1.1 Volt-ampere characteristics of semiconductor diodes
The characteristic of the diode is mainly unidirectional conductivity, which has been explained in the characteristic of PN junction. Now focus on analyzing its volt-ampere characteristic curve and parameters. Figure 2-10 shows the voltage-current relationship of a diode, also called the volt-ampere characteristic curve. The curve is divided into three parts for analysis below.
1.1.1 Forward characteristics
When no voltage is applied across the diode, the current passing through is zero. When a forward voltage U is applied to the diode, a forward current I is generated, but when the voltage is relatively small, the applied voltage is not enough to overcome the resistance caused by the internal electric field to the diffusion movement of carriers.
The diffusion amount of carriers is small, and the forward current is still small at this time, and the resistance of the diode is relatively large (that is, within the dead zone voltage). When the applied voltage reaches a certain value (greater than the dead zone voltage), the internal electric field is greatly weakened, the diode resistance becomes very small, and the forward current increases rapidly (point a in Figure 2-10). This voltage is called the diode’s Valve voltage. It changes with the tube material and temperature: the germanium tube is at 0.20.4V, and the silicon tube is at 0.7~0.8V.
The maximum rectified current of the diode refers to the average value of the maximum forward current allowed by the diode in long-term operation. It is an important parameter of the diode. Be careful not to overload the diode when using it, so as not to damage the tube due to overheating.
Figure 2-10 PN junction volt-ampere characteristic curve
1.1.2 Reverse characteristics
When a reverse voltage is applied to the diode, because there are still a few free electrons in the P-type semiconductor, and there are still a few holes in the N-type semiconductor, these minority carriers can easily pass through the PN junction under the action of the reverse voltage, so Form a reverse current. The reverse current is very small, as shown in Figure 2-10. The reverse current has two characteristics. First, the reverse current increases rapidly with the increase in temperature. This is because when the temperature rises, the thermal movement of molecules inside the semiconductor intensifies, so more pairs of electrons and holes participate in conduction, resulting in an increase in current. Therefore, when the ambient temperature is high, the influence of the reverse current must be considered, especially the germanium tube. Second, as long as the applied reverse voltage is within a certain range, the reverse current basically does not change with the reverse voltage, as shown near point b in Figure 2-10. This is because the number of minority carriers that can be provided at a certain temperature and per unit time is limited and certain. As long as the electric field generated by the applied reverse voltage is enough to attract them to form a current, even if the voltage is high, the number of minority carriers cannot be increased. The reverse current is an important parameter to measure the diode. The large reverse current indicates that the unidirectional conductivity of the tube is poor. Generally, the reverse current of silicon tube ranges from less than 1uA to tens of microamperes (some high-power tubes also reach tens of milliamps), and germanium tubes can reach hundreds of milliamps. It can be seen that the reverse current of the silicon tube is much smaller than that of the germanium tube.
1.1.3 Reverse breakdown voltage
When the reverse voltage continues to increase, the reverse current does not change much at the beginning, but when the reverse voltage increases to a certain level, the reverse current suddenly increases and reverse breakdown occurs, as shown in point c in Figure 2-10. This voltage is called the reverse breakdown voltage. The reason is that the applied voltage is too large, that is, the applied electric field is too strong, which forces the electrons to be pulled out, forcing the number of carriers to rise sharply. The highest reverse working voltage given in some manuals, usually half of the breakdown voltage, is to prevent the diode from being damaged due to reverse breakdown. When choosing a diode, you must pay attention to this limit.
When choosing a crystal diode as a rectifier element, the two values of the maximum rectified current and the maximum reverse voltage are mainly considered.