I. What is the working principle of diodes
A crystal diode is a pn junction formed by a p-type semiconductor and an n-type semiconductor, and space charges are formed on both sides of the interface. layer, and built a self-built electric field. 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.
Second, what is the conduction characteristics 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. Forward characteristics
In electronic circuits, the anode of the diode is connected to the high potential end, and the cathode is connected to the low potential end, and the diode is It will be turned on, 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 electronic circuits, the anode of the diode is connected to the low potential end, and the cathode is connected to the high potential end. There is almost no current flowing in the diode, and the diode is in the off state at this time. 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.
Three, the main parameters of the diode
The technical indicators used to indicate the performance of the diode and the scope of application are called diode parameters. Different types of diodes have different characteristic parameters. For beginners, the following main parameters must be understood:
1. Rated forward working current
refers to the allowable The maximum forward current value passed. Because when the current passes through the tube, the die will heat up and the temperature will rise. When the temperature exceeds the allowable limit (about 140 for silicon tubes and about 90 for germanium tubes), the die will be overheated and damaged. Therefore, the diode should not exceed the rated forward working current value of the diode in use. For example, the rated forward working current of the commonly used IN4001-4007 germanium diode is 1A.
2. The highest reverse working voltage
When the reverse voltage applied across the diode reaches a certain value, the diode will be broken down and the single Electrical conductivity. In order to ensure safe use, the maximum reverse working voltage value is specified. For example, the reverse withstand voltage of IN4001 diode is 50V, and the reverse withstand voltage of IN4007 is 1000V.
3. Reverse current
The reverse current refers to the reverse current flowing through the diode under the action of the specified temperature and the highest reverse voltage. current. The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current has a close relationship with the temperature, about every 10% increase in temperature, the reverse current doubles. For example, 2AP1 type germanium diode, if the reverse current is 250uA at 25, the temperature rises to 35, the reverse current will rise to 500uA, and so on, at 75, its reverse current has reached 8mA, not only lost The unidirectional electrical conductivity will also cause the tube to overheat and be damaged. For another example, the 2CP10 silicon diode, the reverse current is only 5uA at 25, and the reverse current is only 160uA when the temperature rises to 75. Therefore, silicon diodes have better stability at high temperatures than germanium diodes.
