With the development of power electronics technology, the applications of various frequency conversion circuits and chopper circuits are expanding. Whether the main circuit of these power electronics circuits is thyristor with commutation switching off or new power electronics devices with self-switching ability, such as GTO, MCT, IGBT, etc., all need a fast parallel connection. The electrode can reduce the charging time of capacitor by reactive current in the load, and suppress the high voltage induced by the instantaneous reverse of the load current. Due to the continuous improvement of frequency and performance of these power electronic devices, in order to match the switching process, the diode must have fast switching-on and high-speed switching-off capabilities, i.e., short reverse recovery time rr, small reverse recovery current IRRM and soft recovery characteristics.

In high voltage and high current circuit, the traditional PIN diode has good reverse voltage withstanding performance, and when it is in the forward direction, it can turn on a larger current at a very low voltage, showing a low resistance state. However, the existence of a few carriers with large forward injection makes the minority carrier lifetime longer and the switching speed of the diode correspondingly lower. In order to improve the switching speed of the diode, the minority carrier lifetime can be reduced by doping heavy metal impurities and irradiating by electrons, but this will cause the hard recovery characteristics of the diode to varying degrees. The high induction voltage has an important influence on the normal operation of the whole circuit.

1: Working Principle and Influencing Factors

Diodes with very short recovery process, especially those with very short reverse recovery process, are called Fast Recovery Diodes. High-frequency power electronic circuits require not only a fast recovery diode with good forward recovery characteristics, i.e., small forward transient voltage drop and short recovery time, but also a good reverse recovery characteristics, i.e., short reverse recovery time, less reverse recovery charge and soft recovery characteristics.

Opening Characteristic

There is also a process of diode switching on. In the initial stage of switching on, there is a high transient voltage drop. After a certain period of time, the diode can be in a stable state and has a small tube voltage drop. That is to say, the diode shows obvious "inductance effect" in the initial stage of switching on, and can not respond to the change of forward current immediately. During the forward recovery time, the on-going diode has a much larger peak voltage UFP than the steady state. When the forward current rise rate exceeds 50 A/s, there is a high transient voltage drop in some high voltage diodes. This concept is very important in the fast application of buffer circuits.

In addition to the internal mechanism of the device, the inductance effect of the diode is related to the length of the lead and the magnetic material used in the device package. Inductance effect is most sensitive to the change rate of current, so the higher the rise rate of diode current diF/dt, the higher the peak voltage UFP and the longer the forward recovery time.

Turn-off characteristics

All PN junction diodes will store charge in the form of minority carriers when conducting forward current. Minority carrier injection is the mechanism of conductivity modulation, which leads to the decrease of forward voltage drop (VF). In this sense, it is advantageous. However, when a reverse voltage is suddenly applied to the conducting diode, because a large number of minority carriers are stored in the PN junction during conduction, it will take some time to completely withdraw or neutralize these minority carriers by the cut-off time, that is, it will take a period of time to recover the reverse blocking ability. The reverse recovery process is defined as the reverse recovery time (trr). It is noteworthy that the diode is equivalent to a short circuit state before the blocking capability is restored.

Softness factor S is used to describe the rate at which the reverse recovery current disappears from the maximum IRM. Dirr/dt is an important parameter for reverse recovery current. If the dirr/dt is too large, the reverse peak voltage URM will be too high due to the existence of inductance L in the line, and sometimes strong oscillation will occur, which will damage the diode. The influence of dirr/dt on the reverse characteristic can be expressed by the concepts of soft characteristic and hard characteristic. The softening coefficient S can also be expressed as the amplitude of the reverse peak voltage can be predicted by the above formula. Among them, L is the peak voltage applied to the active device when the total inductance URM of the circuit is reversed recovery of the diode. Its value must be less than the voltage rating of the active device, so it is more practical to use di (rec)/dt to express the softness factor. The total time required to deplete charge storage is defined as reverse recovery time trr. As a measure of switching speed, it is a very important parameter when selecting diodes. Trr for general purpose is about 25s. It can be used in rectifiers and circuits with frequencies below 1kHz, but it can be used in choppers and inverters. In the circuit, the fast recovery diode with TRR below 5S must be selected. In some absorption circuits, fast switching on and soft recovery diodes are required.

From the definition of softness factor, it can be seen that it reflects how long the base minority disappears due to recombination in the reverse recovery TB process of the diode. Therefore, the softness factor is closely related to minority carrier lifetime control method, base width and diffusion concentration distribution, component structure and structural parameters. The softness factor will be improved if more residual charges are stationed in the residual base area after the expansion of the space charge area and longer time is stationed.

2: Ways to Improve Performance

Although PIN transistor has good reverse voltage withstanding ability, it produces large reverse peak voltage during reverse recovery because of its poor reverse recovery characteristic, which affects the normal operation of the whole circuit. Secondly, a practical PIN rectifier has an order of magnitude higher forward voltage drop at the instant of switching on than the steady-state voltage drop. The peak voltage can exceed 30V, which is mainly caused by the limited diffusion velocity of minority carriers. It is related to the resistivity and width of N-base material. In order to improve the working characteristics of the diode, related technologies are used in the design and fabrication of the device.

For example, minority carrier lifetime control technology. Because the change of minority carrier lifetime and the control method of minority carrier lifetime affect the frequency characteristics and reverse recovery softness of fast recovery devices, the control technology of minority carrier lifetime is very important. The minority carrier lifetime control technology can be divided into three types according to its characteristics: conventional type, heavy metal doping type and electron radiation type.

1) Conventional type: Conventional type is to control minority carrier lifetime by adjusting the structure parameters of switch tube, including several methods: reducing material resistivity, controlling impurity distribution, and thinning base thickness.

2. Heavy metal doping: In the process of diode manufacturing, consciously choose a suitable deep level heavy metal impurity to diffuse in semiconductor, which can be used to reduce minority carrier lifetime and improve reverse recovery softness. Commonly used heavy metal impurities are gold, platinum, palladium and so on.

3) Electron radiation type: Its characteristic is that the minority carrier lifetime can be accurately controlled by adjusting the dose of electron injection, so that the different requirements of minority carrier lifetime for various electrical parameters of devices can be well coordinated, and it can be carried out after the device is manufactured, making the manufacturing process simple and flexible.

3: New structure

New structures are used to improve the performance of the diodes, such as ideal ohmic contacts. The traditional PIN rectifier uses ohmic contact at the N-N + interface only for multi-sub. Ohmic contact structure is not formed for minority n-n+due to the influence of built-in electric field produced by n-n+high and low junctions. Ideal ohmic contact is an interface that allows both minority and poly to pass smoothly. It is composed of P + region and N + region embedded in each other. In this structure, the hole passes through the interface and the electron passes through the interface. The structure can be obtained by the traditional selective diffusion method or by using Schottky contact instead of the whole p + region, thus eliminating the one-time selective diffusion process.

Recently, amorphous silicon, germanium and boron alloys with high conductivity are deposited on p-type silicon wafers by CVD method as ideal contact layer in the process of making diodes, which makes the diodes have low consumption rectification characteristics in positive bias and fast switching characteristics in bias. The reverse recovery time of the diode with ideal ohmic contact is up to 60 ns, and the leakage current is lower, so that the diode can work at high temperature.

The traditional method of fabricating fast diodes is to reduce minority carrier lifetime by doping gold, platinum or electron irradiation. However, due to the mutual constraints of parameters such as reverse recovery time, reverse peak current, attenuation speed and forward voltage drop, this device has been used in many power electronics applications. Therefore, it is particularly important to fabricate a high-speed diode with short reverse recovery time, low reverse peak current and soft recovery characteristics. For this purpose, different structures of these diodes have been developed, such as concave step "cathode short circuit" structure with auxiliary diodes, cathode short circuit structure, self-tuning emission efficiency and ideal ohmic contact diode (SIOD).

In order to meet the electrical and heat dissipation requirements between the chip and the base, ohmic contact should be achieved at both ends of the diode. Because the anode and cathode of fast soft recovery diode have complex structure and shallow diffusion depth, the traditional ohmic contact process of high power devices, sintering process, will destroy this structure and desired performance. The method to solve this problem is to adopt the ohmic contact process of multi-layer metals. Ohmic contact between anode and cathode is one of the difficulties and key points of this subject. On the basis of the existing theory and practice, the technological process is continuously improved to improve the performance and yield of the diode.