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 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 one-way crystals. By the mid-twentieth century, homemade radio receivers using galena crystals had become quite popular among hobbyists.

As the electronics and 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 trial-and-error. Two development paths were followed—solid-state diodes and vacuum tube diodes. By the mid-twentieth century, inexpensive germanium-based diodes had been developed as solid-state devices. The problem with solid-state diodes was that they could not 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 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 as 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 the 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 electron 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 semiconductors.

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-biased 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 passes 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.

Schottky Diodes. All diodes require at least a small forward bias voltage to work. Schottky 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 Schottky 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 regularly 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. 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 their power efficiency. White light-producing LEDs in home lighting systems decreased power consumption as they used less power than conventional systems. It is also used on television screens, computers, and smartphones. LEDs are also used in agriculture for growth and photomorphogenesis (light-regulated plant growth patterns), thus improving yield and quality. Further research focused on developing and improving Perovskite light-emitting diodes (PeLEDs), which may be green, blue, red, or white.

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 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 laser light production methods, so they have become more common. Most lasers that do not require high-power applications are based on laser diodes. Laser diodes have far-reaching applications in hair removal, medical treatments, fiber-optic data transmission, data storage, barcode scanners, dental surgeries, and more.

Careers and Coursework

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.

Developing new types of diodes requires considerable knowledge of solid-state physics, materials science, and semiconductor manufacturing. Advanced degrees in these fields would often 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 research in this area is concerned with determining how to manufacture or include smaller diodes in integrated circuits. The 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 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 require much knowledge about diodes for technicians who are making semiconductor devices. Such technicians need coursework and training in operating the equipment used to manufacture semiconductors and semiconductor devices, and they must be able to follow directions meticulously when 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 requires little knowledge of the circuit elements themselves, save for the ability to identify them by sight. However, it would be helpful to understand basic diode behavior. Basic coursework in circuits is 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 they are using them. Because electronics are part of most people’s everyday lives, diodes and diode technology will continue to play an essential role in everyday devices. Diodes are simple devices, however, and it is unlikely that the field will advance further in developing basic diodes. 

Specialized devices using the properties of p-n junctions, such as laser diodes, continue to be necessary. Additional uses of p-n junctions are likely, and new types of diodes will develop 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 research continues to explore the possibility of such devices replacing other forms of lighting. For example, OLED (organic LED), which is mercury-free and does not use n-type and p-type semiconductors, is called organic because it is made from carbon and hydrogen. OLEDs have improved image quality, brightness, and power efficiency. Despite their high cost, they are used in mobile phones, digital cameras, tablets, laptops, and televisions. Moreover, several companies worked to bring down the cost of OLEDs. Research efforts also focus on increasing the cooling capacity of LEDs.

Furthermore, light-fidelity (Li-Fi) wireless communication gained popularity in the twenty-first century. Li-Fi uses LEDs to transmit data rather than radio waves, with a speed of over 100 gigabits per second.

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