltage applied to the P-type material and positive voltage applied to the N-type material), the majority carriers are unable to flow, and the minority carriers must overcome the potential barrier, resulting in a very small current flow.

## Can majority and minority concentrations be altered in diodes?

Yes, majority and minority concentrations can be altered in diodes through a process called doping. This involves adding impurities to the semiconductor material to change the concentration of majority and minority carriers. This allows for the creation of different types of diodes with varying properties and behaviors.

## What is the significance of majority and minority concentrations in diodes?

The majority and minority concentrations in diodes are crucial in determining the functionality and behavior of these electronic devices. They allow for the control of current flow and enable diodes to perform tasks such as rectification, switching, and voltage regulation. By altering the concentrations, different types of diodes can be created for various applications in electronics and technology.

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