# ATAV208 Analog Theory > # [[V208 Intro| ◀️ ]] &nbsp;[[V208 Home| Home ]] &nbsp;[[V208 Week 2| ▶️ ]] &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; [[QR V208 Week 1| 🌐 ]] [📝](https://excalidraw.com/) ># [[V208 Week 1#ATAV208 Analog Theory|Week 1]] >- [[V208 Week 1#Atomic Structure of Semiconductors|Atomic Structure of Semiconductors]] >- [[V208 Week 1#Insulators Conductors and Semiconductors|Insulators Conductors and Semiconductors]] >[!jbplus|c-blue]- Lesson Intro >### What > >This week we will review some basic scientific knowledge to support our new learning, and establish some fundamentals about semiconductors. > >### Why > >We will be exploring the action of electrons in semiconductors, and so we must understand these foundational concepts first. > >### Testing > >You will be tested on this material on Assignment 1 and the Midterm/Final. ## Atomic Structure of Semiconductors %%This material is covered in [[T105 Week 1]]. It will be properly integrated at a later date.%% ### Terms #### Background [[Video Voiceover Week 1a|📺 Voiceover 1a]] The material in the next section was covered in [[T105 Home|Basic Electricity]], but let's make sure that we are moving from a solid foundation into this more difficult material. Incidentally, the difficulty this material presents is usually because students are not used to forming their own imagery or conceptualization of the topics discussed. The invisible and multidimensional properties of [[Electricity|electricity]] and [[solid state]] devices make illustration difficult and limited. It may take you a few runs through the material to realize that you have formed a picture of how these things work. Please be patient with this process. Also, be open to different explanations, sometimes a certain way to describe things resonates with you, and helps you to decipher the rest. The linked resources offer many alternatives and extras in the context of the lessons. Let's make sure that we understand a few key terms before moving on:  [[Pasted image 20210515132128.png|➡]] %%#JB update atomic theory stuff to agree with T105 SSGW01 edits once complete%% #### Matter [[Matter]] is the material that makes up our universe. It occupies space, it has [[weight]] (or better, [[mass]]), and can be in the form of [[solid]], [[liquid]], or [[gas]]. You may see [[plasma]] added to this list; as our knowledge increases, the explanations expand! Let's drill down a layer, or zoom in, depending on which analogy suits you. #### Compounds [[Compounds]] are combinations of two or more [[elements]]. When two elements combine to make a compound, an entirely different material can result. If you were to divide a compound again and again, before it breaks down into its separate elements, you would ultimately arrive at… #### Molecules A [[molecule]] is the smallest part of a compound which may exist and still retain the properties of that compound. If you were to zoom in again, or divide this molecule into its component parts, you would have atoms of the component parts or… #### Elements An [[element]] is a substance that cannot be separated into different substances (like we just did with [[Chemical Compound|compounds]]). They can exist by themselves, like for instance copper, hydrogen or carbon. Or, as we've seen, they can combine to form compounds, such as water, which is made up of the elements hydrogen and oxygen. Once again we will zoom in or drill down to… ### Atoms %%==[[Master QB1#Q00418|Q]][[Master QB1#Q00425|Q]][[Master QB1#Q00435|Q]]==%% Atoms are the smallest particle that an element can be divided into and still retain the properties of the element. It consists of a nucleus and orbiting electrons. Now, before we go into atoms, let's reverse the dividing process to make sure we understand this underlying structure: - Like atoms - molecules of elements - Different atoms - molecules of [[Chemical Compound|compounds]] Atoms combine to form molecules. Like atoms combine to form molecules of elements. Different atoms or elements combine to form molecules of compounds.  [[Faculty/Student/Content/ATAV/V208/assets/Pasted image 20210515132949.png|➡]] #### Three Sub-particles %%==[[Master QB1#Q00452|Q]][[Master QB1#Q00454|Q]][[Master QB1#Q00443|Q]]==%% Atoms are composed of three sub-particles: protons, neutrons and electrons. The nucleus contains protons and neutrons. Electrons orbit the nucleus. - **Neutrons** have no electrical charge, or could be referred to as having a neutral charge. - **Protons** have a positive electrical charge. - **Electrons** have a negative charge. Electrons are very small. The mass of a proton is almost 2000 times as much.  [[Faculty/Student/Content/ATAV/V208/assets/Pasted image 20210515133609.png|➡]] #### Atomic Number %%==[[Master QB1#Q00427|Q]][[Master QB1#Q00441|Q]]==%% There are the same number of protons in the nucleus as electrons orbiting the nucleus. The charges in a normal atom are equal and opposite so they cancel each other out, meaning that an atom is a neutrally charged particle.  [[Faculty/Student/Content/ATAV/V208/assets/Pasted image 20210515134238.png|➡]] The atomic number of an atom refers to the number of protons in that atom. The elements in the periodic table are arranged in order of their atomic number. Since the periodic table lists neutral atoms, the atomic number also tells us the number of electrons. Why do we specify a neutral atom? Because this balance can be disrupted, leaving an atom with a positive or negative charge. When this happens, we refer to it no longer as a neutral atom, but as an ion. If the overall charge is positive, that is, the atom is missing one or more electrons, leaving the atom positively charged, it is called a positive ion. Conversely, if an atom has an extra electron, it is called a negative ion. #### Electrons ![[Faculty/Student/Content/ATAV/V208/assets/Pasted image 20210515134123.png]] ##### Orbit %%==[[Master QB1#Q00445|Q]]==%% The circular path along which electrons travel around the nucleus of an atom is called an orbit or shell. These shells are designated by either letters or numbers. For our purposes, it doesn't matter. We are mostly interested in the outside shell, whatever its letter or number. For the simplest atom, Hydrogen, there is one electron in orbit. As we add electrons and their balancing protons in the nucleus, they orbit together. As each orbit fills up, another is created, farther from the nucleus. These orbits each have a capacity, and once full, the atom is very stable. It is when an orbit is something other than full that instabilities in an atom present themselves. ##### Valence shell and valence electrons %%==[[Master QB1#Q00442|Q]][[Master QB1#Q00458|Q]]==%% The outermost orbit (shell) of an electron is known as the valence orbit. Electrons in the valence orbit are known as valence electrons. The force of attraction between the positive charged nucleus and the negatively charged electron decreases with increasing distance. This means that electrons in orbits farther from the nucleus are less tightly bound to the atom than those closer to the nucleus. ##### Free Electrons %%==[[Master QB1#Q00440|Q]][[Master QB1#Q00438|Q]][[Master QB1#Q00426|Q]][[Master QB1#Q00437|Q]][[Master QB1#Q00450|Q]]==%% Certain elements, chiefly metals, are known as conductors because an electric current will flow through them easily. Common examples would be gold, silver and copper. These conductive elements have a valence orbit that is nearly empty. This, and the effect of the distance from the nucleus means that valence electrons are easily given up or received from one atom to another. If energy is applied to these kinds of atoms, the electron can be dislodged from the valence shell. It can gain sufficient energy to jump the gap from the valence band to the conduction band. It then becomes a free electron. This has a number of effects. The atom that lost its valence electron has lost its negative charge as well. This leaves the atom with a positive charge, changing it to a positive ion. The hole that is left represents a positive charge. The electron that is now moving outside of its atom is negatively charged. Remembering that like charges repel, and unlike charges attract, we now have electrical forces in play. The free electron is attracted to positive ions, that is, atoms that are missing the valence electron, and the now positive atoms are similarly attracted to the negatively charged free atoms. This causes the electrons to move, and is known as **Electricity**. ## Insulators, Conductors, and Semiconductors This is probably where we get into new concepts. We remember that electrons moving about is electricity, and we understand the electrical charges and that they are attracted or repelled to each other. ### Conductors But let's take another look at the valence shell. We already saw what happens when the valence shell is almost empty, that is with one valence electron. We saw that these are called conductors. ### Insulators %%==[[Master QB1#Q00444|Q]]==%% If we have a valence shell with almost all of its capacity met, that is, it is almost full, we see different behaviour from the electrons. They tend to work together to ensure the integrity of the atom, seem to maintain a stronger attraction to the nucleus, and thus are less likely to lose an electron to dislodging by an applied energy. So now we have a material that is not prone to electron flow. These are insulators. We can say that materials that have more than half of their valence electrons can be considered insulators. ==Here is where this material diverges and moves on from [[T105 Week 1]].== ### Semi Conductors [[Video Voiceover Week 1b|📺 Voiceover 1b]] Conductors and insulators are the two ends of the spectrum, that is, almost empty valence shell, and almost full valence shell. What if the material has a valence shell that is half full? In the case of both pure silicon and pure germanium, this is four electrons in the valence shell. You will find that it does not conduct as well as a conductor, and that it does not insulate as well as an insulator. Therefore, it is called a semiconductor (semi means half). This is the central concept, or central technological point of this course. Semiconductors have revolutionized our world. #### High Resistance %%==[[Master QB1#Q00420|Q]]==%% Semiconductor material has a very high resistance to current flow in its pure state. It is not stopping current altogether, because that would be an insulator, and it is not letting it flow freely, because that would be a conductor. So, it is somewhere in between. We could describe this material as neither fully conductive nor fully insulative. So, this is because of the number of electrons in the valence shell. What if we were to add just one or two electrons in the mix? Would it conduct better? Indeed it would, and if we removed an electron or two, it would offer even more resistance. #### Silicon and Germanium %%==[[Master QB1#Q00424|Q]][[Master QB1#Q00439|Q]]==%% The two most common materials used in the construction of semiconductors are silicon and germanium. Both of these elements have four valence electrons, that is, exactly half the capacity of the valence shell. Take a look back at the Periodic Table of Elements, and notice that Silicon and Germanium are vertical neighbors in column 14. This is no coincidence, as the table of elements is arranged with properties in mind, and as we'll see, the fact that these both have 4 valence electrons makes them behave similarly. #### Covalent Bonding %%==[[Master QB1#Q00448|Q]][[Master QB1#Q00451|Q]]==%% ![[Pasted image 20210515142451.png|350]] In both Si and Ge atoms the valence electrons are shared with each other to form a covalent bond. This makes the atoms behave as if they had a full valence shell, thereby becoming very stable. So, for our purposes here, you could say that the less it conducts, the more stable it is. This is a narrow view of it, but it will work. #### Intrinsic Material %%==[[Master QB1#Q00430|Q]][[Master QB1#Q00434|Q]][[Master QB1#Q00457|Q]][[Master QB1#Q00423|Q]]==%% A pure piece of Silicon or Germanium is called an intrinsic material. In a pure (intrinsic) semiconductor, because of the effect of covalent bonding, there is a relatively small number of free electrons. As we've seen, they are neither insulators nor good conductors because current in a material depends directly on the number of free electrons. #### Semiconductors and Heat %%==[[Master QB1#Q00455|Q]][[Master QB1#Q00447|Q]]==%% One thing you will see throughout this course is the effect of heat on semiconductors. It affects their design, and requires precautions when using or maintaining them. When no energy is applied, intrinsic silicon is pretty close to an insulator. You see in this graphic that none of the valence electrons have been dislodged into the conduction band. ![[Pasted image 20210515143041.png|350]] Before we go further note the different visual representation here. We have bands that represent the orbits in ascending order. You may like this way of looking at it. You may however prefer to imagine the circular orbits we have seen, and when the electron is dislodged from the valence shell, it is effectively free of the atom. Note that the "conduction band" is hard to represent in the circular, orbital view. It is just the empty space outside the atom. Pure silicon crystal even at room temperature derives heat (thermal) energy from the surrounding air. This causes some valence electrons to gain sufficient energy to jump the gap from the valence band into the conduction band. So the application of even small amounts of heat energy to a semiconductor material changes its conductive properties. Remember this concept, as it will recur as an issue when we discuss more advanced topics on this course. #### Hole Movement %%==[[Master QB1#Q00428|Q]]==%% ![[Pasted image 20210515143049.png|350]] Another important note. Notice the preceding diagram on the right, as the electron leaves the valence shell, it leaves a hole. Understand that when an electron is dislodged from the valence shell, it leaves an absence of the electron's negative charge, which is in effect a positive charge. We have seen how electrons move, and know that to be electrical current flow. If we were to look at the effects of these electrons leaving their valence shells instead of the electrons themselves, we would be looking at holes. These holes appear to move as well, as they are filled and vacated by travelling free electrons. And in fact, they would move in the opposite direction. ![[Pasted image 20210515143433.png|350]] Note that if electrons move in one direction, the holes will move in the opposite direction. Both electrons and holes are referred to as current carriers. This allows us to discuss things in two directions later. #### Conventional Current Flow vs. Electron Flow Theory You may have been taught that current flows from positive to negative. In this course, we will describe electrical current as flowing from negative to positive. What is the story here? ##### Conventional Current Flow Theory When electricity was first discovered, it was imagined that something moved. They supposed that the thing that moved did so to a place where that thing wasn't. In other words, something moved to nothing. They assigned the positive value to the moving thing, and it's absence, logically, as a negative value. And this is a perfectly serviceable way of understanding current flow. ##### Electron Flow Theory %%==[[Master QB1#Q00429|Q]]==%% Except when later we learned more about the atom, we realized that this thing that was moving (the electron) is actually negatively charged. This means that electricity moves from a negative charge to a positive. Which is also a perfectly serviceable way of understanding current flow. Which one is correct? They are both correct. Remember we said that holes move in the opposite direction of electrons. Holes represent the absence of an electron, and so this atom is actually a positive ion. So, conventional current flow, when described as the movement of positive ions, is quite correct. And in fact is used in most of the British Empire (except us) and also in engineering. Positive ions are attracted to negative charges (electrons). However, and often at the college level, it is useful to understand the movement of electrons when learning about electricity, semiconductors in particular. By using electron current flow theory, we can observe electrons doing their thing in a semiconductor, and follow those electrons out into the circuit. If we used conventional current theory, we would talk about electrons moving one way in a device, but current flowing the opposite way in the circuit. This can be very confusing. Yet some like to make that distinction, and I warn you that some youtube videos on this topic may very well mix up conventional and electron flow to endless confusion. Could we describe semiconductors in terms of ions instead and just use conventional flow? Yes, but it is another level of understanding, and is not necessary for our purposes. Understand that these are just two ways of looking at this issue. Electricity still works exactly the same. How it is described is different. So, for this course, we will exclusively describe electricity as electrons flowing from negative to positive. Once we have learned a little more about some circuits, I will demonstrate to you with [[V208 Week 6#Conventional vs Electron Current Flow Theory|this video]] why this debate is not a concern, and why you don't have to worry about this dichotomy. #### Doping We hinted earlier that if we could adjust the number of electrons in a semiconductor material, we could adjust its conductivity. ##### Adding impurities %%==[[Master QB1#Q00421|Q]][[Master QB1#Q00446|Q]]==%% Adding certain impurities to a silicon or germanium crystal increases the number of current carriers. What is an impurity? We are talking about a pure, that is, intrinsic crystal. Any atom that is not of that type makes our pure crystal impure. We call the process of adding impurities to the crystal structure **Doping**. Added impurities create either an excess of electrons or an excess of holes, depending upon the type of impurities added. ##### Extrinsic Material %%==[[Master QB1#Q00453|Q]]==%% We call a crystal that has added impurities extrinsic, which means that any extra mobile electrons or holes are added from an external source. ##### Increased conductivity %%==[[Master QB1#Q00449|Q]]==%% Whether or not we added electrons, or added holes by adding an impurity with an electron deficit or surplus, we now have more free current carriers. This means more electrical flow, and so, we have increased the conductivity of the semiconductor by doping it. ##### Decreased resistivity Your understanding of basic electricity should allow you to conclude that if this material allows more electrical current to flow, it must be demonstrating a decrease in its resistivity. This may seem obvious, but it is an important relationship. #### N-Type Semiconductors ##### Doping to increase electrons %%==[[Master QB1#Q00436|Q]]==%% [[Video Voiceover Week 1c|📺 Voiceover 1c]] N-Type material is the result of doping to increase the number of electrons in the valence shell. Recall that covalent bonding resulted in a full eight-electron valence shell. By introducing atoms with five electrons in their valence shell, we will have an extra electron that has no hole available. This increases the number of free electrons, making the material more conductive. These atoms with five electrons in their valence shell are called pentavalent impurities. (Penta means five). Some that are used in the construction of semiconductors are arsenic, phosphorus, and antimony. To restate it slightly differently: After the covalent bonding with silicon, there is an extra electron in the conduction band, which becomes a free electron. The impurity donates one electron to increase the number of free electrons, without increasing the number of holes. ##### Donor impurity N-Type material is doped with donor impurities, in that the doping material donates extra electrons. This creates an imbalance between electrons and holes. So, our current carriers are not identical now. ##### Majority current carrier - electrons %%==[[Master QB1#Q00433|Q]]==%% We have a majority current carrier of electrons, and the holes constitute the minority current carrier, because there are fewer of them. The conduction in this material is due to electron movement. Because it has a negative majority current carrier, and an overall negative charge, this material is referred to as N-Type. #### P-Type Semiconductors Watch carefully, this section is the opposite of the preceding one. ##### Doping to increase holes P-Type material is the result of doping to increase the number of holes in the valence shell. Recall that covalent bonding resulted in a full eight-electron valence shell. By introducing atoms with three electrons in their valence shell, we will have an extra hole with no electron available. This increases the number of free holes, making the material more conductive. These atoms with three electrons in their valence shell are called trivalent impurities. (tri means three). Some that are used in the construction of semiconductors are Indium and Gallium. To restate it slightly differently: After the covalent bonding with silicon, there is an extra hole in the valence band, which attracts electrons from neighboring orbits, leaving a hole free there. The impurity donates one hole without increasing the number of electrons. ##### Acceptor impurity P-Type material is doped with acceptor impurities, in that the doping material accepts extra electrons. This creates an imbalance between electrons and holes. So, our current carriers are not identical now. ##### Majority current carrier - holes %%==[[Master QB1#Q00422|Q]]==%% We have a majority current carrier of holes, and the electrons constitute the minority current carrier, because there are fewer of them. The conduction in this material is due to hole movement. Because it has a positive majority current carrier, and an overall positive charge, this material is referred to as P-Type. #### Current Flow is same for both N-Type and P-Type %%==[[Master QB1#Q00417|Q]][[Master QB1#Q00456|Q]]==%% Once again, understanding the current flow may be confusing you. This is why we will strictly use **electron current flow theory** for the time being. Current in the external circuit will always consist of electrons moving from the negative side of the battery (surplus of electrons) to the positive side. When we look at P-Type material, and we say that the majority current carrier is holes, it does not change the fact that the electrons are moving as we always said they did, from negative to positive. If we were to apply a voltage to a piece of either N-Type or P-Type material, the current direction would change with the application of voltage. In other words, this doped crystal conducts current both ways, just as a wire or a resistor would. The majority current carriers will be different, but electron flow follows all the rules you are comfortable with. Why is this point being made? Because we are going to change this characteristic very shortly. ## Learning Strategy As we move through the next sections, this will be reinforced, and your confusion may be reduced. As discussed earlier, the difficulty may be in visualizing some of the concepts covered. Take advantage of all the course materials, and do not be afraid to go through the material more than once. Purposeful repetition is a secret weapon for learning. For the first part of the course, each section builds on the last. It is important that you have a fair grasp before moving on too quickly. Contact your prof if any of the concepts here are troublesome. You need to have a grip on them to understand the coming material.