Introduction to Electronic Materials
Introduction to Electronic Materials
From our high school chemistry and physics courses, we know that matter is made up of atoms. Each atom contains protons and neutrons that reside in the atom's nucleus and electrons that orbit the nucleus. Much of the technology we use daily, and often take for granted, depends at a fundamental level on electrons and how they move in an electric circuit. Electrons are negatively charged particles and are attracted to to the atom's nucleus as the nucleus contains positively charged protons. Click on Figure 1 to learn more.
Figure 1: A highly-qualitative sketch of an atom. The nucleus is at the center of the atom and contains the positively-charged protrons and the uncharged neutrons. Surrounding the nucleus is an "electron cloud." As we will see, the electrical properties of a material are largely governed by how the electrons of atoms interact in a material.
In a very general sense, an electric circuit may be thought of as a connection of elements assembled to move charges along specified paths. Of course, there is some ultimate purpose for moving charge. Perhaps we wish to transfer energy from one place to another, or to use a personal device to have a video chat with a friend, or perhaps something so simple as toasting bread at breakfast. How these charges (electrons) move in a circuit, thus creating what is referred to as an electric current, depends on the components and connections that comprise the circuit and the sources that drive the circuit. Some elements allow charges to flow easily, others hinder electron flow, and yet others allow electrons to flow under only very special conditions. The extent to which a component will allow current is, in large measure, related to the materials from which the component is constructed. Later in your studies (e.g. EELE 409: Material Science) you will learn a great deal about various classes of materials. For now, we will take a very simplified look at three basic classes of materials that are important in electronics: conductors, insulators, and semiconductors. In addition to learning a bit about electronic materials, we will briefly consider how the physical geometry of a block of material impacts electrical behavior. Before we proceed any further, let's review the learning outcomes for this reading application.
After completing this reading exercise, you should be able to:
- Identify the basic electrical properties of conductors, insulators, and semiconductors
- Give examples of conductors, insulators, and semiconductors
- Qualitatively describe differences between metallic, ionic, and covalent bonds
- Identify the key geometric features of a bar of material that impact electrical behavior
Conductors
Consider the metal aluminum (Al). You may be aware that Al is a good electrical conductor, that is, it allows electrons to flow freely, or to conduct. Why is this so? An aluminum block, or wire, is made up of Al atoms that are held together by so-called "metallic" bonds. Metallic bonds are characterized by the existence of a sea of electrons as suggested in Figure 2. Clicking on Figure 2 will reveal additional details.
Figure 2: A highly qualitative sketch of how Aluminum atoms come together in a metallic bond. The key point to take from the picture is that the electrons that once belonged to an individual atom are now part of the "sea of electrons" that exist throughout the metal.
In metallic bonds, electrons are able to readily travel from one atom to the next. Since these electrons are able to move quite easily, Al may be used to efficiently transport electrons from one part of a circuit to another. Examples of other materials that act in a similar fashion include copper (Cu), gold (Au), and silver (Ag). Each of these materials is held together through metallic bonding. What metal is chosen for a given application depends not only on its relative ability to allow electron flow, but also on its mechanical properties and cost. Let's take a brief look at electrical conductivity, the metric that allows us to compare how readily various materials allow charge to flow through them.
Electrical Conductivity is the aptly named figure of merit that describes how well suited a material is to support the flow of charge. Metals, allowing charge to flow readily, are characterized by large electrical conductivities. Materials are also characterized by thermal conductivity which describes how readily heat flows in the material. From now on, this reading exercise uses the term conductivity to mean electrical conductivity. Conductivity is denoted with the Greek letter, sigma (σ). At first glance, the units of conductivity are rather curious as suggested below.
Conductivity → σ → Siemens/meter
The "Siemens" in the unit of conductivity is named after a 19th century German inventor; but why 1/meter? To understand the dependence on inverse length, later in this reading application we will take a look at the bar of material and see how physical geometry affects electrical behavior. For now, simply remember the larger a material's conductivity, the more readily it allows electrons to flow.
Insulators
Insulators act very differently with charge than do conductors. Instead of allowing the free flow of electrons, insulators prohibit their flow. Simple table salt, sodium chloride (NaCl) is an interesting example of an insulator. NaCl is held together by what is known as ionic bonding. Ions are charged atoms. An isolated atom has the same number of protons (positive charge) and electrons (negative charge) and thus has zero net charge. Thus alone, sodium (Na) and chlorine (Cl) are uncharged with eleven and seventeen electrons respectively as suggested in the portion of the periodic table shown in Figure 3.
Figure 3: A portion of the periodic table. The upper number in a box is the element's atomic number and corresponds the the number of protons within the atom's nucleus. Since each atom is inherently charge-neutral, this is also the number of electrons associated with the atom. The lower number in each box is the element's atomic mass.
When Na and Cl are brought together to create NaCl, each Na atom tends to give up one of its eleven electrons that the chlorine atoms kindly accept. This creates both positively charged sodium ions (Na+) as each sodium atom loses an electron, and negatively charged chlorine ions (Cl-) as each chlorine atom gains an electron. Due to the attractive forces between positive and negative ions, NaCl forms through ionic bonding. Click on Figure 3 to see more.
In the case of the ionic bonding of NaCl, all of the electrons are tightly bound, there is no "sea of electrons" as in the case of a conductor. Thus NaCl does not readily allow charge flow. Indeed, it takes a significant amount of energy to free the electrons in NaCl before any appreciable amount of charge will flow. It is interesting to note that when salt is dissolved in water, the sodium and chlorine ions are set free. Since ions are charged, their motion can create what we will refer to as electric current. While NaCl is an example or an insulator, it is not used in electric circuits and electronics. Rather, some of the most widely used insulators in modern electronics are various oxides.
While one could describe an insulator by quoting its conductivity, clearly the conductivity of an insulator should be extremely small. Rather than quoting conductivity, insulators are typically characterized by their resistivity (ρ). Resistivity is the inverse of conductivity, that is,
Resistivity = 1/Conductivity
ρ = 1/σ → Ohm-m
The units of resistivity are commonly given in Ohm-meter (or Ohm-cm). We will become very familiar with the unit of "Ohm" which is named after an important German physicist. Comparing the units of conductivity and resistivity, we should recognize that the Siemen is the inverse of the Ohm. Click the continue button to try to identify various materials by either their conductivity or resistivity.
It has been noted that Aluminum is a conductor. Using the fact that the conductivity of Aluminum is 3.77x107 [S/m], attempt to identify the following materials as either conductors or insulators. Pay attention to whether the conductivity or resistivity is given!
Glass → 1x10-11 [S/m]
Copper → 1.678x10-8 [Ohm-m]
SiO2 → 1x1015 [Ohm-m]
As the name suggests, semiconductors are not quite conductors. Instead, they are insulating, but under the right conditions they become reasonably good electric conductors. As in the case of insulators, semiconductors are not characterized by a sea of electrons. Consider again a portion of the periodic table as shown in Figure 4 below.
Figure 4: A portion of the periodic table. Click on the Figure to learn more about how Si atoms bond to form crystal silicon -- one of the most profoundly important electronic materials.
As described in Figure 4, silicon atoms come together to form a crystal. Each silicon atom is connected to four other silcon atoms via a covalent bond, leaving no free electrons as each covalent bond requires two electrons. How can silicon begin to allow electrons to flow in the crystal if none are free? Some of the covalent bonds must be broken! The energy required to free electrons from their bonds in a semiconductor is much less than that required in an insulator. The energy to free electrons may come in the form of heat or light for example, and thus semiconductors may be used to detect changes in temperature and light intensity. At rooom temperature pure silicon has a resistivity of approximately 2.5x103 Ohm-m which yields a conductivity of 4x10-4 Siemens/m. Increasing the temperature 10 degrees from room temperature might increase silicon's conductivity by 50%. While such an increase in conductivity might be useful when developing a temperature sensor, the corresponding conductivity is still quite low. If the goal is to significantly increase the conductivity of a semiconductor such a silicon, changing its temperature is not what is done.
The conductivity of semiconductors may also be dramatically changed through a process known as doping. Doping consists of adding foreign atoms to a previously pure material. For example, phosphorus (atomic number of 15) which has one more electron per atom than does silicon may be added to a silicon crystal to create free electrons without the need for an increase in temperature or light intensity. One only needs to replace one in every ten million silicon atoms with a phoshorous atom to make a large change in silicon's conductivity. Other important semiconductors include Ge and compound semiconductors such as GaAs. Later in this course and in your future studies, you will begin to learn about diodes and transistors, both critically important electronic devices that may be built from doped semiconductors. Diodes allow charge to flow in only one direction and transistors may be used to create efficient switches to control charge flow or may be used to increase the strength (amplify) of a signal.
We have learned that a material's electrical conductivity describes the degree to which a material allows the free motion of charge. But what impact does the geometry of the material have? Consider Figure 5 that shows a sketch of a generic bar of material in which electrons flow left to right. The bar has a length, L, and a cross-sectional area, A. The lightly-shaded plane is an "observation plane" through which the number of electrons passing will be counted every second.
Figure 5: A block of material of length "L" and cross-sectional area "A". Assume electrons are moving along the block in the direction of the arrow. The lightly-shaded plane is the "observation plane" through which the number of electrons passing will be counted.
Let's assume that a constant energy per unit charge is expended to push free electrons from left to right in the bar depicted in Figure 5. Select all the following options that are correct regarding the flow of electrons in the bar of Figure 5 under such a force.