Electricity and Conductivity

Electricity is used to describe a bunch of different things, but isn’t really a “thing” at all. More accurately, electricity describes the relationship between charge and current (BBC, n.d.).

Electric charge is a fundamental property of matter born by two of the particles that make up the basic building blocks of matter – protons and electrons (Encyclopedia Britannica, 2021). To understand how protons and electrons interact with one another, let’s harken back to middle school physics and remember the solar system–like structure of an atom. In our model, each atom has a nucleus made up of positively charged protons and neutral neutrons stuck together in a ball. Surrounding the nucleus are a bunch of negatively charged electrons whizzing about. Atomic structure is held together by the balance between two forces – electromagnetic force and strong force, which we can see from the electron cloud model in Figure 1-1. While many physics textbooks depict electrons orbiting the nucleus of an atom along neat concentric lines, in real life they are much more disorganized and are better pictured as a field or cloud (Williams, 2016).

Electromagnetic force causes opposite charges to attract and similar charges to repel one another. It is the force responsible for keeping electrons close to the nucleus and moving between atoms. Strong force is what holds the neutrons and protons together despite protons having similar charges. In some elements, electrons stay close to the atom’s nucleus, but in other elements, electrons are constantly bouncing around to other nearby atoms. Elements with these more active electrons are called conductors.

A schematic diagram of an atom with four labeled parts. A nucleus, composed of positive protons and neutrons O, is surrounded by a negatively charged electron cloud.

In a conductor like copper, electrons are constantly jumping from atom to atom. Every time an electron in one copper atom jumps to a neighboring copper atom, the transmitting atom and receiving atom each receive a charge – the transmitting atom with one fewer electron (atom 1) now has a positive charge and the receiving atom with one greater electron (atom 2) now has a negative charge. Once atom 1 has a positive charge, the electromagnetic force that causes opposite charges to attract will draw a nearby electron from atom 3, which will quickly jump in to fill the void. Atom 1 will now be neutral while atom 3 is now positive, thus attracting yet another electron from atom 4. We can see this process play out in Figure 1-2.

This process continues constantly in everything you see; we just can’t tell because these movements are happening randomly in all directions and in aggregate cancel each other out. This canceling out effect is why everyday objects don’t have a negative or a positive charge – each thing we encounter may contain billions of positively and negatively charged atoms at any given time, but collectively, each object as a whole is neutral with no charge at all. This is why your couch doesn’t electrocute you every time you sit down! In a neutral state, electrons may jump from atom to atom at random, but what happens if instead of randomly moving from place to place, these electrons were given some guidance?

The diagram exhibits 5 sequential sets of labeled 1 to 4 atoms, which have a positive 1, negative 1, or neither. The final cluster exhibits 4 atoms that are surrounded by labeled 6 to 10 atoms, where some are connected via electrons.

Electric current is what results when electrons flow in the same direction (Science World, n.d.). “Flow” does not mean that electrons themselves are moving along at the speed of the current – this is a common misconception. What is really happening is the process we described previously, with electrons leaving their home atom to join a neighboring atom, which subsequently loses an electron to a 3rd neighboring atom, which loses an electron to a 4th atom, and so on and so forth. The collective effect of these movements is the transmission of electric current along a wire at nearly the speed of light, even though individual electrons travel only a few millimeters a second (Mitchell, n.d.).

To illustrate this process, we can zoom in to see how current is transmitted along a wire (see Figure 1-3). Remember that electromagnetic force causes opposite charges to attract. If we apply a positive charge to one end of the wire (to the left of Atom 1), Atom 1 will lose an electron, which is drawn to the positive charge. Atom 1 is now missing an electron and has a net positive charge. An electron in neighboring Atom 2 is now drawn to the positive charge and jumps to Atom 1, leaving Atom 2 with a positive charge. An electron from Atom 3 now jumps to Atom 2 and the chain goes on and on. Positive charge is “transferred” along the wire from one atom to the next, even though the electrons are moving in the opposite direction. The same basic process is happening with random electron movement in neutral objects like we saw in Figure 1-2, the only difference is that we’ve now given it direction.

A schematic diagram exhibits nine sets, called steps of labelled 1 to 8 atoms in a horizontal orientation. The 1-labeled atom in step 2 loses an electron to another atom, resulting in a highlighted diagonal pattern covering the second to eighth atoms.

You can picture this process by imagining a line of pool balls on a table (see Figure 1-4). If you hit the ball on one end of the line, the ball at the other end will move as though it was hit by you directly. In the same way that the kinetic energy (energy of motion) was transferred from the number 1 ball at the front of the line to the number 15 ball at the back, the energy from the atoms at the front of a copper wire can be transferred to the atoms at the back of the metal wire through an electric current (Beaty, 1999). The strength of an electric current is expressed in units called amperes (amp or A), which measures how many electrons flow past a given point in a single second (Ada, 2013).

The diagram exhibits a line labeled 1 to 15 balls as atoms in a conductor on a pool table. The cue ball is in front of the first ball with the cue stick behind it acting as the voltage E M F.

Figures 1-3 and 1-4 help us picture what’s happening within a wire at an atomic level. If we zoom out a bit and see what’s happening along the wire as a whole, we can see that current and the electrons flow in opposite directions (see Figure 1-5). If this is confusing, a useful heuristic is to remember that conventional current flows from positive charge to negative charge. We know that electrons (-) are attracted to the positive charge (+) and thus flow from negative to positive, so the current must flow in the opposite direction. By creating a charge differential, we’ve initiated the chain reaction of electron movement between atoms that allows current to flow along a wire. The difference in charges between two objects is called electric potential and is responsible for the flow of charge through a circuit.

A schematic diagram exhibits atoms moving rightward due to the electron flow, where the clockwise cycle of current direction and electron movements is observed, and against the conventional current flow.

So how exactly do we get electrons to form a current from one point to another? The answer lies in voltage (V), also called electromotive force (EMF or E) (Nave, 2000). You can think of voltage like the water pressure in a hose, except that instead of pushing water out onto your lawn, it is pushing electrons to move from point A to point B (Nussey, 2019). Technically speaking, voltage exists wherever there is a difference in charge between any two places.

If your cell phone has a negative charge and your charger has a positive charge, a voltage exists between them. If your dog has a positive charge and your cat has a negative charge, a voltage exists between them. If your boss has a negative charge and your car has a positive charge, a voltage exists between them. Though it is highly unlikely that your boss or your dog has a net positive or negative charge, what’s important is the charge differential. If we connect your cell phone to your charger, dog to cat, or boss to car with a conductor like a copper wire, an electric circuit will form through which electric current can flow.

A circuit is any closed loop between a source of voltage like a battery, a conductive wire, and other electrical elements (Rice University, 2013). Don’t let the fancy terminology scare you – if you shuffle across the carpet in winter and touch your friend, you are forming a circuit. Current flows (in the form of charges) from you, through your friend, and to the ground. Ouch!

Another name for voltage is potential difference, which describes the amount of work required to bring a positive charge from one point to another (Electrical Potential, 2020). An object’s electric potential is determined by its charge condition at a given point in time. A positively charged object is considered to have a higher potential than one that is negatively charged. If we connect an object with a higher potential to an object with a lower potential using a conductor, the electrons will flow from the low potential body to the high potential one, while current flows from high potential to low. The greater the difference in charges between the two objects, the greater the voltage. We can see potential difference illustrated in Figure 1-6.

Remember how current flows along a wire – with a positive charge moving along from one end to the other as electrons are transferred between neighboring atoms in the opposite direction. To start this chain reaction and transfer positive charge along the wire, we need an initial difference in charges between two parts of the circuit. Why else would the electron from Atom 1 in our diagram shift to the left and keep the current moving in the same direction? Electric current cannot flow without voltage – it is this potential difference that allows electricity to light our homes, heat our water, and do the work we need to power our lives.

A schematic diagram exhibits the random flows of atoms under no current flow. Another diagram exhibits atoms following the electron flow with the application of high potential, positive and low potential, negative on the sides.

If we imagine any two bodies with a difference in charges, whether they be the two ends of a battery powering a light bulb or between the key and the thunder clouds in Benjamin Franklin’s famous kite experiment, a potential difference exists between them. We can activate this potential difference by connecting them with some sort of conducting material or a direct connection between the objects themselves, thus forming a circuit. For a circuit, this might mean connecting one end of the battery with the other, which causes current to flow from one end of the circuit to the other and turns on the light. For Ben Franklin, this could be a connection between a cloud and the key at the end of his string. In both cases, positive charge flows from the object with higher potential (positive end of the battery or the thunder cloud) to the object with lower potential (negative end of the battery or the key).

Franklin was trying to prove that lightning was an electrical discharge that can be redirected safely into the ground and away from flammable structures. We can see a famous drawing of his daring work published in 1876 by one of America’s most prolific printmaking firms Currier & Ives in Figure 1-7. The experiment led to the invention of lightning rods that have protected buildings and people to this day (Currier & Ives, 2009).

A painting of Benjamin Franklin flying his kite in an open field during a thunderstorm together with his son. Two houses at a considerable distance from each other are observed in the background as well as various trees and plants.

Rather than depending on potential difference in the natural environment, batteries work by lifting the charge and therefore potential of one end of the circuit (+) relative to the other end (-). Current will not stop flowing until all of the surplus negative charges on the negative part of the circuit have flowed to the positive side of the circuit and the battery is empty. We can see this illustrated in Figure 1-8.

If you’ve ever wondered why there’s a positive (+) and negative (-) side to a household battery, it marks each end necessary to form a potential difference and power whatever appliance you’re using it for. The materials that make up a battery enable it to create this electronic potential – a cathode made of a material “in need” of electrons forms the positive (+) side of the battery (where the current leaves or electrons enter) and an anode made of a material “with excess” electrons forms the negative (-) side of the battery (where the current enters or electrons leave) (US Department of Energy, n.d.). The potential difference between each end is stored as chemical energy, which is then converted into electrical energy as the battery is used.

A schematic diagram exhibits the components needed to light bulb. A battery with a high potential charge is connected to a bulb, which is then connected to the battery's low potential charge, via a conducting wire.

Current and voltage tell us how electricity works in principle, but to understand what electricity can really do, we need to talk about Power. Power describes the work done by an electric circuit when an electric current is converted into some form of useful energy. This useful energy could be a plethora of things – motion, light, heat, sound, satellite signal, etc. When your speaker plays your favorite song or your bedside lamp helps you read before bed, they are turning electrical power into useful energy. Power is measured in watts (W). One watt measures the amount of work executed by an electronic circuit in which one amp of current is “pushed” by one volt (Electronics Tutorials, 2021).

Remember, amps measure the number of electrons flowing across a given point over time (current), while volts measure the amount of pent-up electrical pressure between two points due to potential difference (voltage). You can see a summary of these three forces, as well as electrical resistance, in Figure 1-9.

A table has 2 columns and 4 rows. The columns are labeled unit and definition, while the leftmost rows are labeled current, voltage, power, and resistance.

The relationship between Power (P), Voltage (E), and Current (I) is reflected in Joule’s Law, which was named after the English physicist James Prescott Joule who discovered it in 1840 (Shamieh, n.d.). You don’t need to memorize this equation, it’s just good to understand how the three are related and how voltage and current can work together to create something we can use (power). If you do plan on using this equation, it’s important to use the correct units for power (watts), voltage (volts), and current (amperes).

Joule’s Law: Power (P) = Voltage (E) x Current (I)

We can think of power, voltage, and current like the flow of water from a water heater to a shower head. Charge, in our analogy, is like the water itself – it’s what moves through the system to get stuff done. We can pretend voltage is like water pressure and current is the flow of water (or charge) throughout the system. If we multiply the water pressure (voltage) by the flow of water (current), we deliver the power that comes out the other end – the more power, the better the shower. We can see this analogy visualized in Figure 1-10.

A diagram of the flow of current from a water heater to the head of a person. Equations of voltage, E multiplied by current I = power P and water pressure multiplied by water flow = shower power are exhibited above.

Electricity describes the relationship illustrated by Joule’s Law. It is the phenomena in which voltage is applied to drive current, which happens when electrons are pushed to flow in the same direction. This happens at large scale in your local utility company that ships power to your home. But let’s face it, that's not really the interesting part to everyday consumers. The challenge is to figure out how to harness this current to do something useful. For that, we need a way to control the current we have created. All these flowing electrons (current) must be channeled through a material of some kind to make them useful as electricity. We need materials with conductivity.

Conductivity is a measure of how easily electric current can pass through a material. The key to performing useful work with electricity is to control conductivity – allowing current to flow in some cases and restricting it in others. That overhead light in your room would be much less useful if it was just on all the time or off all the time. Turning the current on and off is critical.

Electrical wiring is commonly made of a conductor, like copper, encased by an insulator, like rubber (See Figure 1-11). The insulator protects the wire and the surrounding environment by absorbing the excess electrical energy given off by the conductor. Semiconductors, containing properties from both conductors and insulators, are better able to control the flow of electricity, allowing engineers to create smaller, more intricate systems.

The diagram exhibits multicolored wires, which are protected by rubber inner and outer sheaths, made of a copper conductor within rubber insulation. A close-up shot of electrical wiring is on the right.

By harnessing the properties of each of these types of materials, engineers can build elaborate systems that store and send information, solve complex problems, and perform all kinds of tasks that make modern technology possible.

Since the semiconductor is the key to this technology, let’s learn a little more about what a semiconductor is and how they are built.

Silicon – The Crucial Semiconductor

There exists a large variety of semiconductor materials, each with varying levels of conductivity. Though other semiconductors, like germanium and gallium arsenide (GaAs), are also used in electronic devices, the vast majority of electronics are made using an element called Silicon (Si14 for all you periodic table enthusiasts out there). Silicon has numerous advantages that make it an ideal material for building computer chips – in addition to useful mechanical and thermal properties, it is inexpensive and abundant. Comprising roughly 30% of the Earth’s crust, silicon is the second most abundant element in Earth’s crust after oxygen and can be found in sand, rocks, clays, and soils (Templeton, 2015). We can see a picture of purified silicon in Figure 1-12.

A photograph of purified silicon on a patterned floor. Its reflective surface is observed to feature many edges.

A Quick Semiconductor History

Before we dive deep into the details of the semiconductor design and manufacturing technology, it can be helpful to know just a little bit about its history and key inventors. Once scientists discovered the semiconductor properties of silicon, they were able to build simple transistors, which are basically switches that prevent current from moving forward or allow it to pass. By arranging transistors in intricate patterns, they realized they could selectively guide current along a path of their choosing and make it do some useful work along the way. For about a decade after the first transistor was invented in 1947, semiconductor design and manufacturing were slow, cumbersome, and costly.

Individual transistors and other components had to be manufactured independently, then fit together manually using “flywire” connections, which are basically metal wires connecting transistors to one another one at a time. A complete transistor circuit could literally fill an entire room. This was not going to be the basis for any kind of world-altering technological revolution.

All this changed in 1959, which can be officially observed as the beginning of the semiconductor revolution (doubtful we can get a national holiday and a day off out of it, but still worth noting). This was due to two key events. First, the invention of the integrated circuit by Jack Kilby at Texas Instruments, and Robert Noyce at Fairchild Semiconductor allowed hardware designers to fit a bunch of transistors together on a single chip (Nobel Media, 2000) (Kilby, 1964). Second, the invention of planar manufacturing by Jean Hoerni at Fairchild Semiconductor allowed chip companies to fabricate multitudes of components at the same time and on the same substrate (semiconductor base material, kind of like the foundation of a house but for computer chips) (Nobel Media, 2000) (Hoerni, 1962). Kilby received a Nobel Prize for Physics in 2000 for his work. Hoerni (deceased, 1924–1997) never received a Nobel Prize, but is widely recognized for his contributions. The importance of these core innovations – the integrated circuit (IC) and planar manufacturing process – cannot be overstated. They serve as the foundation of the design and manufacturing-based value chain that semiconductors and computers are built with to this day. We will cover each in detail in Chapters 3 and 4.

Semiconductor Value Chain – Our Roadmap

In trying to discuss a complex technical topic like semiconductors, it can be a real challenge to decide how exactly to tell the story. Should we start with “Once upon a time…” and proceed chronologically up to today? Should we start with the smallest elements like atoms and electrons, and work our way up to huge systems like computers and cars?

In Understanding Semiconductors, we’ve decided to tell the story just like a semiconductor company operates – from deciding what products to build, to designing and manufacturing them, to packaging and integrating them into the system.

We call this sequence the Semiconductor Value Chain, and it will serve as our roadmap for our entire journey. We’ll go on a few detours to discuss some fundamentals and a couple of special topics, but for the most part, everything that follows in this book can be tied back to a part of this core sequence and can serve as a mental foundation on which to build your understanding of the industry as a whole.

For all you visual learners out there, Figure 1-13 features a step-by-step conceptual framework of the Semiconductor Value Chain, from design to delivery.

The diagram exhibits six illustrated steps namely customer need and market demand, chip design, fabrication and manufacturing, I C packaging and assembly, system integration, and product delivery.

For the most part, semiconductor companies concentrate on Steps 2–5. And, in fact, some companies (called ‘fabless’ companies) really only do step 2. They design the chip, and then outsource most of the other steps. Companies start by forecasting market demand or collecting orders from downstream device companies and focus their energy on building chips that can satisfy the needs of their customers. Since the industry’s inception in the 1940s, this value chain has remained, conceptually, relatively stable. At the same time, however, the organization and business strategies of how each step is done have been incredibly dynamic, driven by innovative companies competing to provide the best performing chips and highest quality products. We will use this value chain to guide our discussion of key sub-processes and anchor our understanding of each step to the bigger picture.

Performance, Power, Area, and Cost (PPAC)

For semiconductor companies focused on the design, manufacturing, assembly, and integration portions of the value chain (steps 2, 3, 4, and 5), the goal is to achieve the Highest Performance, using the Lowest Power and the Smallest Area possible. These three key design metrics are typically measured in clock frequency (Hz), watts, and nanometers (nm), respectively. Each semiconductor design may trade off one of these for the others, depending on the application. For example, a team designing a chip for a server in a data center with plenty of space and an industrial grade power source may focus on performance, while not caring as much about size or power. A team designing a chip for a battery-powered cell phone, however, may be concerned more about power and size than performance. For any given application, the goal is to optimize a chip design along these three constraints at the Lowest Cost and in the Shortest Time frame possible. We can better picture how these factors relate to one another in Figure 1-14.

Each chip must balance PPAC constraints to provide an optimal solution. Design teams must keep in mind the problem they are trying to solve and the application the circuit they are building is meant to address. For example, plugged-in devices like desktop computers may not have to optimize for power consumption as much as battery-powered laptops. Laptops, in turn, may have greater flexibility on area than smaller, hand-held devices like cell phones. It is important not to forget time as a key constraint as well – it may be worth lower performance if you can cut your design cycle short and beat your competitors to market. There are virtually limitless applications of circuits, all with unique performance, power, area, cost, and time constraints – the important thing is to understand the trade-offs.

In the back-and-forth of semiconductor product development, the marketing and business teams always want the best of all three metrics: highest performance, lowest power, and smallest area. In making trade-offs, the engineering teams frequently respond with, “You pick two, we get to pick the other one.” While this is a bit snarky from the engineering teams, it does highlight the trade-offs required. For example, if you really must have the highest performance and smallest area, the laws of physics will limit your efficiency on power.

A diagram exhibits the interconnections among power, performance, area, and cost via a three-dimensional triangle. Each has a bulleted list of its own aspects.

Who Uses Semiconductors?

Before we move into the more technical details of how chips are designed and fabricated, it’s important we don’t lose sight of why we’re building them in the first place. The Semiconductor Industry Association is one of the key trade groups in the semiconductor industry, and they define six different categories for end-use applications (see Table 1-1).

Table 1-1

SIA Framework – End-Use Applications

A table has 2 columns and 6 rows. The columns are labeled market and specific applications, while the leftmost rows are labeled consumer, automotive, computing, industrial, communications, and government.

Within these categories, the most recent 2021 report by the World Semiconductor Trade Statistics (WSTS) and Semiconductor Industry Association (SIA, 2021), states that “Communications” and “Computer & Office” accounted for the greatest proportion of semiconductor sales, combining for nearly two thirds of industry revenues. But as cars become more electrified, and as industrial operations become more and more automated, analysts expect the Automotive and Industrial and Instrument categories to grow in the future. We can see a breakdown of what end-use applications semiconductors are used for in Figure 1-15.

A pie chart has 6 slices. Computing holds the biggest slice with 32% followed by communications with 31%, industrial and consumer with 12% each, automotive with 11%, and government with 1%.

Each of these applications has different PPAC requirements and drivers based on their unique purpose, which impacts each stage of the design, manufacturing, and assembly processes.

Summary

In this chapter, we examined how electricity and conductivity work together to make semiconductor technology possible. We discovered what semiconductors are, what they do, and why silicon is the most useful one. We introduced the Semiconductor Value Chain that converts ideas and raw materials into chips and finished products. Finally, we reviewed the key PPAC parameters that shape how chips are optimized for their intended purpose and what those real-world applications actually are.

The ideal chip is high performance, requires low power, and takes up little space. Accomplishing all three would be ideal, but cost and time force us to make tough choices. Each step of the value chain is essential to get a chip from concept to customer – at every stage, companies fight for profits and market share by balancing these factors more effectively than their competitors.