Semiconductors

-Aashirwad jindalPhysics Project..

to heema mamIf we knew what it was we were doing, it would not be called research, would it?Albert Einstein

Welcome to this basic tour of semiconductor physics! Two of our most excellent guides, Sally Con and Jerry Manium, will take you through.

Sally and Jerry explain things in different ways. Sally tries to be correct, and likes to stick to the facts. Jerry is easy-going, and uses examples from the everyday world around us. let Sally and Jerry explain a few things!

Introduction

History..

Let's begin this journey into the world of semiconductors with a look at the history books. In the early 1900s, not much was known of the world at an atomic level, and even less so at the subatomic level. Physics, to a large extent, still calmly followed classical rules. But new discoveries likeRntgen'sx-rays, Thomson's electron andRutherford'sdiscovery of the atomic nucleus made it clear that new rules were needed. Scientists likePlanck,Einstein,Bohr,PauliandHeisenberg, to name a few, all contributed to the development and understanding necessary for the creation of the new paradigm of quantum physics. The development of quantum physics also laid the ground for 'Solid State Physics' which is a discipline explaining the internal atomic structure and the electronic properties of the materials that we see in our everyday life such as metals, plastics, glass, etc.

History..

[Type sidebar content. A sidebar is a standalone supplement to the main document. It is often aligned on the left or right of the page, or located at the top or bottom. Use the Text Box Tools tab to change the formatting of the sidebar text box.]

Before we start, it would be a good idea to clarify what electricity is. Electricity can be seen as a stream of electrons. Electrons are tiny particles with a negative charge. So, roughly explained, electricity is a stream of electrons flowing from one point to another.

ElectricityA good way to explain an electric current passing through a cable would be to imagine a pipe filled with marbles that exactly fit the pipe. If we push a marble into the pipe in one end, the motion would be distributed, each marble pushing its neighbor, so that almost instantly a marble in the other end would be pushed out of the pipe.

Conductivity..!!

[Type sidebar content. A sidebar is a standalone supplement to the main document. It is often aligned on the left or right of the page, or located at the top or bottom. Use the Text Box Tools tab to change the formatting of the sidebar text box.]

Why semiconductors..???

Semiconducting materials..!!

Semiconductors can be made of a single material or a combination of several different materials. In early semiconductor devices germanium was often used. However in today's semiconductor industry, silicon is commonly used.

Silicon is very easy to find in Nature. Ordinary sand, like on the beach or in the desert for example, is nothing more than one silicon atom combined with two oxygen atoms. However, if you want silicon in its pure form suitable for the production of, for instance, computer chips it has to be purified in a carefully monitored process.

One of the main reasons for the popularity of silicon is that it is stable and can be heated to a rather high degree without loosing its material characteristics. This means that engineers can be sure it will perform according to their plans, even under quite extreme conditions.

Semiconducting materials cont.Just to make sure we avoid misunderstandings, when we talk about silicon, we don't mean silicone spelled with an "e" at the end, 'cause that is a material mostly known for its use in human implants.

To understand the principles of semiconductors, it is good to first understand the basics of atoms and energy levels within atoms. So that's where we're going to start.

[Type sidebar content. A sidebar is a standalone supplement to the main document. It is often aligned on the left or right of the page, or located at the top or bottom. Use the Text Box Tools tab to change the formatting of the sidebar text box.]

Silicon and its molecules..

If we look at the solid material of Silicon we will see that it is built from a huge number of Silicon atoms that are brought together. When the atoms interact with each other, the atomic shells of each atom interacts with the atomic shells of neighboring atoms. On an energy scale, the overlapping energy shells of all the separate atoms form energy bands that are similar to the energy shells in the single atom. Between the bands no electrons are allowed. In a simplified way, it is almost as if the solid material is an enlargement of the single atom.

Bands and their theories...

To continue our journey, you don't need to fully understand what Sally just said. But what you do need to understand is, that the highest energy band that is occupied by electrons in a material is called thevalence band, just like in the single atom where the highest shell occupied by electrons is called thevalence shell. The band with energy one step higher than the valence band is theconduction band. The energy gap between these two bands, where no electrons are allowed, is called theband gap.

If you think of the energy bands as steps in a staircase then the band gap is the area between the steps. You can put your foot on the first step of the stairs and you can put it on the second, but you can never put it somewhere between the first and second.

Bands and their theories... cont

A electron-hole pair..!!A very important feature of the semiconductor material is the electron-hole pair. To get a semiconductor to conduct a current, we must make an electron jump from an occupied to an unoccupied energy level. When it does this it leaves a hole (an empty state). This hole can be filled by another electron, which itself leaves a new hole. Therefore, we could say that both the hole and the electron contribute to the conductivity as they move around in the material. The hole is like a positive charge (lack of negative), the electron is negative.

It's a little bit like this simple puzzle game where you move pieces around to form an image. The moving pieces correspond to the electrons, of course.

Conduction in different types of materialsAs mentioned earlier, the semiconductor has a conducting capacity somewhere between the conductor and the insulator. If we look closer at the materials we can see why they behave like this. Before we go on, note that contrary to what its name may suggest, the conduction band is not the only band where conduction of a current may occur. Conduction is equally possible in the valence band.

In a good conductor like a metal, the highest energy band with electrons (valence band) is only partially filled. This means that the electrons can accelerate. In other words, they gain energy so that they can transfer to higher energy levels that are empty. Simply put, in a conductor there is plenty of room for the electrons to jump from an occupied state to an empty one.

Conduction in different types of materials

Conduction in different types of materials

Conduction in different types of materialsIf you felt that Sally's explanation of the conduction properties in different materials was crystal clear, you can skip the following part. But if you're still a little unsure of how it works, I will try to show you another way of looking at this phenomenon. To help my explanation, I am going to use the unrealistic cup with the water-filled compartments again. The compartments equal the energy bands of the material and the water equals the electrons. This time the cup only has two compartments, one for the valence band and one for the conduction band.

Conduction in different types of materials

In a conductor, the valence band is only partially filled. This means that, in our cup, we are going to have the valence compartment half-filled with water. If we tip the cup from side to side, we will see that it is easy for the water to move back and forth, just as it is easy for the electrons to move within the conductor.

A semiconductor at low temperature is an insulator because there is no place for the electrons to go to. The valence compartment is filled and no matter how we tip the cup there is no room for the water to move into. At room temperature, the heat (energy) makes the atoms vibrate slightly, enough for a few of the electrons to break their bonds and jump into the conduction band. If we take some water (electrons) from the valence band and move it to the conduction band, we will have place for the water (electrons) to move in both bands. If we tip our cup, water will move both in the valence and conduction band. Thus, in a semiconductor at room temperature, a small current will flow.

Conduction in different types of materials cont

In an insulator, the valence band is completely filled, and as a result no electrons can move. In the cup, no water will move no matter how we tip it. The band gap between the valence and the conduction band is huge. To move water (electrons) from our valence compartment to the conduction compartment, we would need to add such an amount of energy that our cup (material) would be close to breaking before any water (electrons) would begin to move between the compartments.

[Type sidebar content. A sidebar is a standalone supplement to the main document. It is often aligned on the left or right of the page, or located at the top or bottom. Use the Text Box Tools tab to change the formatting of the sidebar text box.]Doping Now we are going to talk about doping. Maybe the word makes you think of athletes taking illegal drugs to perform better. Although doping in sports is outrageous, the parallel between that and doping of semiconductors is not too far-fetched. In both cases you have something pure, like an athlete or a semiconducting material, and add something foreign to it to change its performance.

Doping..cont..So, in the process of doping you add a tiny amount of atoms from another material to the pure semiconductor. By doing so, you can drastically increase its ability to conduct a current. There are two forms of doping,pandn.pstands for positive andnfor negative. Finally, two words that are good to know: a pure non-doped semiconductor is calledintrinsic, while a doped semiconductor material is calledextrinsic.

Pure semiconductors..Before we look at examples of doped semiconductors, let's look at how the silicon atoms in pure silicon interact to form the crystal structure of the material. In pure silicon, each atom has four valence electrons and these are shared with four neighboring silicon atoms to make four double bonds. Now each atom will have a completely filled valence shell of eight electrons. At low temperature this bond is very stable, completely filling the valence band and thus making conduction impossible. Here is a model of the structure of pure silicon:

In a pure semiconductor at low temperature, the valence layer is completely filled with electrons and the conduction band is empty. That would be equal to one filled and one empty compartment in my cup. The water (electrons) can't move because there is no empty space.

P dopingp-doping is when you add atoms with less valence electrons to the semiconductor so that the material gets a shortage of electrons in the crystal bonds. This way positive holes that can transport current are formed. The materials that add holes are called acceptors because they accept electrons from the surrounding atoms. In a p-type semiconductor the major carrier of current are the holes, not the electrons.

The p in p-doping stands for positive. This is because compared to the atoms in the semiconductor material the added atoms have fewer negative valence electrons. In the p-doped semiconductor the higher conduction band is empty, but there will be holes in the valence band.In the cup, this means that we remove some water from the valence compartment. In other words, we form air bubbles (positive holes) in the water. Now if we tip the cup, there is room for the water (electrons) to move in one direction and for the created holes (lack of electrons) to move in the opposite direction (just like bubbles would do in water).

N-dopingIn the process of n-doping you add atoms with one extra valence electron to the pure semiconducting material. This creates a situation where there are extra electrons that are just loosely bound in the crystal. The amount of energy needed to get these electrons to jump to the conduction band so that a current may pass is very small. The materials that add electrons are called donors. This is simply because they donate electrons to the semiconductor. In the n-type semiconductor the major carrier of current is the negative electrons.

The n in n-doping stands for negative. This is because compared to the atoms in the semiconductor material the added atoms have more negative valence electrons. In the n-doped semiconductor, the valence band is full so there is no room for the electrons to move there. Instead, the extra electrons move into the conduction band.In our cup, we can see that no water will move in the full valence compartment. Instead, the extra water (electrons) added will move within the conduction compartment.

Semiconductors-the future

In a world where computers become faster and faster each year, semiconductor components, like chips and transistors, must be made smaller and smaller. This means that we will eventually reach a limit on how much faster and more effective the Silicon based technique can be made (in fact, devices operating with just a single electron have already been demonstrated). "What happens then?" you might ask yourself. Well we don't know for sure, but today's scientists are working hard to find new materials or to improve old ones. In the future, large molecules might do the work that transistors do today. This field is called Molecular Electronics. So hopefully (if you like information technology, that is) computers can continue to evolve for a long time to come.

Circuit diagramCharacteristics of transistor..

Transistor as amplifier.

Transistor as switch

Full wave rectifier.

Circuit diagrams..half wave rectifier.

Circuit diagramlogic gatesIntegrated circuit

1 | physics project.