P-Type Semiconductor

Semiconductors are those materials that possess the conductivity between conductors (generally metals) and non-conductors or insulators (such as ceramics). Types of semiconductor are Intrinsic and Extrinsic Semiconductor. The extrinsic P-Type Semiconductor is formed when we add trivalent impurity to a pure semiconductor in a small amount, and as a result of that, a large number of holes are created in it. A large number of holes are found in the semiconductor material by the addition of trivalent impurities like Gallium and Indium.

Here, we will discuss p type semiconductor and the types of charge on p type semiconductor. We will determine whether p type semiconductor is positive or negative.

What is a P-Type Semiconductor?

A P-Type semiconductor is an intrinsic semiconductor that is doped with boron (B) or indium (In). Silicon of Group IV element has four valence electrons and the boron of Group III element has three valence electrons. If a small amount of boron is doped to a single crystal of silicon, then the valence electrons will be insufficient at one position to bond silicon and boron, resulting in holes that lack electrons. When a voltage is applied in this state, the neighbouring electrons move to the hole. The place where an electron is present becomes a new hole, and the holes that appear to move to the "–" electrode are in sequence.

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Types of impurities that are produced by P-Type semiconductor are known as acceptor impurities because each atom of them create one hole which can accept one electron.

• Mainly due to holes

• Entirely neutral

• I = Ih and nh >> ne

The majority is the number of Holes and the Minority is the number of Electrons.

When a pure semiconductor is doped with a trivalent impurity like (B, Al, In, Ga ) then, the three valence electrons of the impurity bonds with three of the four valence electrons of the semiconductor. This leaves an absence of an electron (or hole) in the impurity. With the increase in the number of impurities, holes (the positive charge carriers) are also increased. Hence, it is called a P-Type semiconductor.

Crystal as a whole is neutral, but the acceptors present makes an immobile negative ion. As conduction is due to a large number of holes, the holes in the P-Type semiconductor are majority carriers and electrons are minority carriers.

Is P-Type Semiconductor Positive or Negative?

p−type semiconductors have a larger hole concentration as compared to electron concentration. In P-Type semiconductors, holes refer to majority carriers and electrons refer to minority carriers. p−type semiconductors are created by doping intrinsic semiconductor having acceptor impurities. A common p−type dopant used for silicon is boron or gallium. There will be no charge on the semiconductor. So charge on a P-Type semiconductor is neutral.

The number of free electrons is exactly equal to the total number of holes and positively charged ions in a semiconductor, whether intrinsic or doped, it is electrically neutral.

P-Type Material 

In a pure (intrinsic) like Si or Ge semiconductor, each nucleus uses its four valence electrons to form four covalent bonds with its neighbours (as shown in the below diagram). Each ionic core consists of the nucleus and non-valence electrons. It has a total charge of +4, and it is surrounded by 4 valence electrons. Since there are no excess electrons or holes. In this case, the number of electrons and holes present at any given point in time will always be equal.

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An intrinsic semiconductor. We should note that each +4 ion is surrounded by four electrons.

Now, if one of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, of Group 3 element like Boron (B) or Gallium (Ga), then the electron-hole balance will be changed. This impurity will only be able to contribute three valence electrons to the lattice, therefore leaving one excess hole (we can see figure below). As we know that holes will "accept" free electrons, so a Group 3 impurity is also called an acceptor.

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A semiconductor doped with an acceptor. An excess hole is now present.

As we know an acceptor donates excess holes, which are considered to be positively charged. A semiconductor when doped with an acceptor is called a P-Type semiconductor. Here, "p" stands for positive. We should keep in mind that the material as a whole remains electrically neutral. In a P-Type semiconductor, the current is largely carried by the holes, which exceed the free electrons. In this situation, the holes are the majority carriers, and the electrons are the minority carriers.

How P-Type Semiconductor Formed?

The Si semiconductor is a tetravalent element and the common structure of crystal includes 4 covalent bonds formed by the 4 outer electrons. In Si, generally doping is done with group III & V elements. Group III elements include 3 outer electrons that work like acceptors when we use dope Si.

Once an acceptor atom changes a tetravalent Si atom within the crystal then an electron-hole can be created. It is one kind of charge carrier that is used for generating electric current within the semiconducting materials.

The charge carriers in this semiconductor are positively charged and it moves from one atom to another within semiconducting materials. The trivalent elements that are added to an intrinsic semiconductor will create positive electron holes within the structure. For example, a-Si crystal doped with group III elements like boron will create a P-Type semiconductor. The whole no. of holes can be equal to the no. of donor sites (p ≈ NA). The majority charge carriers of this semiconductor are known as holes whereas minority charge carriers are known as electrons.

Energy Band Diagram of P-Type Semiconductor

Given below diagram represent the energy band diagram of a P-Type Semiconductor:

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A large number of holes or vacant space present in the covalent bond is created in the crystal by adding the trivalent impurity. A small or we can say a minute quantity of free electrons is also available in the conduction band.

They are produced when thermal energy at room temperature is given to the germanium crystal that forms electron-hole pairs. The holes are more in number as compared to the number of electrons in the conduction band. It is because of the predominance of holes over electrons then the material is called a P-Type semiconductor. Here, the word “p” stands for positive material.

Conduction Through P-Type Semiconductor

In a P-Type semiconductor, we can create a large number of holes by adding the trivalent impurity. When we apply a potential difference across this type of semiconductor the figure is shown  below:

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In p type semiconductor the holes are present in the valence band and they are directed towards the negative terminal. As the current flows through the crystal it passes through holes, which are carriers of positive charge. Therefore, this type of conductivity is known as P-Type conductivity. In a P-Type conductivity, the valence electrons move from one covalent bond to another.

The conductivity of an n-type semiconductor is nearly double that of p type semiconductor. The electrons available in the conduction band of the n type semiconductor are much more movable as compared to holes available in the valence band in a P-Type semiconductor.

The mobility of holes is poor as they are more bound to the nucleus.

Even at room temperature, the electron-hole pairs can be formed. These free electrons that are available in minute quantities also carry a little amount of current in the P-Type semiconductors.

In a P-Type semiconductor, the number of holes in the valence band is more, whereas the numbers of free electrons in the conduction band are less. So, current conduction is mainly because of holes in the valence band. Free electrons in the conduction band contain a little current. 

The P-Type semiconductor is formed when we add trivalent impurity to the pure semiconductor. Similarly, when a Pentavalent impurity is added to the pure semiconductor n-type semiconductor is obtained.

Conclusion

So, from the above discussion, we can conclude that a P-Type semiconductor is condensed due to the fact that these are positive semiconductors because of extra holes. The impurity present within the semiconductor accepts atoms with free electrons (dopants). This forms a ‘hole’ within the semiconductors. In a p type semiconductor, the majority of charge carriers move from higher potential to lower potential. The impurity added in this type of semiconductor has the ability to take an electron hence they are known as acceptor atoms.