Abstract : This paper introduces a fast soft recovery diode with a diffused double-base region structure. The diode base region consists of a traditional lightly doped substrate N- region and a diffused, heavily doped N- region (buffer base region). Experimental results show that the reverse recovery softness factor of this diode is improved to about 1.0, which is a significant improvement over traditional PIN diodes. Keywords : diode, fast, soft recovery New Development of Soft Recovery Diode: Diode with Double Base Regions by the Diffusion Zhang Haitao, Zhang Bin (Power Electronics Factory of Tsinghua University) (PO Box. 1021, Beijing, China, Post Code: 102201) Abstract: This paper introduces a fast and soft recovery diode with double base regions by diffusion. The base design of the diode consists of two regions: a conventional lightly doped substrate N⁻ and a more heavily doped region N₁ (buffering region) by diffusion. The test results indicate that the reverse recovery softness of the diodes is increased to about 1.0, and is more improved than the conventional PIN diode. Key words: diode, fast, soft recovery 1 Introduction PIN diodes, widely used in power circuits, have high reverse breakdown voltage, and under high forward current density, the forward voltage drop is small due to the base region conductance modulation effect. In order to improve the withstand voltage, the traditional PIN diode adopts a deep diffusion slow-change structure, which results in a large amount of stored charge before turn-off, which prolongs the reverse recovery time. In order to reduce the voltage drop, this high voltage diode usually needs to be designed as a base region punch-through structure to reduce the base region, thereby making the reverse recovery characteristics harder and less suitable for the development of power electronics technology. In order to shorten the reverse recovery time of the diode, improve the reverse recovery softness, and at the same time make the diode have a higher withstand voltage, adding an N-type buffer base region on the basis of the traditional PIN diode is a better solution. That is, the base region of the diode is composed of a lightly doped N-substrate region and a more heavily doped N region. Among the diodes designed abroad, there are also those that adopt a double base region structure, but the formation of their N buffer base regions is all achieved by epitaxial process [1]. Since our laboratory does not have epitaxial equipment, although there is epitaxial equipment in China, the current level of high-resistivity thick film epitaxy is still difficult to meet the requirements of high-power diodes. Therefore, we decided to try to use the traditional diffusion process to make an N buffer base region structure in order to improve the reverse recovery softness of the diode. 2. Reverse Recovery Analysis of Fast Recovery Diodes 2.1 Reverse Recovery Process Analysis A diode with a short reverse recovery process is called a fast recovery diode. High-frequency power electronic circuits require fast recovery diodes to have short reverse recovery times, low reverse recovery charge, and soft recovery characteristics. All PN junction diodes store charge in the form of minority carriers when conducting forward current. Minority carrier injection is the mechanism of conductance modulation, which leads to a reduction in forward voltage drop (VF), and in this sense, it is advantageous. However, when a reverse voltage is applied to a conducting diode, because a large number of minority carriers are stored in the base region during conduction, it takes a certain amount of time to completely extract or neutralize these minority carriers before turning off. That is, the recovery of reverse blocking capability takes a certain period of time, and this process is called the reverse recovery process. The time taken for this process to occur is defined as the reverse recovery time (trr). Figure 1 shows a schematic diagram of the current and voltage waveforms during the reverse recovery process of a diode. Wherein, IRM is the reverse recovery peak current, VRM is the reverse peak voltage, QR is the reverse recovery charge, and trr is the reverse recovery time. These parameters are very important parameters in device design and application. Starting from time t=tf, the diode that has been conducting is subjected to a reverse voltage VR, and the forward current IF that was originally conducting decreases at a rate of diF/dt. This rate of change of current is determined by the reverse voltage and the inductance in the switching circuit, and most of the applied reverse voltage drops across the circuit inductance. That is (1) When t=t0, the current in the diode drops to 0. Before this, the diode is forward biased and the current is forward current. After time t0, the forward voltage drop decreases slightly, but it is still forward biased, and the current begins to flow in reverse, forming the reverse recovery current irr. t0 to t1 is called the minority carrier storage time ta. During this period, a portion of the internal stored charge is rapidly extracted through the reverse direction, the reverse current of the diode rises from zero to its peak value IRM, and the PN junction voltage decreases slightly but is still forward. Due to the low voltage on the diode, the power loss on the diode is very small during ta. However, power dissipation in switching devices is high because the power switching device carries the entire reverse current of the diode while simultaneously bearing the full voltage of the circuit, and the reverse current of the diode during the ta period is quite large. After t>t1, the space charge region begins to form, minority carriers disappear through recombination, and the reverse recovery current decreases rapidly at a rate of dirr/dt, generating a high electromotive force in the line inductance. This electromotive force, along with the power supply voltage, is applied to the diode and the power switching device connected in antiparallel to it, so the diode and power switching device withstand a very high reverse voltage VRM. The time from t1 to t2 is called the recombination time tb. The test standard specifies that t2 is determined by the intersection of the line connecting the currents at 0.9IRM and 0.25IRM on the time axis. The time from t0 to t2 is called the reverse recovery time trr, and the ratio of tb to ta is the softness factor S of the diode. It can be seen that the more minority carriers remain in the base region of the diode after t1, i.e., the longer the recombination time tb, the larger the softness factor of the diode. 2.2 Reverse recovery parameter analysis The softness factor S is usually used to describe the softness and hardness characteristics of reverse recovery: (2) The rate of decrease of reverse recovery current dirr/dt is also a very important parameter. If dirr/dt is too large, due to the inductance L in the circuit, the reverse peak voltage VRM will be too high, and sometimes strong oscillations will occur, causing damage to the diode or switching device. The relationship between VRM and dirr/dt is as follows. (3) Where L is the total inductance of the circuit, and VL is the additional induced electromotive force. The relationship between dirr/dt, VRM, IRM and QR and softness factor S can be derived as follows: As can be seen from Equations 3 to 7, increasing the softness factor S of the diode can significantly reduce the reverse recovery dirr/dt, IRM and VRM. For example, for the same reverse recovery time trr, if the softness factor S is increased from the current 0.3 to 1.0, then dirr/dt and the additional induced potential VL can be reduced by 70%, and IRM and QR can be reduced by 35%, which will greatly improve the reliability and stability of power devices and circuits. 2.3 Factors Affecting the Softness Factor As defined, the softness factor reflects the time it takes for minority carriers in the base region to disappear due to recombination during the reverse recovery tb process of a diode. Therefore, the softness factor is closely related to the minority carrier lifetime control method, the base region width and diffusion concentration distribution, the device structure and structural parameters, etc. Residing more residual charge in the remaining base region after the space charge region expansion, and retaining it for a longer time, will improve the softness factor. 2.4 Main Methods to Improve the Softness Factor Improving the softness factor mainly involves two aspects: first, selecting a suitable minority carrier lifetime control method. Among various minority carrier lifetime control methods, gold-plated devices have a larger softness factor, but larger leakage current and poor high-temperature characteristics; while electron irradiation devices have the smallest softness factor, but good high-temperature characteristics. Therefore, it is necessary to weigh the pros and cons and make a compromise based on the actual situation. Secondly, efforts should be made to improve the diode's structural design. There are various diode structures that can improve the softness factor, such as self-adjusting emission efficiency structures, concave stepped cathode short-circuit structures, auxiliary diode structures, epitaxial diode structures, and epitaxial double-base structures. Among these, diodes with epitaxial double-base structures are the most effective. 3. Structure and Working Principle of Double-Base Diodes The main characteristic of a double-base diode is that its base region consists of two parts: the N- region and the N- region. The N- region is the lightly doped substrate region of the original silicon wafer, while the N- region is a heavily doped region with a doping concentration higher than the N- region but much lower than the N+ region. We call the N-base region the buffer base region or buffer layer. The N-region is usually obtained using epitaxial methods; due to limitations, we used the traditional diffusion method. The double-base structure can significantly improve the diode's softness because the impurity concentration in the buffer layer is higher than that in the substrate, which significantly slows down the expansion of the depletion region after reaching the buffer layer during reverse recovery. Thus, after a small amount of carrier storage time, a large number of carriers remain unrecombined or unremoved in the buffer layer, increasing the recombination time and thus improving the diode's softness factor. To achieve this, two conditions must be met: first, the N-base region of the diode must be sufficiently narrow to ensure that the space charge region at rated voltage can expand into the buffer layer; second, the concentration of the buffer layer should be appropriate—not too high to ensure conductivity modulation, but not too low to prevent the space charge region from penetrating the buffer layer. The cross-sectional structure and concentration distribution of this dual-base diode are shown in Figures 2 and 3. The diode's anode is composed of a lightly doped P-region and a heavily doped P+ region. This P-P+ structure can control the hole injection effect, thereby controlling the self-regulating emission efficiency and shortening the reverse recovery time. The impurity concentration and junction depth of the P-region and P+ region are mainly considered to ensure that the diode not only has sufficient reverse blocking capability but also exhibits low hole injection efficiency at low current densities and high injection efficiency at high current densities. Low hole injection efficiency reduces the concentration of injected carriers on the anode side, leading to earlier pinch-off at the PN junction when transitioning to the blocking state. Under the specified current commutation rate di/dt, the reverse peak current is small. Simultaneously, the presence of a relatively large number of carriers in the neutral region of the base improves the softness factor. The cathode of the diode is the same as that of a typical diode, consisting of a heavily doped N+ region. 4. Experimental Scheme and Process Flow For ease of experimental comparison, we designed four experimental schemes: structurally, two types of diodes with a double-base region structure and two types of traditional PIN diodes with a standard structure; minority carrier lifetime control methods included gold diffusion and electron irradiation. This resulted in four combined schemes. All four schemes used original silicon wafers with identical resistivity and thickness. Except for the traditional PIN diode which lacks an N-buffer layer, the diffusion concentrations and junction depths of P, P+, and N+, as well as the processing procedures, were completely identical, resulting in the same final wafer thickness. Based on the die design requirements, the process flow for the double-base region diode is shown in the figure below. 5. Experimental Results Experiments were conducted on the above four schemes, and the experimental results were tested and compared. The test conditions were as follows: the forward peak voltage drop VFM was tested under the condition of forward peak current IFM = 1000A; the reverse recovery time trr and the reverse peak recovery current IRM were tested under the conditions of forward current IFM = 80A, reverse voltage VR = 100V, and forward current decrease rate diF/dt = 40A/μs; and the leakage current IRRM under the reverse repetitive peak voltage VRRM = 2000V was tested at room temperature and 150℃ respectively. The test results are shown in Table 1. The data in the table are the average values ​​of the data measured for multiple samples. The numbers represent the classification numbers of different structures and processes, and their meanings are as follows: 1. Double-base structure diode, gold expansion controls minority carrier lifetime; 2. Double-base structure diode, electron irradiation controls minority carrier lifetime; 3. Traditional PIN diode, gold expansion controls minority carrier lifetime; 4. Traditional PIN diode, electron irradiation controls minority carrier lifetime. As shown in Table 1, dual-base diodes (types 1 and 2 in the table) have shorter reverse recovery times, shorter reverse recovery peak currents, and shorter dihr/dt compared to traditional PIN diodes (types 3 and 4 in the table), and their softness factor is also significantly improved. However, their forward voltage drop is slightly larger, especially for diodes using electron irradiation to control minority carrier lifetime. Compared to diodes using electron irradiation to control minority carrier lifetime, diodes using gold-plating methods have smaller forward voltage drops, shorter reverse recovery times, shorter reverse recovery peak currents, larger softness factors, and smaller dihr/dt. However, their reverse leakage current is larger. Therefore, for diodes with dual-base structures, gold-plating is the best method for controlling minority carrier lifetime. Table 1. Test Results of Dual-Base Diodes | Category Number | Sample Quantity | Forward Voltage Drop VFM (V) | Reverse Recovery Time trr (μs) | Softness Factor S=tb/ta | Reverse Recovery Peak Current IRM (A) | dirr/dt (A/μs) | Leakage Current IRRM (mA) (@ VRRM=2000V) | The leakage current data for categories 1 and 2 at 150℃ are the lowest values ​​among the samples. 6. Conclusion Dual-base diodes exhibit shorter reverse recovery times, shorter reverse recovery peak currents, and shorter dirr/dt compared to traditional PIN diodes, and significantly improve the softness factor. It is evident that the N-buffer layer has a significant impact on the reverse recovery characteristics of the diode. However, at the same time, the reverse leakage current of the buffer layer diode is significantly larger than that of the traditional PIN diode, especially at high temperatures. Due to time constraints, we have only conducted a batch of relatively systematic comparative experiments, and many issues have not yet been further explored. We expect that through further optimization of the design and improvement of the process, the reverse leakage current problem of the dual-base region structure diode can be improved, and the reverse recovery softness can be further enhanced.