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Basic structure and working principle of fast recovery diode FRD


Generally, semiconductor devices with rated current exceeding 1 ampere are called power semiconductors. They block voltages ranging from a few volts all the way up to tens of thousands of volts. Among the many power semiconductor devices, the power diode is a relatively simple device, but it is also a common basic device that is most widely used in power circuits. It is not only used in simple rectifier circuits, but also often used as freewheeling diodes (FWD for short). In simple single-phase half-bridge inverter circuits, it is matched with controllable switching devices and used in anti-parallel to continue to maintain inductance. current on the load, preventing voltage spikes due to sudden drops in current. The use of this kind of FWD exists in many applications such as switching power supplies, variable frequency power supplies, uninterruptible power supplies, and variable frequency motor drives.


The fast recovery diode has the characteristics of high withstand voltage and low leakage current during reverse blocking, and low forward on-state resistance and large current. Since it is used as a switch, it is generally required to have a faster switching speed. In addition, proper selection of the characteristics of the freewheeling diode, especially the reverse recovery characteristics, such as reverse recovery time and reverse recovery softness, can significantly reduce the power consumption of switching devices, diodes and other circuit components, and reduce the power consumption caused by the freewheeling current. Voltage spikes, electromagnetic interference caused by diodes, thereby minimizing or even eliminating the absorption circuit.


The difference between power diodes and ordinary diodes is that they have the characteristics of large rated working current and high blocking voltage. Therefore, in order to achieve high withstand voltage, the ordinary PN junction structure cannot be used, but the P-i-N structure is generally used.


The diode of the P-i-N structure adds an i region between the ordinary PN junctions, which refers to the lightly doped semiconductor region.


Since the PN junction bears the voltage drop through the expansion of the depletion region, it can be known from the Poisson equation that the smaller the net charge density in the depletion region, the smaller the electric field slope, and because the electric field peak is at the PN junction Therefore, after the semiconductor on the lightly doped side is depleted, the field drops slowly, and the voltage is the integral of the electric field at the position, which can withstand a larger voltage. Therefore, it is best to have an intrinsic region between the PN junctions, so that the electric field can be flat for a certain distance to achieve a larger withstand voltage. i represents the meaning of the intrinsic region, but the intrinsic semiconductor is unrealistic in the actual process, so the i region is actually a lightly doped semiconductor region.


Usually, this i-region is a lightly doped N-region for two main reasons: First, by using a process of neutron transmutation, a very uniform N-type doping with low doping concentration can be produced; At the same given voltage level, the thickness of the device made of P+N junction is thinner than that of N+P junction, and the power consumption of the device is approximately proportional to the square of their thickness. The P+N junction region of the P-i-N structure has been mentioned before, and the necessity of the remaining N region is introduced below.


For the contact between the semiconductor and the metal electrode, since the N-type semiconductor is not as easy to form a good ohmic contact with the metal as the P-type semiconductor can be low-doped, it will produce a higher contact resistance when the doping concentration is lower than 1019cm-3. , therefore, the i region cannot be directly connected to the metal electrode, so as to avoid a large voltage drop and excessive power consumption. This problem can be solved by adding a heavily doped N+ layer on the side of the lightly doped i region. This also forms the basic structure of the power diode P-i-N. Due to the concentration of power lines at the junction corners of the PN junction in the reverse biased state, the withstand voltage of a simple P-i-N diode is usually much lower than that of the corresponding ideal parallel plane junction. In order to improve its withstand voltage characteristics, through the introduction of junction termination technology, the use of partial pressure field rings, guard rings, field plates, or the use of mesa structures, positive bevel, and negative bevel terminals to improve terminal efficiency and achieve maximum withstand voltage.


The low forward resistance of the fast recovery diode is that when the PN junction is forward biased, a large amount of plasma is injected into the i region. conductance modulation. However, the large amount of these injected excess carriers will greatly slow down the turn-off time of the device when the PN junction is reverse biased, that is, when the diode is turned off. Since the i region usually has a certain thickness to maintain the withstand voltage, the minority carriers stored in the device need to drift and recombine to disappear when the device is working in forward bias, but this takes a certain amount of time, which also forms The reverse recovery process of a power diode. Since fast recovery diodes are usually used in anti-parallel with other switching devices, the reverse recovery process seriously restricts the high performance of the device, and greatly affects the operating frequency and performance of other devices and the entire system, so it needs to be minimized and eliminated.


This same problem can be solved by adding a heavily doped N+ layer on the side of the doped i region. This also forms the basic structure of the fast recovery diode P-i-N.


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