Diode Technology

Summary

Diodes act as one-way valves in electrical circuits, permitting electrical current to flow in only one direction and blocking current flow in the opposite direction. The original diodes used in circuits were constructed using vacuum tubes, but these diodes have been almost completely replaced by semiconductor-based diodes. Solid-state diodes, the most commonly used, are perhaps the simplest and most fundamental solid-state semiconductor devices, formed by joining two different types of semiconductors. Diodes have many applications, such as safety circuits to prevent damage by inadvertently putting batteries backward into devices and in rectifier circuits to produce direct current (DC) voltage output from an alternating current (AC) input.

Definition and Basic Principles

A diode is perhaps the first semiconductor circuit element that a student learns about in electronics courses, though most early diodes were constructed using vacuum tubes. It is very simplistic in structure, and basic diodes are very simple to connect in circuits. They have only two terminals, a cathode and an anode. The very name diode was created by British physicist William Henry Eccles in 1919 to describe the circuit element as having only the two terminals, one in and one out.

Classic diode behavior, that for which most diodes are used, is to permit electric current to flow in only one direction. If voltage is applied in one direction across the diode, then current flows. This is called forward bias. The terminal on the diode into which the current flows is called the anode, and the terminal out of which current flows is called the cathode. However, if voltage is applied in the opposite direction, called reversed bias, then the diode prevents current flow. A theoretical ideal diode permits current to flow without loss in forward bias orientation for any voltage and prohibits current flow in reverse bias orientation for any voltage. Real diodes require a very small forward bias voltage in order for current to flow, called the knee voltage. The terms threshold voltage or cut-in voltage are also sometimes used in place of the term knee voltage. The electronic symbol for the diode signifies the classic diode behavior, with an arrow pointing in the direction of permitted current flow, and a bar on the other side of the diode signifying a block to current flow from the other direction.

Though most diodes are used to control the direction of current flow, there are many subtypes of diodes that have been developed with other useful properties, such as light-emitting diodes and even diodes designed to operate in reverse bias mode to provide a regulated voltage.

Background and History

Diode-like behavior was first observed in the nineteenth century. Working independently of each other in the 1870s, American inventors Thomas Alva Edison and Frederick Guthrie discovered that heating a negatively charged electrode in a vacuum permits current to flow through the vacuum but that heating a positively charged electrode does not produce the same behavior. Such behavior was only a scientific curiosity at the time, since there was no practical use for such a device.

At about the same time, German physicist Karl Ferdinand Braun discovered that certain naturally occurring electrically conducting crystals would conduct electricity in only one direction if they were connected to an electrical circuit by a tiny electrode connected to the crystal in just the right spot. By 1903, American electrical engineer Greenleaf Whittier Pickard had developed a method of detecting radio signals using the one-way crystals. By the middle of the twentieth century, homemade radio receivers using galena crystals had become quite popular among hobbyists.

As the electronics and the radio communication industries developed, it became apparent that there would be a need for human-made diodes to replace the natural crystals that were used in a trial-and-error manner. Two development paths were followed: solid-state diodes and vacuum tube diodes. By the middle of the twentieth century, inexpensive germanium-based diodes had been developed as solid-state devices. The problem with solid-state diodes was that they lacked the ability to handle large currents, so for high-current applications, vacuum tube diodes, or thermionic diodes, were developed. In the twenty-first century, most diodes are semiconductor devices made of silicon, with thermionic diodes existing only for the rare very high-power applications.

How It Works

Thermionic Diodes. Though not used as frequently as they once were, thermionic diodes are the simplest type of diode to understand. Two electrodes are enclosed in an evacuated glass tube. Because the thermionic diode is a type of vacuum tube, it is often called a vacuum tube diode. The geometry of the electrodes in the tube depends on the manufacturer and the intended use of the tube. Heating one of the electrodes in some fashion permits electrons on that electrode to be thermally excited. If the electrode is heated past the work function of the material of which the electrode is fabricated, the electrons can come free of the electrode. If the heated electrode has a more negative voltage than the other electrode, then the electrons cross the space between the electrodes. More electrons flow into the negative electrode to replace the missing ones, and the electrons flow out of the positive electrode. Current flow is defined opposite to electron flow, so current would be defined as flowing into the positive electrode (labeled as the anode) and out of the negative electrode (labeled as the cathode). However, if the voltage is reversed, and the heated electrode is more positive than the other electrode, electrons liberated from the anode do not flow to the cathode, so no current flows, making the diode a one-way device for current flow.

Solid-State Diodes. Thermionic diodes, or vacuum tube diodes, tend to be large and consume a lot of electricity. However, paralleling the development of vacuum tube diodes was the development of diodes based on the crystal structure of solids. The most important type of solid-state diodes are based on semiconductor technology.

Semiconductors, such as silicon, germanium, and gallium arsenide, are neither good conductors nor good insulators. The purity of the semiconductor determines, in part, its electrical properties. Extremely pure semiconductors tend to be poor conductors. However, all semiconductors have some impurities in them, and some of those impurities tend to improve conductivity of the semiconductor. Purposely adding impurities of the proper type and concentration into the semiconductor during the manufacturing process is called doping the semiconductor. If the impurity has one more outer shell electron than the number of electrons in atoms of the semiconductor, then extra electrons are available to move and conduct electricity. This is called a negative doped or n-type semiconductor. If the impurity has one fewer electrons than the atoms of the semiconductor, then electrons can move from one atom to another in the semiconductor. This acts as a positive charge moving in the semiconductor, though it is really a missing electron moving from atom to atom. Electrical engineers refer to this as a hole moving in the semiconductor. Semiconductors with this type of impurity are called positive doped or p-type semiconductor.

What makes a semiconductor diode is fabricating a device in which a p-type semiconductor is in contact with an n-type semiconductor. This is called a p-n junction. At the junction, the electrons from the n-type region combine with the holes of the p-type region, resulting in a depletion of charge carriers in the vicinity of the p-n junction. However, if a small positive voltage is applied across the junction, with the p-type region having the higher voltage, then additional electrons are pulled from the n-type region and additional holes are pulled from the p-type region into the depletion region, with electrons flowing into the n-type region from outside the device to make up the difference and out of the device from the p-type region to produce more holes. As with the thermionic device, current flows through the device, with the p-type side of the device being the anode and the n-type side of the device being the cathode. This is the forward bias orientation. When the voltage is reversed on the device, the depletion region simply grows larger and no current flows, so the device acts as a one-way valve for the flow of electricity. This is the reverse bias orientation. Though reverse bias diodes do not normally conduct electricity, a sufficiently high reverse voltage can create electric fields within the diode capable of moving charges through the depletion region and creating a large current through the diode. Because diodes act much like resistors in reverse bias mode, such a large current through the diode can damage or destroy the diode. However, two types of diodes, avalanche diodes and Zener diodes, are designed to be safely operated in reverse bias mode.

Applications and Products

P-n junction devices, such as diodes, have a plethora of uses in modern technology.

Rectifiers. The classic application for a diode was to act as a one-way valve for electric current. Such a property makes diodes ideal for use in converting alternating current into direct-current circuits or circuits in which the current flows in only one direction. In fact, the devices were originally called rectifiers before the term diode was created to describe the function of these one-way current devices. Modern rectifier circuits consist of more than just a single diode, but they still rely heavily on diode properties.

Solid-state diodes, like most electronic components, are not 100 percent efficient, and so some energy is lost in their operation. This energy is typically dissipated in the diode as heat. However, semiconductor devices are designed to operate at only certain temperatures, and increasing the temperature beyond a specified range changes the electrical properties of the device. The more current that passed through the device, the hotter it gets. Thus, there is a limiting current that a solid-state diode can handle before it is damaged. Though solid-state diodes have been developed to handle higher currents, for the highest current and power situations, thermionic, or vacuum tube diodes, are still sometimes used, particularly in radio and television broadcasting.

Shottky Diodes. All diodes require at least a small forward bias voltage in order to work. Shottky diodes are fabricated by using a metal-to-semiconductor junction rather than the traditional dual semiconductor p-n junction used with other diodes. Such a construction allows Shottky diodes to operate with extremely low forward bias.

Zener Diodes. Though most diodes are designed to operate only in the forward bias orientation, Zener diodes are designed to operate in reverse bias mode. In such an orientation, they undergo a breakdown and conduct electric current in the reverse direction with a well-defined reverse voltage. Zener diodes are used to provide a stable and well-defined reference voltage.

Photodiodes. Operated in reverse bias mode, some p-n junctions conduct electricity when light shines on them. Such diodes can be used to detect and measure light intensity, since the more light that strikes the diodes, the more they conduct electricity.

Circuit Protection. In most applications of diodes, they are used to take advantage of the properties of the p-n junction on a regular basis in circuits. For some applications, though, diodes are included in circuits in the hope that they will never be needed. One such application is for DC circuits, which are typically designed for current to flow in only one direction. This is automatically accomplished through a power supply with a particular voltage orientation such as a DC source, power converter, or battery. However, if the power supply were connected in reverse or if the batteries were inserted backward, then damage to the circuit could result. Diodes are often used to prevent current flow in such situations where voltage is applied in reverse, acting as a simple but effective reverse voltage protection system.

Light-Emitting Diodes (LEDs). For diodes with just the right kind of semiconductor and doping, the combination of holes and electrons at the p-n junction releases energy equal to that carried by photons of light. Thus, when current flows through these diodes in forward bias mode, the diodes emit light. Unlike most lighting sources, which produce a great deal of waste heat in addition to light (with incandescent lights often using energy to produce more heat than visible light), most of the energy dissipated in LEDs goes into light, making them far more energy-efficient light sources than most other forms of artificial lighting. Unfortunately, large high-power applications of light-emitting diodes are somewhat expensive, limiting them to uses where their small size and long life characteristics offset the cost associated with other forms of lighting. However, the development of blue LED in 2014 enabled the development of white light in combination with green and red LEDs. The discovery of white LEDs had a broad impact on various electronics industries due to its power efficiency. The use of white light-producing LEDs in home lighting systems brought down power consumption as it used less power than other conventional systems. It is also used in the screens of televisions, computers, smartphones, etc. LEDs are also used in agriculture for growth and photomorphogenesis (light-regulated plant growth patterns), thus improving yield and quality.

Laser Diodes. Very similar to light-emitting diodes are laser diodes, where the recombination of holes and electrons also produces light. However, with the laser diode, the p-n junction is placed inside a resonant cavity and the light produced stimulates more light, producing coherent laser light. Laser diodes typically have much shorter operational lifetimes than other diodes, including LEDs, and are generally much more expensive. However, laser diodes cost much less than other methods of producing laser light, so they have become more common. Most lasers not requiring high-power application are based on laser diodes. Further research in the field continues. For example, in 2020, Japanese electronics company Panasonic had developed a high-powered blue beam.

Careers and Course Work

The electronics field is vast and encompasses a wide variety of careers. Diodes exist in some form in most electronic devices. Thus, a wide range of careers come into contact with diodes, and therefore a wide range of background knowledge and preparation exists for the different careers.

Development of new types of diodes requires considerable knowledge of solid-state physics, materials science, and semiconductor manufacturing. Often advanced degrees in these fields would be required for research, necessitating students studying physics, electrical engineering, mathematics, and chemistry. However, diode technology is quite well evolved, so there are limited job prospects for developing new diodes or diode-like devices other than academic curiosity. Most of this area of study is simply determining how to manufacture or include smaller diodes in integrated circuits. Institutions such as Georgia Institute of Technology and the University of Colorado Boulder offer related courses.

Electronics technicians repair electrical circuits containing diodes. So, knowledge of diodes and diode behavior is important in diagnosing failures in electronic circuits and circuit boards. Sufficient knowledge can be gained in basic electronics courses. A two-year degree in electronic technology is sufficient for many such jobs, though some jobs may require a bachelor's degree. Likewise, technicians designing and building circuits often do not need to know much about the physics of diodes—just the nature of diode behavior in circuits. Such knowledge can be gained through basic electronics courses or an associate's or bachelor's degree in electronics.

Manufacturing diodes does not actually require much knowledge about diodes for technicians who are actually making semiconductor devices. Such technicians need course work and training in operating the equipment used to manufacture semiconductors and semiconductor devices, and they must be able to follow directions meticulously in operating the machines. An associate's degree in semiconductor manufacturing is often sufficient for many such jobs. Manufacturing circuit boards with diodes, or any other circuit element, does not really require much knowledge of the circuit elements themselves, save for the ability to identify them by sight, though it would be helpful to understand basic diode behavior. Basic course work in circuits would be needed for such jobs. Aspirants can work as biophotonics engineers, display technologists, and computational physicists.

Social Context and Future Prospects

Diodes exist in almost every electronic device, though most people do not realize that they are using diodes. Because electronics have been increasing in use in everyday life, diodes and diode technology will continue to play an important role in everyday devices. Diodes are very simple devices, however, and it is unlikely that the field will advance further in the development of basic diodes. Specialized devices using the properties of p-n junctions, such as laser diodes, continue to be important. It can be anticipated that additional uses of p-n junctions may be discovered and new types of diodes developed accordingly. Because the p-n junction is the basis of diode behavior and is the basis of semiconductor technology, diodes will continue to play an important role in electronics for the foreseeable future. LEDs produce light very efficiently, and work is proceeding to investigate the possibility of such devices replacing many other forms of lighting. For example, OLED (Organic LED), which are mercury-free and do not use n-type and p-type semiconductors, are called organic because they are made from carbon and hydrogen. OLEDs have improved image quality, brightness, and power efficiency. They are used in mobile phones, digital cameras, tablets, laptops, and televisions despite their high cost. Moreover, several companies worked to bring down the cost of OLEDs. The efforts to increase the cooling capacity of LEDs are also under research.

Furthermore, in the twenty-first century, an effort to expand the use of Li-Fi (light fidelity wireless communication that uses LEDs to transmit data rather than radio waves, with a speed of over 100 Gigabits per second) is growing.

Bibliography

Gibilisco, Stan. Teach Yourself Electricity and Electronics. 5th ed. New York: McGraw-Hill, 2011. Comprehensive introduction to electronics, with diagrams. A chapter on semiconductors includes a good description of the physics and use of diodes.

Held, Gilbert. Introduction to Light Emitting Diode Technology and Applications. Boca Raton, Fla.: Auerbach, 2009. A thorough overview of light-emitting diodes and their uses. The book also includes a good description of how diodes in general work.

"LED Lighting History, Why Is the Blue LED Special?" LED by Vision, 24 Mar. 2017, www.ledbyvision.co.uk/article/led-lighting-history-why-is-the-blue-led-special. Accessed 13 Jul. 2021.

"LEDs: State of the Union." Arrow Electronics, 31 May 2021, www.arrow.com/en/research-and-events/articles/leds-state-of-the-union. Accessed 10 Jul. 2021.

Paynter, Robert T. Introductory Electronic Devices and Circuits. 7th ed. Upper Saddle River, N.J.: Prentice Hall, 2006. An excellent and frequently used introductory electronics textbook, with an excellent description of diodes, different diode types, and their use in circuits.

Razeghi, Manijeh. Fundamentals of Solid State Engineering. 3d ed. New York: Springer, 2009. An advanced undergraduate textbook on the physics of semiconductors, with a very detailed explanation of the physics of the p-n junction.

Schubert, E. Fred. Light Emitting Diodes. 2d ed. New York: Cambridge University Press, 2006. A very good and thorough overview of light-emitting diodes and their uses.

Turley, Jim. The Essential Guide to Semiconductors. Upper Saddle River, N.J.: Prentice Hall, 2003. A brief overview of the semiconductor industry and semiconductor manufacturing for the beginner.

"What Exactly Is LiFi?" LiFi.co, 2020, lifi.co/what-is-lifi. Accessed 10 Jul. 2021.