WatElectronics.com

A crystal that has its conduction value in between conductor and insulator is termed as the semiconductor.It can be formed by the addition of impurities. It can be referred to as either p-type or n-type. Hence its conduction is based on the types and the amount of impurity added. These solids conduction value is proportional to the temperature value. As the temperature increases its conduction value increases as temperature degrades then the conduction of the semiconductor tends to decrease.

Semiconductor and its Types

These semiconductors are further classified into two basic types based on their purity and the doping value, For more detailed comparison between the two semiconductor types, refer to the article – Difference Between Intrinsic and Extrinsic Semiconductor

(1) Intrinsic Semiconductor

The semiconductor in its pure form is termed an intrinsic semiconductor. It is also related to its energy gaps for the silicon in its pure form the energy gap will be 1.1 electron volts and for germanium, it is 0.72 electron volts. In this type semiconductor at room temperature, the number of carriers and number of holes is equal to each other indicating the neutral condition.

(2) Extrinsic Semiconductor

Once the impurity is added to the semiconductor then its purity gets affected and also there is the increment in the charge carriers. These types of semiconductors are known as extrinsic semiconductors. Further, this is classified into two types’ p-type and n-type. In these types, the numbers of electrons are more in n-type whereas the number of holes in p-type.

What is a P-Type Semiconductor?

If the intrinsic semiconductor is doped with an electron acceptor in order to make it as a p-type semiconductor. The electron acceptor is responsible for the formation of a hole by accepting an electron from the lattice.

As a result, majority carriers in the p-type semiconductor formed are holes. In this way, a p-type semiconductor is defined based on its electron acceptor capability.

Formation of P-type Semiconductor

In order to form a p-type semiconductor the basic step is to dope intrinsic semiconductor with trivalent impurity. In this type, the valence shell consists of three electrons requires further one more electron.

This is possible by sharing the electron. As it is accepting electrons it is generally referred to as acceptor. The acceptor impurities are Boron, indium, gallium. Once these are added to either silicon or germanium p-type semiconductors are formed.

Let us take boron as the trivalent impurity so that we can add it to the silicon in order to make it extrinsic. Everything here is completely based on the concept of sharing.

The covalent bond concept is preferred during accepting the electrons. Boron atomic number is five. Based on its valence shell electronic configuration concept the numbers of valence electrons present in boron are three.

Hence each valence electron forms a covalent bond with the neighboring silicon atoms. The number of bonds formed with a number of silicon atoms is based on the number of valence electrons present.

It is evident that to boron there are four neighbouring silicon atoms as per that there must be four electrons but there are only three valence electrons present. Then the absence of fourth electron or the vacancy of the electron is termed as a hole.

It indicates that one electron is accepted by the boron atom. The vacancy or the need of electron can be fulfilled. The number of vacancies and the acceptance of electrons are proportional to each other.

Now it is clear that boron comes under trivalent impurity because it is responsible for the formation of holes in the semiconductor. Therefore doping pure form of silicon with boron leads to the formation of p-type semiconductor.

Example of P-Type Semiconductor

Trivalent impurities such as boron or gallium are commonly used in silicon as doping impurity. Then silicon doped with boron or gallium is a perfect example for a p-type semiconductor.

Whether silicon is doped with gallium or indium the process is also can be represented by utilizing the same concept of boron and silicon.

In this way, the semiconductors that are rich in holes as there carriers formed by the trivalent impurities comes under the list of p-type semiconductors.

Whether silicon or germanium if they are added with a trivalent impurity that belongs to p-type of the semiconductor.

Conduction in P-Type Semiconductor

When the external supply of voltage is given to the p-type semiconductor there majority carriers present in valence band tends to move towards the negative terminal of the supply and the minority carriers that are electrons present in the conduction band move towards the positive terminal.

However, the concentration of electrons is less in the conduction band and the majority of holes are present in the valence band.

Hence the current in the p-type is because of majority carriers in valence band a little amount of current is in conduction band because of few electrons that are their minority carriers.

Energy Diagram of P-Type Semiconductor

As it is doped with trivalent impurity there are a huge number of holes formed in the p-type. Hence it has a majority concentration of holes and minority concentration of electrons in it.

P-type because of majority a-holes it referred to as a positive type. As it is a p-type semiconductor the Fermi level is present near to the valence band rather than conduction band.

Once the impurity is inserted in the pure semiconductor numerous amounts of holes are formed in the valence band. There are thermal excitation’s also in the semiconductors because of this same amount of electrons are also present in the conduction band.

As always majority wins, more amount of hole compared to electrons makes p- types majority carriers as holes.

As the energy band diagram suggests there is less number of electrons in conduction band in p-type compare to that of n-type making the state of conductivity of n-type double than that of p-type.

The reason behind this is the excitation’s will release a tremendous amount of energy and also enhances the flow of current making to mold it purposefully based on requirements.

As the doping concept introduced in extrinsic and made it p-type and n-type. The formation of these types made changes in the development of modern electronics.

For example, it can be LASER, LED or a BJT everything is interlinked to each other. In what ways do you think p-type is preferred over n-type?

When a p-type and n-type interfaced together that leads to the formation of a PN Junction. The commonly suggested semiconductor for this p-n junction is either silicon or germanium based on its preferred electrical properties and its abundance. Basically the p-type and n-type of a semiconductor is formed due to doping impurities.

There is an interesting concept evolved from it clearly states that both the p and n regions are conducting until the formation of a barrier. Once it has been formed the p and n regions become neutral in terms of conduction. This evolving concept has been utilized for the development of modern electronics.

The Theory Behind The Formation of PN Junction

Based on the required conditions one can easily say that there are two types of materials involved in a single crystal to interface in such a way that PN Junction has formed. P-type is formed because of trivalent doping impurity.

These impurities accept free electrons and they become negatively charged ions. Similarly n-type is formed by doping it with pentavalent impurity and donates the free electrons in order to become positively charged ions.

Rather than using the same material if we use different semiconductors for the formation of the PN Junction there will be a phenomenon of scattering that inhibits the flow of charge carriers. Because of this reason single crystal with doping concept is preferred.

Charge Representation of P-Type and N-Type

As the p and n-type semiconductor have been interfaced or fused together in such a way that at the center there must be action between majorities of the charge carriers that tend to the formation of the junction in between them.

Now, there are charges in both p and n-types. P-type consists of holes as majority charge carriers and n-type have electrons as the majority. When these types are in contact with each other there majority carriers tend to move and the process of diffusion occurs. This is only possible when n-type and p-type both are formed on a single crystal.

As the charges flow from higher concentration to lower concentration and the process impacts in such a way that some charge has been imposed at the center on the impure ions. These carriers form a layer that is static and adjacent towards the junction. Then this static layer becomes so strong in terms of electric charge that towards n-type electrons lied statically while p-type has holes for that purpose.

However, the concentration will be on the major part but there will be some minority carriers in each region showing the impact of their presence. As p-type has minority carriers as electrons it tends to move and crosses the PN Junction and gets combined in n-type region. A similar concept is also applied for n-type minority carriers. As a result, the regions tend to attain the condition of neutrality.

This makes the junction to attain a state of equilibrium. In this state, the donor tends to oppose holes and the acceptors repel the electrons. This phenomenon leads to the formation of a zone called “potential barrier”. The barrier doesn’t have any free carriers left it means that all the free electrons are combined by respective holes in it.

Interaction of N-Type and P-Type

As a result, there must not be any free carriers across both p and n-types junction. Then both p and n regions are now termed as neutral regions. In other words, these free carriers are depleted. Hence the region of the p-n junction is termed as Depletion region and the charge will be neutral on both sides.

In the formation of depletion layer n-type loosen its free electrons while p-type has lost free holes. Both the sides have some ions and its nature is impure. This results in the existence of an electric field in it. The problem is, in order to overcome the barrier it requires some extra charge. Hence the electric field generated due to the process of diffusion without any application of power supply is can be given as

Here ND represents the donor impurity concentration; NA represents the acceptor’s impurity concentration. Where ni refers to the intrinsic concentration.

Once the barrier is formed at the center there is the existence of the other two processes. Firstly, the concentration of the carriers is different and when the holes tend to diffuse from p-type to n-type there some current has been generated at the junction termed as diffusion current. Secondly, because of this depletion, there is some movement in minority carriers in such a way that electric field is imposed because of this electron from p-side tends to flow towards n-side resulting in drift current.

Diffusion current is possible only by the movement of majority carriers whereas drift is possible by the influence of minority carriers. After the formation of a barrier if charges require movement then it is possible by applying it with suitable voltages. This concept is termed as biasing. Without any power supply, there is the presence of some potential in the semiconductor based on its relevance.

If it is silicon then it has 0.6-0.7 volts as the basic potential across the depletion region. In case germanium is used then it has 0.3-0.35 volts. As it is referred to potential barrier it opposes the flow of charge carriers on both sides of the junction. The PN Junction barrier is formed during the process of manufacturing itself compared to germanium silicon has the highest barrier potential. This potential is highly sensitive to the variations in the temperature, the element or material utilized in its fabrication as well as the concentration of doping required based on the type and the factors.

The most major factor of inbuilt potential inside the interfaced PN Junction is responsible for repelling the flow of electrons and holes once the barrier has formed. The flow of charged particle in such interfacing will always be in a unified direction. This is a huge application

These PN Junction devices are considered as the fundamental units for any type of transistors, a few modifications in it can be used for various purposes based on requirements. Now, these are the basics that made p-n junction more popular in all the aspects termed for modern electronics. Have you ever thought of how and when these types of p or n-types are replaced and utilized in any other respective equipment?

John Dalton in 1800 was the one who explained regarding the law on multiple proportions. Atom represents the fundamental units that can be divided into subatomic particles. This leads to the development of atomic models. In 1898 Thomson proposed a model by stating that mass of the atom is equally distributed or spread over it but Rutherford’s scattering experiment demolishes the statement in 1909.

Hence the existence of the nucleus with some electric charge at the center has been estimated along with the revolving of electrons around it. Further Bohr’s model unable to explain the structure of atom that consists of multiple electrons and it disobeys Heisenberg uncertainty principle. Later in 1926 Schrodinger’s equation has been proposed. As per the model based on quantum, it’s clear that numbers of electrons are arranged in the respective shells. Atom size measured in terms of angstrom that is 10^-10 meter.

What Structure Does Atom Consists?

Each atom has its nucleus and the electrons bound to it. Protons and neutrons present inside the nucleus are known as nucleons. Mostly the nucleus is responsible as the major constituent for the mass of the atom. The structure of the atom mainly surrounds around electron, proton, and neutron. Atom’s behavior is relevant to the orbital in which electrons are revolving and the chemical properties are determined by the shells.

The General Structure of Atom

An electron is negatively charged, a proton is positive in charge whereas a neutron is neutral. If the atom has equally balanced electrons and protons in that case atom become neutral otherwise it leads to the formation of an ion. Based on the highest content of electrons and protons one can describe either the charge of the atom is positive or negative.

Electromagnetic force binds electrons and nucleus.  Protons and neutrons consist of nuclear force to attract each other inside the nucleus. The presence of a number of protons indicates the atomic number. Similarly, a number of neutrons describe the isotopes. Magnetic properties of the element can relate to the presence of the number of electrons in it. Atoms interaction and distraction towards each other leads to the physical changes in the environment. This can be done by chemical bonding between them.

There exists a quantum state in which proton must be present in the nucleus but it should differ for every proton that is it follows Pauli’s exclusion principle. Therefore protons, neutrons , and electrons are categorized under fermions. As the charge has been classified neutrons compared to protons are heavier in terms of mass. Electrons have a mass of 9.109382911*10^-28 gram. Proton or neutron comparison with the electron is 1,836 times heavier.

As the electron tends to rotate in order to obtain stability it jumps from one orbital to another one this makes the behavior of the atomic spectra so unpredictable. This technique is known as a quantum leap. When it occurs from a higher level to the next lower level there is an emission of radiation in terms of photons.

Properties of Atom:

Based on the number of protons, electrons and neutrons atom properties can be described:

(1) Atomic number:

Presence of the number of protons in the nucleus derives the atomic number it relates to the chemical properties of an atom. The symbol used for the representation of an atomic number is Z. it is clearly stated that atomic number is influenced by the positively charged particles. If the elements consist of the equal number of protons and electrons results in a neutral atom. Carbon is one of the neutral atoms.

Example: Z = 6, represents carbon.

(2) Atomic mass:

Mostly the weight or mass of the atom is dependent on its nucleus. Nucleus weight is dependent on the sum of the masses of proton and neutrons. This type of mass is known as atomic mass. The symbol A denotes atomic mass. If the neutrons and protons are different in number then it is termed as Isotopes.

Example:

Hydrogen is the best suitable example that can relate to this condition. Based on neutrons it is classified as

(a) Protium: It consists of 99.985% of the hydrogen atom with only one proton in it.

 

(b) Deuterium: It has one proton and one neutron.

 

(c) Tritium: It possesses two neutrons and a proton it. Naturally, this condition is not possible one has to make it based on requirement.

 

 

(3) Radioactive:

The variations in the number of protons and neutrons lead to the changing in a nucleus. Then the respective atom is known as radioactive. This condition is nothing but an unstable nucleus. At this stage, the atoms will be in the same state until it becomes stable. The elements that have an atomic number more than the limit of 82 are said to be radioactive.

Example: Lead

(4) The spin of electrons:

There is a property regarding spinning of electrons as it comes under fermions it takes only half of the integer spin. Example helium is a chemical element based on its number of protons its atomic number is two. In order to occupy in the same orbit, it should be spin in different directions.  This results in the movement of electric charge so that they are considered as a magnet in tiny shape. Electrons do have a magnetic moment and it is almost 9.28*10^-24 joule per tesla.

(5) Electric charge:

Atoms that involved in the movement of electron and in the formation of bonds in order to achieve stability results in the formation of an ion. In this process the atom that is a giver becomes positive and the one who takes it becomes negative.

(6) Relative atomic mass:

As the numbers of protons are equivalent to the numbers of neutrons in the carbon making it a neutral atom the relativity in atomic mass is comparatively average per mass of the one carbon atom.

Summary:

This article explains the basics of atoms and the way its structure has been classified based on the concept of orbital, basing on subatomic particles chemical properties of atoms can be explained. Mainly its objective is to give the basic structure details and deal with the basic properties. The way electron, proton, neutron lies in the atom and in the way its masses are related and the charges of the fermions have been discussed. There is a strong influence of charge particles in the atom. Based on the charges the classification of ions and the effect of increase and decrease of subatomic particles results have been overviewed. The energies associated in between nucleus and electrons have been discussed.

Hence the atoms are the fundamental units of any basic structure its classification on subatomic particles has been helped in figuring out the presence of various chemical elements in the periodic table. Based on the classification we can deal with the properties of basic atoms. Generally we use lead pencils in our day to day activities have you ever get a thought of it how the leads are joined and made as a solid pencil?

Semiconductors are materials which exhibit properties of both conductors and insulators. The semiconductors are of two types, Intrinsic and Extrinsic Semiconductors. The intrinsic semiconductors are the one with no doping and therefore called as pure semiconductors. The extrinsic conductors are doped with impurity atoms. Based on the type of impurity added they are classified as: N-type and P-type Semiconductors.

What is an N-type Semiconductor?

A N-type semiconductor is defined as a type of extrinsic semiconductor doped with a pentavalent impurity element which has five electrons in its valence shell. The pentavalent impurity or dopant elements are added in the N-type semiconductor to increase the number of electrons for conduction.

Doping in N-type Semiconductor

The N-type semiconductor is doped with pentavalent impurity elements. The pentavalent elements have five electrons in the valence shell. The examples of pentavalent impurities are Phosphorus (P), Arsenic (As), Antimony (Sb). The pentavalent impurity is added in a very minute fraction in the N-type semiconductor such that the crystal structure of the original intrinsic semiconductor is not disturbed. The pentavalent impurity atom makes covalent bonds with four silicon atoms and one electron is not bonded with any silicon atom. Each pentavalent impurity atom donates one electron to the N-type semiconductor hence it is called as a Donor impurities. Thus, there are more number of electrons in the N-type semiconductor.

N-type Semiconductor Example

An intrinsic semiconductor material like Silicon (Si) has 14 electrons with a configuration of 2,8,4 and Germanium (Ge) has 32 electrons with a configuration of 2,8,18,4. Each atom requires 8 electrons in its valence shell to be stable. Hence intrinsic semiconductor atoms have covalent bonds based on sharing the electrons of a nearby atom to achieve 8 electrons to balance their atomic structure.

 

A N-type semiconductor is created by doping this pure silicon crystal lattice with a pentavalent impurity element like Antimony (Sb). In an N-type semiconductor the atom of pentavalent impurity element Antimony (Sb) is in between silicon atoms. The Silicon atoms have four electrons in the valence shell. Each of the silicon atom creates a covalent bond with an electron of the prevalent impurity atom.

The Antimony (Sb) impurity element electron form covalent bonds with only four silicon atoms. The fifth electron of the impurity atom is not bonded with any semiconductor atom in the crystal lattice. This electron is loosely bonded to its parent impurity atom. Thus, as external voltage or heat is applied this fifth electrons easily breaks its bond with the parent atom and takes part in conduction.This fifth electron majorly contributes to the current in an N-type semiconductor.In the N-type Semiconductor the electrons become the majority carrier.

Energy Diagram of n-Type Semiconductor

 

The Energy diagram represents two energy bands, the valence band and the conduction band. The electrons in the valence band in the energy diagram represent the electrons which are in the valence band of the atom and they are still bonded to the parent atom. The electrons in the conduction band in the energy diagram represent atoms which take part in conduction. The energy gap between the valence and conduction band is called as the forbidden band or band gap.

In an N-type semiconductor because of the pentavalent impurity a number of loosely bonded electrons are available in the lattice structure. As the voltage is applied, these electrons break free from the covalent bonds andare ready to conduct. These electrons are depicted in the conduction band.When a certain amount of voltage is applied, these electrons gain energy to cross the forbidden gap and leave the valence bandto enter into the conduction band. A very less number of holes are formed in the valence band as the electron leaves valence band to enter conduction band. The Fermi level is near the conduction band as more number of electrons enter the conduction band.

Conduction through N-type Semiconductor:

The conduction through an N-type semiconductor is majorly caused by the electrons. The pentavalent donor impurity has imparted extra electrons to the lattice structure. As a voltage is applied or the semiconductor is subject to external heat the electrons gain energy. The electrons break covalent bonds and more electrons are released into the conduction band. The electron which breaks away from its covalent bond, leaves a void space or hole in its place.

As the negatively charged electron leaves a hole, this empty space attracts other electrons. Hence the hole is considered to be positively charged. Thus the N-type conductor has two types of carriers,negatively charged electrons and positively charged holes. In an N-type semiconductor the electrons are greater in number and hence they are termed as the majority carriers and the holes are termed as minority carriers as they are less in number. The current constituted in an N-type semiconductor by electrons is called a majority carrier current and the current constituted by holes is called the minority charge current.

When a covalent bonds break and the electrons leaves a hole in its place, some other electron breaks away from its covalent bond and gets attracted towards this hole. Thus the electron and holes move in opposite directions. The electrons get attracted towards positive terminal of the battery and holes are attracted towards negative terminal of the battery. As the electrons and holes travel through the lattice current starts following in the N-type semiconductor.

The N-type extrinsic semiconductor has majority carrier as electrons and hence has greater conductivity. Because of this reason the N-type Semiconductor in combination with a P-type semiconductor is used to manufacture all the major semiconductor components and devices. The basic components like PN Diode, Bipolar Junction Transistor and Field effect Transistors have their working based on the properties and characteristics of N-type semiconductor.

A conductor has a number of charge carriers, which are ready to take part in conduction after a certain voltage or heat is applied. Non-conductors do not have any free charges to take part in conduction even after applying an external voltage or heat. The materials which exhibit properties of both conductors and non-conductors are called the semiconductors. The Semiconductor materials behave as an insulator for a certain voltage level and conduct only after the specific voltage level is applied at the input. The commonly available semiconductor materials are Silicon (Si) and Germanium (Ge). The compound semiconductors are prepared by alloying different elements, one of the examples is Gallium Arsenide (GaAs).

There are two types of semiconductors namely intrinsic and extrinsic semiconductors. The classification is based on the type and concentration of carriers that majorly contribute to the flow of current these types of semiconductors.

Silicon and Germanium are tetravalent elements. It means that they have four electrons in its outermost shell. Each atom of Silicon forms a covalent bond with four neighboring Silicon atoms. Thus, with the help of sharing of atoms the lattice crystal structure of the semiconductor is formed.

Intrinsic Semiconductor :

An intrinsic semiconductor is the purest form of the semiconductor. Doping element or impurity is not added to the intrinsic semiconductor. The electrons are bonded to the parent semiconductor atom, but at a certain voltage or heat is applied, these valence electrons, they leave the parent atom are moved freely in the lattice. Such free electrons constitute current in the intrinsic semiconductor material. The electron from the valence band crosses the forbidden gap to enter into the conduction band. This electron leaves a positive hole or void in its place in the valence band. This void or positive space is termed a ‘hole’.

Extrinsic Semiconductor:

An extrinsic semiconductor is formed by doping an intrinsic semiconductor. Doping is a process where a very small fraction of impurity atom is added to the intrinsic semiconductor. The extrinsic semiconductors are of two types based on the doping elements used.

Types of Extrinsic Semiconductor are N-type and P-type.

• N-type Semiconductor : The intrinsic semiconductor is doped with Pentavalent impurity element. Such an impurity element has five electrons in the valence shell. The elements like are Phosphorus (P), Arsenic (As), Antimony (Sb) is the pentavalent impurities used for N-type semiconductor.
• P-type Semiconductor: The intrinsic semiconductor is doped with the Trivalent impurity element. Such an impurity element has five electrons in the valence shell. The elements like Boron (B), Gallium (G), Indium (In), Aluminium (Al) is used for P-type semiconductors.

Key Differences between Intrinsic and Extrinsic Semiconductor

The intrinsic and extrinsic semiconductors can be differentiated on the following parameters:

1) Crystal Lattice Structure:

The schematic of an intrinsic Silicon (Si) semiconductor is shown in Figure 1 which depicts the covalent bonds in neighboring Silicon atoms.

Each Silicon atom shares its four electrons with four neighboring silicon atoms to form covalent bonds.

 

Figure 3. P-type semiconductor with acceptor impurity

The N-type Extrinsic semiconductor in Figure 2 shows a pentavalent impurity atom of Antimony (Sb) along with the free electron which is freely roaming in the crystal structure and it is ready to conduct. The P-type Extrinsic semiconductor in Figure 3 shows a trivalent impurity atom of Boron (B) along with a void space formed in the covalent bond with a neighboring silicon atom. This hole attracts electrons and participates in conduction.

2) Doping Level:

Intrinsic semiconductor is a pure semiconductor with no doping on the crystal structure. There is an equal number of holes and electrons in an intrinsic material. This is termed as electrical neutrality.

An external semiconductor is a doped intrinsic semiconductor. An N-type semiconductor is doped with a pentavalent impurity and a P-type semiconductor is doped with a Trivalent impurity. The pentavalent impurities are called as Donor impurities as they give an extra electron to the lattice of the semiconductor. The Trivalent impurities are termed as Acceptor impurities as they create a void or positive hole in the crystal structure which can accept an electron.

3) Carrier Concentration:

In intrinsic semiconductors, there is an equal number of holes and electron concentration as no doping is added. Intrinsic conductors have lower conductivity compared to the extrinsic semiconductor.

In an N-type semiconductor, electrons are called the majority carriers as they are more in number and holes are termed as minority carriers. The conduction in an N-type of semiconductor majorly results from the electrons which are majority carriers.

In a P-type semiconductor, holes are called the majority carriers as they are more in number and electrons are termed as minority carriers. In a P-type of semiconductor conduction results primarily because of the holes which are majority carriers.

4) Fermi Level:

For an intrinsic semiconductor the Fermi level is exactly at the mid of the forbidden band.energy band gap for Silicon (Ga) is 1.6V, Germanium (Ge) is 0.66V, Gallium Arsenide (GaAs) 1.424V. In an extrinsic semiconductor

For n-type and p-type extrinsic semiconductors the Fermi levels are a function of doping level density.

In an N-type semiconductor, the Fermi level is near the conduction band as it has more electrons. In a P-type semiconductor, the Fermi level is near valance band as it has more of the holes.

5) Effect of temperature:

In an intrinsic semiconductor at there are no electrons in the conduction band. As the temperature reaches room temperature of the electrons gain energy to cross the valence band and reach the conduction band.

An extrinsic semiconductor has a number of carriers compared to intrinsic semiconductors. Increase in temperature will increase the conductivity of extrinsic semiconductors as more number of carriers are released for conduction.

6) Uses:

Although the intrinsic semiconductor is a pure semiconductor it is not used for practical manufacturing as has low conductivity. The number of free charge carriers is less hence it has higher resistance to conduction of charges.

Whereas an extrinsic semiconductor has greater conductivity as it has a number of free charge carriers. Hence external semiconductors are preferred for practical manufacturing of semiconductor components and devices.

Semiconductor, as the name suggests is a kind of material whose shows properties of both conductors and insulators. A semiconductor material requires a certain level of voltage or heat to release its carriers for conduction. These semiconductors are classified as ‘intrinsic’ and ‘extrinsic’ based on the number of carriers. The intrinsic carrier is the purest form of semiconductor and an equal number of electrons (negative charge carriers) and holes (positive charge carriers). The semiconductor materials most profoundly used are Silicon (Si), Germanium (Ge), and Gallium Arsenide (GaAs). Let us study the characteristics and behavior of these types of semiconductors.

What is an Intrinsic Semiconductor?

The Intrinsic semiconductor can be defined as chemically pure material without any doping or impurity added to it. The most commonly known intrinsic or pure semiconductors available are Silicon (Si) and Germanium (Ge). The behavior of the semiconductor on applying a certain voltage is dependent on its atomic structure. The outermost shell of both Silicon and Germanium have four electrons each. To stabilize each other nearby atoms form covalent bonds based on the sharing of valence electrons. This bonding in the crystal lattice structure of Silicon is illustrated in figure 1. Here it can be seen that the valence electrons of two Si atom pair together to form a Covalent Bond.

At all the covalent bonds are stable and no carriers are available for conduction. Here the intrinsic semiconductor behaves as an insulator or non-conductor. Now if the ambient temperature comes close to the room temperature of the covalent bonds start breaking. Thus the electrons from the valence shell are released to take part in conduction. As more number of carriers are released for conduction the semiconductor starts behaving as a conducting material. The energy band diagram given below explains this transition of carriers from the valence band to the conduction band.

The Energy band diagram

The Energy band diagram shown in figure 2(a) depicts two levels, Conduction Band and Valence Band. The space between the two bands is called the forbidden gap

 

 

When a semiconductor material is subjected to heat or applied voltage few of the covalent bonds break, which generates free electrons as shown in figure 2 (b). These free electrons get excited and gain energy to overcome the forbidden gap and enter the conduction band from the valence band. As the electron leaves valence band, it leaves behind a hole in the valence band. In an intrinsic semiconductor always an equal number of electrons and holes will be created and hence it exhibits electrical neutrality. Both the electrons and holes are responsible for conduction of current in the intrinsic semiconductor.

What is an Extrinsic Semiconductor?

The extrinsic semiconductor is defined as the material with an added impurity or doped semiconductor. Doping is the process of deliberately adding impurities to increase the number of carriers. The impurity elements used are termed as dopants. As the number of electrons and holes is greater in extrinsic conductor it exhibits greater conductivity than intrinsic semiconductors. Based on the dopants used the extrinsic semiconductors are further classified as ‘N-type semiconductor’ and ‘P-type semiconductor’.

N-type Semiconductors:

The N-type semiconductors are doped with pentavalent impurities. The pentavalent elements are called so as they have 5 electrons in their valence shell. The examples of pentavalent impurity are Phosphorus (P), Arsenic (As), Antimony (Sb). As depicted in figure 3, the dopant atom establishes covalent bonds by sharing four of its valence electrons with four neighboring silicon atoms. The fifth electron remains loosely bound to the nucleus of the dopant atom. Very less ionization energy is required to set free the fifth electron so that it leaves valence band and enters the conduction band. The pentavalent impurity imparts one extra electron to the lattice structure and hence it is called as the Donor impurity.

P-type Semiconductors:

P-type semiconductors are doped with the trivalent semiconductor. The trivalent impurities have 3 electrons in their valence shell. The examples of trivalent impurities include Boron (B), Gallium (G), Indium (In), Aluminium (Al). As depicted in figure 4, the dopant atom establishes covalent bonds with only three neighboring silicon atoms and a hole or vacancy is generated in the bond with the fourth silicon atom. The hole acts as a positive carrier or space for the electron to occupy. Thus the trivalent impurity has imparted a positive vacancy or hole which can readily accept electrons and hence it is called an Acceptor impurity.

 

Carrier Concentration in Intrinsic Semiconductor

The intrinsic carrier concentration is defined as the number of electrons per unit volume in the conduction band or the number of holes per unit volume in the valence band. Due to the applied voltage, the electron leaves the valence band and creates a positive hole in its place. This electron further enters the conduction band and takes part in the conduction of current. In an intrinsic semiconductor, the electrons generated in the conduction band is equal to the number of holes in the valence band. Therefore the electron concentration (n) is equal to the hole concentration (p) in an intrinsic semiconductor.

Intrinsic carrier concentration can be given as:

n_i=n=p
Where,n_i : intrinsic carrier concentration
n : electron-carrier concentration
p : hole-carrier concentration

Conductivity of Intrinsic Semiconductor

As the intrinsic semiconductor is subjected to heat or applied voltage the electrons travel from valence band to conduction band and leave a positive hole or vacancy in the valence band. Again these holes are filled by other electrons as more covalent bonds are broken. Thus the electrons and holes travel in the opposite direction and the intrinsic semiconductor starts conducting. The conductivity increases when a number of covalent bonds are broken thereby more electrons are holes are released for conduction. The conductivity of an intrinsic semiconductor is expressed in the terms of mobility and concentration of the charge carriers.

The expression for the conductivity of an intrinsic semiconductor is given expressed as:

σ_i=n_i e(μ_e+μ_h)
Where σ_i: conductivity of an intrinsic semiconductor
n_i : intrinsic carrier concentration
μ_e: mobility of electrons
μ_h: mobility of holes