In this blog post, we’ll take a simple look at everything from what a semiconductor is—including its energy band structure and the principles of electrical conductivity—to the formation process of n-type and p-type semiconductors, as well as the various applications of semiconductors, such as diodes and transistors.
We often come across semiconductors in various media, such as newspapers and news reports. In videos, we see researchers wearing white lab coats and safety goggles conducting experiments with circular semiconductor wafers, or machines in automated facilities producing semiconductors. Observing these research and production processes may spark curiosity about what semiconductors actually are, how they work, and in which fields they are used. Even if you search the internet to satisfy these curiosities, the results are often dominated by technical content that is difficult to understand; therefore, this article aims to help readers understand the subject through simpler explanations.
Before understanding the principles of semiconductors, it’s important to note that solids can be broadly classified into conductors, semiconductors, and insulators based on their electrical properties, with semiconductors exhibiting electrical characteristics intermediate between those of conductors and insulators. Therefore, let’s first examine conductors and insulators before exploring semiconductors, which possess intermediate properties. Conductors are materials that conduct electricity well; most metals fall into this category. In contrast, insulators are materials that do not conduct electricity well; common examples found in our daily lives include wood and rubber. The reason conductors conduct electricity well while insulators do not can be explained by the energy band structure of solids.
Energy bands consist of the valence band, the band gap, and the conduction band. The valence band is the energy region primarily occupied by electrons; the band gap represents the region between the valence band and the conduction band where electrons cannot exist stably due to the energy difference; and the conduction band is the energy region where electrons can move once they acquire sufficient energy. Electrons can generally exist only in permitted energy regions, such as the valence band or the conduction band, and cannot exist in a stable state within the band gap. The phenomenon of electric current flow can be understood from two main perspectives. The first is when electrons in a material gain energy and move into the conduction band, and the second is when vacancies that electrons can occupy are created in the valence band. For the first case, it is sufficient to understand that electrons move to the conduction band and thus participate in electrical conduction; the second case is easier to grasp if you imagine a crowded room where people have difficulty moving, but once a few people leave, others can move through the empty spaces. The vacancy created when an electron leaves the valence band is called a hole.
Applying this concept to conductors and insulators, conductors have a band structure in which the energy levels where electrons can move are closely spaced, or where the valence band and conduction band overlap; thus, electrons can participate in electrical conduction relatively easily. However, in insulators, a very wide band gap exists between the valence band and the conduction band, so a very large amount of energy is required for an electron to move into the conduction band. Since it is difficult to continuously supply such a large amount of energy under realistic conditions, almost no current flows in insulators.
Semiconductors possess electrical properties intermediate between those of insulators and conductors; in their band structure, a band gap exists between the valence band and the conduction band, but its width is narrower than that of insulators. Therefore, semiconductors require less energy to move electrons into the conduction band than insulators do; while current does not flow as freely as in conductors, it flows more readily than in insulators. Another characteristic of semiconductors is that their electrical properties can vary significantly depending on factors such as temperature and the addition of impurities.
After understanding the energy band structure of semiconductors, researchers conducted extensive studies to control their performance and increase their electrical conductivity. One method devised to increase electrical conductivity is to introduce small amounts of impurities into a pure semiconductor. This process is called doping. For example, in the case of pure silicon, arsenic can be introduced between the silicon atoms, or elements such as indium can be added. By adding small amounts of impurities in this way, the electrical properties of silicon can be intentionally altered.
In the case of arsenic, since it has one more valence electron than silicon, when it is introduced between silicon atoms, it forms bonds with the surrounding silicon atoms but still has one electron remaining. This remaining electron is in a state where it can move relatively more easily than electrons involved in bonds, and it can participate in conduction even with a small amount of energy. An increase in the number of electrons that can easily participate in conduction means an increase in electrical conductivity. A semiconductor in which impurities capable of donating electrons are introduced to improve electrical conductivity—as opposed to a pure semiconductor—is called an n-type semiconductor. In this context, impurities that donate electrons, such as arsenic, are called donors.
Conversely, in the case of indium, since it has one fewer valence electron available for bonding than silicon, when it is inserted between silicon atoms, one of the positions that would normally form a bond remains vacant. This relates to the second of the two cases of current flow discussed earlier in the energy band structure of solids. When a vacancy is created in the valence band where an electron can escape, surrounding electrons fill that vacancy, triggering a chain reaction of electron movement that contributes to electrical conductivity. Such an electron vacancy is called a hole, and a semiconductor designed so that holes act as the primary charge carriers is called a p-type semiconductor. A p-type semiconductor is one in which the electrical properties have been altered by introducing an element with fewer electrons available for bonding than in a pure semiconductor; an impurity that creates holes, such as indium, is called an acceptor.
A diode is a representative device that utilizes these n-type and p-type semiconductors in everyday life; let’s take a brief look at diodes. A diode is fundamentally composed of a structure formed by joining an n-type semiconductor and a p-type semiconductor. If the (+) terminal is connected to the p-type semiconductor side and the (-) terminal to the n-type semiconductor side, a forward bias is formed, allowing current to flow relatively easily; conversely, if the (-) terminal is connected to the p-type semiconductor side and the (+) terminal to the n-type semiconductor side, a reverse bias is formed, and under normal conditions, almost no current flows. Therefore, diodes have the property of allowing current to flow primarily in one direction. By utilizing this property, we can construct rectifier circuits that convert alternating current (AC)—where the direction of the current constantly changes—into direct current (DC) that flows in a single direction. Diodes are used not only for rectification but also in various fields such as power supply circuits, signal processing, and light-emitting diodes (LEDs).
Another representative example of the practical application of n-type and p-type semiconductors is the transistor. Transistors can typically be configured as NPN transistors with an n-p-n structure or PNP transistors with a p-n-p structure. Transistors offer the advantage of controlling larger currents or power, or amplifying signals, using small input signals; these characteristics are utilized in various electronic devices, including audio amplifiers and speaker systems. Furthermore, transistors are not only used to amplify signals but also serve as switches to control the flow of current, making them core components in countless electronic devices, including today’s computers and smartphones.
So far in this article, we’ve explored what semiconductors are, how they work, and their applications. While we often encounter information about semiconductors in the media, much of it focuses on difficult research conducted in laboratories or complex production processes, which might have led you to think that semiconductors are somewhat detached from our daily lives. However, semiconductors are used everywhere in our daily lives—in speakers, smartphone memory and processors, computers, automobiles, and home appliances—and they are so widely utilized that it is difficult to find a modern electronic device that does not use them. Therefore, I hope that after reading this article, readers will feel more familiar with semiconductors and develop a greater interest in them.
