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The electric light bulb is an important symbol of human civilization. It brings light and convenience to our life. But you know what? Behind the light bulb lies a wonderful story about hot electron emission and vacuum tubes, which not only reveal the mysteries of electrons, but also open the door to the electronic age.

Edison effect: an accidental discovery that the spherical glass lampshade on the bulb is designed to protect the filament from oxidation. But Edison found that there was a layer of black on the glass shell after the bulb had been used for a long time. He wondered what it was. After thinking about it, I think that some impurities on the filament may be heated by electric current and evaporated, and then deposited on the glass spherical shell.

To test his conjecture, he added a piece of metal that did not come into contact with the carbon wire in the bulb, hoping it would absorb some of the material evaporated by the carbon wire. However, he was surprised to find that a weak current could be detected when an ammeter was connected between the metal sheet and the carbon filament while heating the filament.

This was an incredible thing at that time. Can electricity travel from the air? In fact, Edison discovered the phenomenon of hot electron emission, but he did not understand its principle at that time and did not study it in depth. He just used it as a by-product to measure the power of the light bulb. He called it the "Edison effect" and applied for a patent.

Thomson effect: the birth of the electron in the more than ten years after Edison discovered this effect, people have not carried out in-depth research and use of it. It was not until 1897 that Joseph John Thomson, a British physicist, did a historic experiment that revealed the secret behind the Edison effect.

Thomson placed a heated cathode and an anode in a vacuum tube and applied a high-voltage electric field between them. He observed that a negatively charged stream of particles (later called cathode rays) was emitted from the cathode and that the particles could be deflected by an applied electric or magnetic field. By measuring the deflection angle and intensity of the particle flow, Thomson calculated the mass-to-charge ratio of these particles and came to the astonishing conclusion that these particles were much lighter than hydrogen atoms and had the same amount of negative charge.

Thomson believes that these particles are the basic units that make up atoms, what we call electrons today. He called the discovery the "Thomson effect" and won the 1906 Nobel Prize in physics for it. The Thomson effect not only proves that the atom has an internal structure, but also reveals a new perspective of the relationship between the microscopic and macroscopic properties of matter.

Fleming effect: the birth of vacuum tubes the Thomson effect gives people a new understanding of the Edison effect: electrons are released from the surface of the heated cathode! So, can we use these electrons to achieve some useful functions? In 1904, British physicist John Ambrose Fleming gave an affirmative answer.

Fleming was the chief engineer of Edison in the UK and was involved in projects such as improving carbon filament light bulbs and radio communications equipment. He noticed that the use of an ordinary diode rectifier (consisting of two diodes connected in opposite directions) in a radio receiver would cause a lot of signal loss, so he thought of using the Edison effect to improve the performance of the rectifier.

He encapsulated two metal sheets in a vacuum glass tube, one as the cathode and the other as the anode. He applied a DC voltage to the cathode to heat and emit electrons, and a high-frequency AC voltage to the anode to attract and repel electrons in plus or minus half a week, respectively. In this way, when alternating current passes through the device, it is turned into direct current.

Fleming called the tube with two electrodes a vacuum diode, which has both rectification and detection functions. It is the first electronic device in human history. Fleming applied the invention to radio detection and obtained a patent in the United Kingdom on November 16, 1904.

Electronic Triode: the birth of the Amplifier Fleming's vacuum diode has the function of rectifying and detecting, but it cannot amplify the signal because it has only two electrodes and cannot control the size of the electron flow. In order to amplify the signal, people begin to try to add an electrode to the vacuum tube to change the path and intensity of the electron flow. This is the origin of the electronic Triode.

The electronic transistor was invented by American physicist Lee de Forrest in 1906. He added a metal mesh structure to Fleming's vacuum diode as the third electrode, called the gate. The gate is located between the emitter and the collector, which can modulate and control the electron flow emitted by the emitter. When the gate adds a negative voltage, it hinders or reduces the electron flow to the collector; when the gate adds a positive voltage, it promotes or increases the electron flow to the collector. In this way, the current of the collector can be controlled by changing the voltage of the gate.

De Forrest called his invention Audion and used it for radio reception and amplification. He found that when the gate receives a weak AC signal, the collector outputs a larger AC signal, and the phase and waveform between the two are consistent. This is the principle of signal amplification. De Forrest used the invention in the fields of wireless telephone, wireless telegraphy and broadcasting, and patented it in the United States on January 29, 1907.

Although de Forrest's Audion has amplification function, it still has some defects, such as low output power, high distortion, high noise and so on. In order to improve the performance of Audion, people begin to optimize and innovate its structure and materials. For example, in 1913, American engineer Irving Langmuir added a water-cooled system to the Audion to enable vacuum tubes to withstand higher temperatures and pressures and increase output power; in 1915, British engineer Robert von Ribowski used oxide cathodes in Audion to improve emission efficiency and reduce thermal noise In 1916, American engineer Edwin Armstrong used a negative feedback loop in Audion to reduce distortion and improve stability.

After these improvements, the electronic Triode has become a kind of amplifier with excellent performance and wide application. It is not only used in the fields of wireless communication, broadcasting and telephone, but also in the fields of acoustics, instruments, computers and radar. It has also given birth to a series of new vacuum tube devices, such as Tetrode, pentode, hexagon and so on. Until the late 1950s, with the emergence and development of transistors and other semiconductor devices, the electronic Triode was gradually replaced.

Transistor: although the semiconductor revolutionary electronic transistor has many advantages, it also has some disadvantages, such as large size, high power consumption, short life, low working frequency and so on. In order to overcome these shortcomings, people began to look for a new amplifier, which can be smaller, more power-saving, more durable and faster.

The transistor was invented in 1947 by John Bardeen, William Shockley and Walter Bratton of Bell Laboratories in the United States. they used the characteristics of semiconductor materials to connect two or three different doped semiconductor regions together. a new type of amplifier is formed. There are two basic types of transistors: bipolar transistors and field effect transistors. Bipolar transistors have two structures, NPN and PNP. There are also two structures of field effect transistors, namely junction field effect transistors and metal oxide semiconductor field effect transistors.

Transistors work in a similar way to electronic transistors, using a small signal to control a large signal. The difference is that the electronic transistor uses the voltage on the gate to control the electron flow between the emitter and the collector, while the transistor uses the current on the base to control the current between the emitter and the collector (bipolar transistor). Or use the voltage on the gate to control the current between the source and the drain (field effect transistor). Transistors have many advantages over electronic transistors, such as small size, low power consumption, long life, high working frequency and so on.

The invention of transistor has triggered a revolution in semiconductor technology, which not only replaces the application of electronic transistor in the fields of wireless communication, broadcasting and telephone, but also opens up new fields such as computer, microwave communication and satellite navigation. It has also given birth to a series of new semiconductor devices, such as diodes, photodiodes, laser diodes, solar cells and so on. Up to now, transistors are still one of the most important and widely used semiconductor devices.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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