“Among Diode Electronic components, a device with two electrodes that only allows current to flow in a single direction, many of which are used to apply its rectification function. The most common function of a diode is to allow current to flow in only one direction (called forward bias) and block it in the reverse direction (called reverse bias). semiconductor diodes are used in almost all electronic circuits. They play an important role in many circuits. They are one of the earliest semiconductor devices and are widely used. The following mainly introduces the conductive characteristics, classification and working principle of semiconductor diodes, let’s take a look!
Conductive Properties of Diodes
The most important characteristic of diodes is unidirectional conductivity. In a circuit, current can only flow in from the positive pole of the diode and out from the negative pole. The forward and reverse characteristics of the diode are described below through a simple experiment.
1. Positive characteristics
In an electronic circuit, the positive electrode of the diode is connected to the high potential end and the negative electrode is connected to the low potential end, and the diode will conduct. This connection method is called forward bias. It must be explained that when the forward voltage applied across the diode is very small, the diode still cannot conduct, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (this value is called “threshold voltage”, the germanium tube is about 0.2V, and the silicon tube is about 0.6V), the diode can be turned on directly. After turning on, the voltage across the diode remains basically the same (about 0.3V for germanium tubes and 0.7V for silicon tubes), which is called the “forward voltage drop” of the diode.
2. Reverse characteristics
In an electronic circuit, the anode of the diode is connected to the low-potential end, and the cathode is connected to the high-potential end. At this time, almost no current flows through the diode, and the diode is in the cut-off state. This connection method is called reverse bias. When the diode is reverse biased, there will still be a slight reverse current flowing through the diode, known as leakage current. When the reverse voltage across the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose its unidirectional conduction characteristics. This state is called diode breakdown.
How Diodes Work
The crystal diode is a pn junction formed by a p-type semiconductor and an n-type semiconductor, and a space charge layer is formed on both sides of the interface, and a self-built electric field is built. When there is no applied 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 and is in an electrical equilibrium state.
When there is a forward voltage bias in the outside world, the mutual suppression of the external electric field and the self-built electric field increases the diffusion current of the carriers and causes the forward current.
When there is a reverse voltage bias outside, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current I0 that is independent of the reverse bias voltage value within a certain reverse voltage range.
When the applied reverse voltage reaches a certain level, the electric field strength in the space charge layer of the pn junction reaches a critical value, resulting in a multiplication process of carriers, a large number of electron-hole pairs, and a large reverse breakdown current. , known as the breakdown phenomenon of the diode.
Type of Diode
There are many types of diodes. According to the semiconductor materials used, they can be divided into germanium diodes (Ge tubes) and silicon diodes (Si tubes). According to their different uses, they can be divided into detection diodes, rectifier diodes, Zener diodes, switching diodes, etc. According to the die structure, it can be divided into point-contact diodes, surface-contact diodes and planar diodes. Point-contact diodes use a very thin metal wire to press the surface of a smooth semiconductor wafer, and a pulse current is applied to make one end of the contact wire and the wafer firmly sintered together to form a “PN junction”. Because it is a point contact, only a small current (not more than tens of milliamps) is allowed to pass through, which is suitable for high-frequency small current circuits, such as radio detection.
The “PN junction” area of the surface-contact diode is large, allowing a large current (several amperes to tens of amperes) to pass through, and is mainly used in the “rectifier” circuit that converts alternating current into direct current.
Planar diode is a special silicon diode, which can not only pass a large current, but also has stable and reliable performance, and is mostly used in switching, pulse and high-frequency circuits.