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A diode is a semiconductor device with two terminals, allowing current to flow in one direction. It is widely used for rectification and signal processing in electronic circuits.

A diode is an electronic device made of semiconductor materials such as silicon, selenium, or germanium. It consists of two electrodes: the positive electrode, also known as the anode, and the negative electrode, known as the cathode. When a forward voltage is applied across the two terminals of the diode, it conducts current; when a reverse voltage is applied, the diode blocks the current. The conduction and blocking of the diode can be likened to the action of a switch, turning on and off.

Diodes exhibit unidirectional conductivity, allowing current to flow from the anode to the cathode when conducting. As one of the earliest semiconductor devices, diodes find widespread applications, particularly in various electronic circuits. By judiciously connecting diodes with resistors, capacitors, inductors, and other components, diverse circuit functions can be realized, including AC rectification, modulation signal demodulation, limiting, clamping, and voltage regulation.

Whether in common radio circuits, household appliances, or industrial control circuits, the footprint of diodes can be found, showcasing their versatility and indispensability in modern electronics.

Structure and composition:

A diode is a semiconductor device consisting of a PN junction, electrode leads, and a protective casing. It is fabricated by employing various doping techniques and utilizing the diffusion process to create a P-type semiconductor and an N-type semiconductor on the same semiconductor substrate, typically silicon or germanium. At the interface between the two regions, a space charge region called the PN junction is formed.

The electrode lead connected to the P-region is called the anode, while the one connected to the N-region is called the cathode. Due to the unidirectional conductivity of the PN junction, when the diode is forward biased, current flows from the anode through the diode to the cathode.

The circuit symbol of a diode is depicted in Figure 1. It consists of two terminals: the anode, connected to the P-region and considered the positive terminal, and the cathode, connected to the N-region and considered the negative terminal. The direction of the triangular arrow indicates the direction of forward current. The voltage across the diode is represented by VD.

How it works

The main principle of a diode is to utilize the unidirectional conductivity of the PN junction. When leads and packaging are added to the PN junction, it becomes a diode.

A crystal diode consists of a PN junction formed by P-type and N-type semiconductors, with space charge layers formed on both sides at the interface, creating a self-built electric field. In the absence of an external voltage, the diffusion current caused by the difference in carrier concentration on both sides of the PN junction is equal to the drift current caused by the self-built electric field, resulting in an equilibrium state.

When a forward bias voltage is applied externally, the external electric field and the self-built electric field counteract each other, causing an increase in the diffusion current of carriers and resulting in a forward current. When a reverse bias voltage is applied externally, the external electric field and the self-built electric field are further strengthened, leading to a reverse saturation current within a certain reverse voltage range, independent of the reverse bias voltage value.

When ICs applied reverse voltage reaches a certain level, the electric field strength in the space charge layer of the PN junction reaches a critical value, leading to the multiplication of carriers and the generation of a large number of electron-hole pairs, resulting in a significantly large reverse breakdown current, known as the breakdown phenomenon of the diode. The reverse breakdown of the PN junction can be categorized into Zener breakdown and avalanche breakdown.

Principle of PN Junction Formation

P-type semiconductor is created by doping a small amount of trivalent impurities (e.g., boron) into intrinsic semiconductor (a completely pure and structurally intact semiconductor crystal). As 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 created in the crystal. When an electron from adjacent covalent bonds gains energy, it may fill this vacancy, making the boron atom an immobile negative ion. Meanwhile, the original silicon atoms form holes due to the lack of an electron, but the entire semiconductor remains neutral. In this type of semiconductor, hole conduction is dominant, with holes being the majority carriers and free electrons being the minority carriers.

The formation principle of N-type semiconductor is similar to that of P-type semiconductor. It involves doping pentavalent atoms (e.g., phosphorus) into intrinsic semiconductor, forming covalent bonds with silicon atoms and creating free electrons. In N-type semiconductor, electrons are the majority carriers, while holes are the minority carriers.

Therefore, by doping trivalent and pentavalent impurity elements into two different regions of the intrinsic semiconductor, P-type and N-type regions are formed. Based on the characteristics of N-type and P-type semiconductors, it can be inferred that at their junction, there exists a difference in the concentrations of electrons and holes. Both electrons and holes will diffuse from the region of higher concentration to the region of lower concentration, disrupting the original electrical neutrality at the junction.

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