Early diodes mainly included "Cat's Whisker" Crystals and vacuum tubes (referred to as "Thermionic Valves" in the UK). Nowadays, the most common diodes are mostly made of semiconductor materials such as silicon or germanium. The materials of diodes are generally conductors (generally low-priced elements such as copper, iron, and aluminum), insulators (generally high-priced elements such as inert gases or polymer materials such as plastics and rubbers), and semiconductors (whose conductivity is between conductors and insulators such as silicon, germanium, and gallium arsenide). They have very wide applications. Especially in various electronic circuits, diodes and components such as resistors, capacitors, and inductors are reasonably connected to form circuits with different functions, which can realize multiple functions such as rectifying alternating current, detecting modulated signals, limiting and clamping, and stabilizing power supply voltage. Traces of diodes can be found in both common radio circuits and other household electrical products or industrial control circuits.
Structural Composition of Diode
The diode, also known as a crystal diode, is mainly composed of a hole-type P-type semiconductor and an electron-type N-type semiconductor combined to form a PN junction. By using different doping processes and through diffusion, the P-type semiconductor and the N-type semiconductor are fabricated on the same semiconductor (usually silicon or germanium) substrate, and a space charge region is formed at their interface, which is called a PN junction.
The electrode that leads out from the P region is called the anode, and the electrode that leads out from the N region is called the cathode. Due to the unidirectional conductivity of the PN junction, when the diode is conducting, the current direction is from the anode through the inside of the tube to the cathode.
A diode has two electrodes. The electrode led out from the P region is the positive pole, also called the anode; the electrode led out from the N region is the negative pole, also called the cathode. The direction of the triangular arrow indicates the direction of the forward current. The text symbol of the diode is represented by VD.
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Ordinary diode |
voltage regulator diode |
light-emitting diode |
photodiode |
varactor diode |
Working Principle of Diode
The main principle of a diode is to utilize the unidirectional conductivity of the PN junction. Adding leads and packaging to the PN junction becomes a diode. A crystal diode is a PN junction formed by a P-type semiconductor and an N-type semiconductor. At the interface, a space charge layer is formed on both sides, and a built-in electric field is established. When there is no external voltage applied, due to the diffusion current caused by the carrier concentration difference on both sides of the PN junction and the drift current caused by the built-in electric field being equal, an electrical equilibrium state is reached.
When there is a forward voltage bias from the outside, the mutual suppression effect of the external electric field and the built-in electric field increases the diffusion current of carriers and causes forward current. When there is a reverse voltage bias from the outside, the external electric field and the built-in electric field are further strengthened, forming a reverse saturation current that is independent of the reverse bias voltage value within a certain reverse voltage range.
When the applied reverse voltage is high enough, the electric field strength in the space charge layer of the PN junction reaches a critical value, resulting in a carrier multiplication process, generating a large number of electron-hole pairs, and producing a very large reverse breakdown current, known as the breakdown phenomenon of the diode. The reverse breakdown of the PN junction can be divided into Zener breakdown and avalanche breakdown.
diode-Junction Diode Symbol and Static I-V Characteristics
Formation Principle of PN Junction: P-type and N-type Semiconductors
P-type semiconductor is obtained by doping a small amount of trivalent element impurities, such as boron, into an intrinsic semiconductor (a completely pure and structurally complete semiconductor crystal).
Since boron atoms have only three valence electrons, they form covalent bonds with surrounding silicon atoms. Due to the lack of one electron, a vacancy is generated in the crystal. When electrons on adjacent covalent bonds gain energy, they may fill this vacancy, making boron atoms into immobile negative ions. While the covalent bond of the original silicon atom lacks one electron, forming a hole. However, the entire semiconductor remains neutral. In this P-type semiconductor, hole conduction is dominant. Holes are the majority carriers, and free electrons are the minority carriers.
The principle of forming an N-type semiconductor is similar to that of a P-type. Doping pentavalent atoms, such as phosphorus, into intrinsic semiconductors. After doping, it forms covalent bonds with silicon atoms and generates free electrons. In N-type semiconductors, electrons are the majority carriers, and holes are the minority carriers.
Therefore, by doping trivalent and pentavalent impurity elements in two different regions of intrinsic semiconductor, P-type and N-type regions are formed. According to the characteristics of N-type semiconductors and P-type semiconductors, it can be known that at their junction, there is a concentration difference between electrons and holes. Electrons and holes both diffuse from regions with high concentration to regions with low concentration. Their diffusion disrupts the electrical neutrality at the original junction.
diode-Increase in the Depletion Layer due to Reverse Bias
Unidirectional Conductivity of PN Junction
When a forward voltage V is applied externally to the PN junction, under the action of this external electric field, the equilibrium state of the PN junction is broken. Holes in the P region and electrons in the N region both move toward the PN junction. Holes neutralize the negative ions in the P region of the PN junction, and electrons neutralize the positive ions in the N region of the PN junction. This narrows the PN junction. As the external electric field increases, the diffusion movement is further enhanced, and the drift movement is weakened. When the external voltage exceeds the threshold voltage, the PN junction is equivalent to a resistor with a very small resistance, that is, the PN junction is conducting.
diode-reduction in the Depletion Layer due to Forward Bias
Main Classification of Diode
According to usage
Diodes can be divided into ordinary diodes and special diodes. Ordinary diodes include detection diodes, rectifier diodes, switching diodes, and voltage regulator diodes; special diodes include varactor diodes, photodiodes, and light-emitting diodes.
varactor diodes
Classification by frequency
The most basic classification method. According to their characteristics, diodes are divided into rectifier diodes, switching diodes, Schottky barrier diodes, Zener diodes, and high-frequency diodes for high frequencies. In addition, Zener diodes are generally used as protection elements. However, with the precision of peripheral circuits and the miniaturization of applications, higher-performance protection elements - TVS (Transient Voltage Suppressor) are required.
transient voltage suppressors(TVS)
Zener diodes
According to the semiconductor materials used
They can be divided into germanium diodes (Ge tubes) and silicon diodes (Si tubes).
Classified by atomic structure
They are mainly divided into the currently mainstream planar type and the high-voltage-resistant mesa type.
Planar diodes： when planar diodes are used as switching tubes in pulse digital circuits, the PN junction area is small. When used for high-power rectification, the PN junction area is large.
Voltage regulator diodes: voltage regulator diodes are a special kind of surface-contact semiconductor silicon diodes with the function of stabilizing voltage.
Classification by shape
Diodes come in various shapes, generally divided into plug-in type and surface mount type.
Main Applications of Diodes
In almost all electronic circuits, semiconductor diodes are used. The use of semiconductor diodes in circuits can protect circuits and extend circuit life. The development of semiconductor diodes has made integrated circuits more optimized and played a positive role in various fields. Diodes have many functions in integrated circuits and maintain the normal operation of integrated circuits. The following is a brief introduction to the role of diodes in the following four circuits.
In digital and integrated circuits, the unidirectional conductivity of diodes is used to realize the conduction or disconnection of circuits. This technology has been widely used. Switching diodes can well protect circuits and prevent circuits from being burned out due to short circuits and other problems. They can also realize the functions of traditional switches. Switching diodes also have a characteristic of very fast switching speed. This is incomparable to traditional switches.
In electronic circuits, limiting circuits are often used to process various signals. It is used to allow signals to selectively transmit a part of the signals within a preset level range. Most diodes can be used as limiters, but sometimes special limiting diodes are needed, such as when protecting instruments.
Zener diodes are usually used in voltage-stabilizing circuits.
Varactor diodes are often used in varactor circuits to realize automatic frequency control, tuning, frequency modulation, and scanning oscillation of circuits.
Light-emitting diodes are generally used as backlight sources for screens or display and lighting applications in electronic products. Light-emitting diodes are widely used in automobiles and large machinery. Application in coal mines. Due to the characteristics of light-emitting diodes such as higher efficiency, lower energy consumption, longer lifespan, and stronger luminosity compared to ordinary light-emitting devices, miners' lamps and underground lighting equipment use light-emitting diodes. As decorative lights for cities. More details: Led Emitting Diode
Texas Instruments LP3985IM5X-4.7/NOPB
Texas Instruments LP3985IM5-4.7/NOPB
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