Classification of Solids into Conductors, Semiconductors and Insulator

Added: 2026-06-01

[00:00:08]Hello everyone, welcome to my channel physics for engineers.

[00:00:14]I am Dr. Thulra Burabelli, associate professor and head department of basic sciences school of sciences and humanities SR University Wangal Telangana India.

[00:00:29]Today we will be discussing about the classification of solid materials into three categories conductors, semiconductors and insulators.

[00:00:46]Particularly we will be focusing on the properties electrical properties and electronic band structure of these materials.

[00:00:59]And then we will dis uh discuss about the semiconductors and their classification.

[00:01:10]Once again I welcome you to the course.

[00:01:17]Have a wonderful learning journey with physics for engineers.

[00:01:31]Now that we have introduced the semiconductors, let us first understand how materials are classified based on their electrical behavior.

[00:01:46]In nature, the materials can generally be divided into three groups.

[00:01:55]conductors, semiconductors and insulators.

[00:02:00]The conductors are the materials through which the electricity can flow very easily.

[00:02:10]I can give some examples like the conventional copper, aluminum, silver and gold.

[00:02:20]These are the metals which can process the high electrical conductivity.

[00:02:28]In this materials, the electrons can move freely when the electric field is applied.

[00:02:37]This is why the copper is commonly used in electric wiring.

[00:02:45]At the other extreme we have insulators examples glass, rubber, plastic, ceramics and dry wood.

[00:03:07]So in this materials the electrons are tightly bound to the atoms and they cannot move freely. So the electrical conductivity is very difficult in insulating materials.

[00:03:27]Therefore the current flow is extremely limited in insulators.

[00:03:35]Between these two categories lies semiconductors.

[00:03:41]Semiconductors conductivity is between the conductors and insulators.

[00:03:52]It is higher than the insulators and lower than the conductors.

[00:04:02]The most common semiconducting materials are silicon and germanmanium which are available freely in the nature.

[00:04:13]So that these two semiconductors are also known as intrinsic semiconductors.

[00:04:23]What makes the semiconductor special is that the conductivity can be controlled by changing temperature or adding impurities or by applying external electric field.

[00:04:45]We can control the conductivity of this semiconductors.

[00:04:51]we can increase or decrease their conductivity.

[00:04:56]The controllable conductivity is the reason why the semiconductors became the foundation of the modern electronics.

[00:05:11]Every smartphone, laptop, computer like computer processors or chips, memory chips, LEDs, solar cells and communication devices that relies on these semiconductor materials.

[00:05:33]From the energy band pers perspective, the materials again classified as the overlapping of valency band and conduction band refers to the conductors.

[00:05:53]Whereas in insulators the band gap is very large.

[00:06:00]The conduction band and valency band are separated with a large forbidden gap.

[00:06:10]Whereas in semiconductors the small energy band gap is approximately one electron volt.

[00:06:21]The small energy gap allows electrons to move into the conduction band under suitable conditions.

[00:06:36]Think of a conductor as an open highway where vehicles move freely.

[00:06:46]An insulator is like a road blocked with a huge barriers.

[00:06:53]A semiconductor is a controlled deck where traffic can be regulated according to our requirements.

[00:07:08]In the next you can see the band gap structure in the diagram below.

[00:07:19]You can see here the conductors and semiconductors and insulators.

[00:07:30]In conductors the energy band gap I mean the conduction band and valency band are wl whereas in insulators there is a huge band gap it is about 5 electron volts whereas in the semiconductors the energy gap or forbidden gap is lies between insulators and conductors.

[00:08:03]You can see how the electrons are exiting from the valency band to conduction band and they become free.

[00:08:15]When the electron is exited from the valency band to conduction band, that creates vacant positions in the valency band.

[00:08:28]Those vacant positions are referred as holes.

[00:08:35]So finally the conduction band is fully occupied with electrons and the valency band is occupied with holes.

[00:08:50]So in the next slide we will discuss about the concept of energy band diagrams.

[00:09:05]To understand why conductors, semiconductors and insulators behave differently, we need to understand the energy band theory.

[00:09:16]When atoms cast together to form a solid, the individual energy levels combine to form energy band.

[00:09:30]Two important bonds are valency band and conduction band.

[00:09:37]The valency band contains electrons involved in bonding.

[00:09:44]The conduction band contains electrons that are free to move and contributes to the electric conduction.

[00:09:58]The separation between these two is called energy band gap or forbidden energy gap in the solid materials.

[00:10:12]Let us compare the three types of materials as shown in the diagram below.

[00:10:23]First let us discuss about the conductors where either the conduction band overlaps the valency band or the conduction band is partially filled.

[00:10:41]Therefore the electrons can move easily with very little energy input.

[00:10:50]So the electric electron transportation takes place easily so that it results a good electrical conductivity.

[00:11:02]So this explains why metals conduct electricity efficiently.

[00:11:11]Coming to the next part. In the semiconductors, the semiconductors process a small band gap. Usually it is less than 2 electron volts.

[00:11:28]Because the cap gap is small, thermal energy available at room temperature can uh excite some of the electrons into the conduction band.

[00:11:46]Therefore, these excited electrons can contribute to the current flow.

[00:11:55]Coming to the insulators, the insulators have a large band gap greater than 4 electron volts.

[00:12:16]The room temperature thermal energy is not sufficient to exite many electrons across this energy gap.

[00:12:29]As a result, the electrical conductivity remains very low.

[00:12:37]This simple difference in band gap determines the electrical behavior of materials.

[00:12:50]Engineers exploit this principle extensively.

[00:12:58]For example, LED colors depends on the semiconductor band gap.

[00:13:05]Solar cell conversion converts the sunlight into the electricity by exciting electrons across the band gap.

[00:13:17]So the rate of conversion of solar energy into electricity by the solar cell is also depends on the band gap of the semiconductor.

[00:13:31]Similarly, the transistors operate by controlling electrons movement between the these two energy bands.

[00:13:45]Therefore, we can say that the energy band theory is one of the most important concept in semiconductor physics and electronics.

[00:14:04]So in this slide we will discuss about the concept of electrical conductivity.

[00:14:13]So how the electrons transport from the valance band to conduction band and how the electricity or electric transport takes place.

[00:14:28]The conductivity tells us about how easily electric current flow through a material.

[00:14:39]Different materials exhibit enominously different conductivity values. Let us say if you take a conductor the conductivity is in the order of 10^ 6 ohms per per cm.

[00:15:03]The same case if you take the insulator it is less than 10^ -6 ohms per cm.

[00:15:18]So for metals the conductivity can reach values around higher level whereas the insulators are very small.

[00:15:36]So the semiconductors occupies the middle region between these two extremes. It is between conductors and insulators.

[00:15:52]But what determines the conductivity? How we can find the conductivity?

[00:16:02]You can see in the diagram you can see the flow of electrons. How the electrons are transporting in a material.

[00:16:16]The conductivity depends primarily on two factors.

[00:16:22]One is the carrier density and the second one is mobility of charge carriers.

[00:16:35]The carrier density refers to the number of charge carriers available in the conductor for conduction.

[00:16:47]The second carrier mobility.

[00:16:53]The carrier mobility measures how easily charge carriers move through a material when an electric field is applied.

[00:17:06]The availability of charge carriers is important.

[00:17:10]At the same time, the mobility of charge carriers also very very important. These two factors affects the electrical conductivity and we can determine the conductivity using these two factors.

[00:17:25]Mathematically the conductivity depends on both carrier concentration and mobility of charge carriers.

[00:17:37]Even if many carriers are present but the mobility is very poor.

[00:17:50]So which results a reduction in electrical conductivity.

[00:17:57]Similar way the excellent mobility is not useful if there are very few carriers available.

[00:18:10]Both the things are very much needful and equally important.

[00:18:19]This is why the semiconductors engineering focuses on controlling both carrier concentration and mobility of charge carriers.

[00:18:36]For example, doping increases the carrier density.

[00:18:45]Material purification improves mobility.

[00:18:51]So the modern semiconductor industries spend billions of dollars in optimizing these properties to produce the faster computer processes and more efficient electronic devices.

[00:19:14]Understanding the conductivity is crucial because almost every electronic component is designed by controlling carrier density and mobility of charge carriers.

[00:19:31]In this slide further emphasizes the relationship between the conductivity, carrier density and mobility of charge carriers.

[00:19:42]Let us imagine two highways.

[00:19:46]The first one is there are many cars but severe traffic congestion.

[00:19:56]The second one there is very few cars but the road is completely clear.

[00:20:06]Neither highways provides optimal transportation.

[00:20:18]Similarly, electrical conductivity depends on both the number of uh charge carriers and their mobility.

[00:20:32]If the carrier density is high but the mobility is poor that results a limited electrical conductivity.

[00:20:46]Similarly, if the mobility is uh excellent but very few carriers exist, still the electrical conductivity is low.

[00:21:00]The ideal situation occurs when both carrier density and mobility are high.

[00:21:10]This is exactly what engineers try to achieve in semiconductor devices.

[00:21:19]For example, in a silicon transistor, doping is used to increase the carrier concentration.

[00:21:29]The advanced fabrication techniques reduces defects and improves the mobility.

[00:21:38]As semiconductor technology advances from nanometers to atomic scale, the controlling mobility becomes increasingly important.

[00:21:55]Today's high performance processes contains billions of transistors where electron transportation must occurs extremely efficient.

[00:22:10]Therefore, understanding the balance between the carrier density and mobility is essential for designing next generation electronic devices.

[00:22:29]Whenever you hear the word such as highspeed electronics, low power devices, efficient semiconductor materials.

[00:22:43]Remember that the conductivity, carrier density and mobility of charge carriers are playing a crucial role behind these sense.

[00:22:57]You can see here the different ranges of this conductivity values are given. If you see the metals for silver it is silver, copper and iron for all metals it is 10^ 7 order.

[00:23:18]Whereas in semiconductors it is about 10^ -4 to 10^ -6 order.

[00:23:26]Whereas in insulators if you see particularly ceramics like sime glass or concrete or aluminium oxide it is 10^ - 10 to 10^ -3 order. Whereas in polymers it is in the order of 10^ -14 to 10^ -16.

[00:23:54]This ceramics and polymers comes under the category of insulators.

[00:24:11]Think about the diagram.

[00:24:23]Here it is designed to think.

[00:24:41]Let us consider a simple question here.

[00:24:47]Why do why do we not make computer chips using copper?

[00:24:55]Though the computer uh copper is a conductor which has the electrical more electrical conductivity.

[00:25:08]We are not using it in computer chips.

[00:25:18]Even though it is better conductor than the silicon, we are using silicon based semiconductor or semiconductor based chips.

[00:25:33]At first glance, the copper seems like the ideal material because it's conducts electricity extremely well.

[00:25:48]However, there is a problem. The copper conductivity cannot be controlled effectively. That is one thing. Other thing is once the current starts flowing it continues to flow freely. That is the problem with the semicondu conductors.

[00:26:12]And the other thing is here we can understand from the diagram as we mentioned the availability of charge carriers is one of the important factor.

[00:26:25]If you see the silicon in silicon there are four electrons available in the outermost orbital but in the copper only one electron available in the outermost orbital. The valency electron is one in copper.

[00:26:46]There are four valency electrons in the silicon.

[00:26:53]As per our previous discussion, if the electrons, the valency electrons are more, free electrons are more, the electrical conductivity should be more.

[00:27:07]But here it is quite reverse.

[00:27:11]The conductivity of copper is more far far high and compared to the silicon.

[00:27:20]But the availability of charge carriers in the outermost orbital is high in silicon.

[00:27:33]On the other hand, the silicon behaves differently by adding impurities, applying voltage or changing operating conditions.

[00:27:52]Engineers can precisely control the conductivity of the silicon or any kind of semiconductor.

[00:28:04]This controllability allows silicon to function as a switch and many other applications also.

[00:28:20]So the modern digital electronics relies on billions of tiny switches called transistors.

[00:28:32]Those all transistors made up of these semiconductor materials.

[00:28:40]A transistor must turn on or off reliably.

[00:28:49]So if we cannot control the current, if we cannot control the conductivity of a metal, it which cannot be used as a switch.

[00:29:02]So the conductor cannot perform this switching action effectively because the conductivity cannot be controlled in metals.

[00:29:18]Semiconductors can because we can control the current flow in semiconductors.

[00:29:28]The another question is why the semiconductor device become more conductive when temperature increases whereas metals become less conductivity with respect to the temperature.

[00:29:47]The conductivity of metals decreases with increasing temperature. Whereas in semiconductors the conductivity will be increases with increasing temperature.

[00:30:00]This opposite behavior is one of the defining characteristic of semiconductors and will be discussed in the subsequent slides.

[00:30:17]The ability to ask such questions and analyze the underlining physics is what distinguishes an engineer from an technician.

[00:30:36]A technician cannot think beyond but the engineer can think and design the new materials.

[00:30:47]The engineer can discover the new materials for advanced applications.

[00:30:56]Physics is not about memorizing formulas.

[00:31:01]It is about understanding why materials behaves the way they do and how we can use those behaviors to create useful technologies.

[00:31:18]But an engineer should understand and they they they should be in the position to apply this physics knowledge to innovate new things or new technologies.

[00:31:36]Keep these questions in mind as we move forward because they will help you to uh appreciate the remarkable properties of semiconductors.

[00:32:02]and conclude the classification of semiconductors.

[00:32:06]And here the semiconductors behaves differently like they can behave like insulators at extremely low temperatures and the conductivity will be increases with increasing temperature.

[00:32:28]Thank you very much for your kind attention and once again I welcome you to the exciting learning journey in physics for engineers.

[00:32:42]Thank you.