The ROHM agent tells you that diodes are not actually one-way conductive

发表于2020-07-31

  1. I believe everyone knows that diodes play a very important role in electronic circuits. It has a very important characteristic, which is unidirectional conductivity. In other words, current can only flow from the positive electrode (anode) to the negative electrode (cathode). We can use this characteristic of the diode to achieve rectification, detection, amplitude limiting, protection and other operations.


    But is the diode really unidirectional? Agent Roma tells you, no!


    Each type of diode has detailed operating parameters, such as maximum forward current, maximum reverse voltage, reverse recovery time, reverse current and so on.

    Reverse current? Doesn't it mean that it is unidirectionally conductive? How can there be a reverse current? This starts with the structure and principle of the diode...


    The main reason for the reverse current is the existence of junction capacitance; when the diode passes alternating current, it can turn alternating current into direct current due to the unidirectional conductivity of the diode, but at the same time because of its own capacitive effect (although the capacitance is small) negative half The periodic alternating current will flow in the opposite direction of the diode (parasitic capacitance) to form a reverse current. The magnitude of the reverse current is determined by the diode junction capacitance. At the same time, it is closely related to the reverse recovery time. The larger the junction capacitance, the longer the reverse recovery time (because it takes time to charge and discharge the capacitor). This also directly limits the operating frequency of the diode. When the frequency reaches a certain level, the unidirectional conductivity of the diode will completely disappear.


    What exactly is junction capacitance? Why is there junction capacitance? How is the junction capacitance generated? Is there a way to eliminate the junction capacitance?


    To clarify this problem, we must analyze it in terms of structure and principles. There are two types of capacitors, diffusion capacitance and barrier capacitance.

    Diffusion capacitance: A differential capacitance effect exhibited by the p-n junction when it is forward biased. Pn junction diffusion capacitance is the capacitive effect caused by the charge storage of unbalanced minority carriers (unbalanced minority carriers) in the neutral region on both sides of the pn junction; simply put, this is because the PN junction is applied with a forward voltage, because the P/N The change of the charge concentration difference in the zone leads to the accumulation and release of charge. This accumulation and release process is equivalent to the charging and discharging of a capacitor, which is determined by the special material and structure of the diode, so the diffusion capacitance cannot be eliminated.


    Barrier capacitance: When a reverse voltage is applied to the PN junction, since the reverse cut-off current of the PN junction cannot directly flow through the diode, the two electrodes of the PN junction are relatively insulated, which happens to constitute the prototype of a capacitor: two metals in any insulator All conductors have a capacitive effect, and the capacity of the capacitor is inversely proportional to the distance between the two poles and proportional to the area of the two poles.

    In other words, the larger the electrode area of the diode, the larger the junction capacitance. For example, a surface contact rectifier diode has a larger electrode contact surface, which is suitable for passing large currents. But at the same time, as the electrode area increases, its junction capacitance will also increase, which limits its operating frequency.


    The smaller the PN junction area, the smaller the junction capacitance, and the smaller the current it can pass. For example, point-contact detector diodes do not need to flow too much current. The electrode area is very small, and the junction capacitance can be strictly controlled to ensure the quality of high-frequency detection.


    Based on the above content, you now know what is going on. If there is a diode demand, please contact Lianfengtai, and we will reply to you as soon as possible!