H + Irradiation for Reverse Recovery Softness and Reliability of Power p-i-n Diodes for Snubberless Applications P. Cova, R. Menozzi, M. Portesine Abstract - We describe a coupled experimental-numerical study of the effect of proton irradiation on the inductive turn-off of fast- recovery p-i-n diodes for snubberless applications. The goal is to avoid the large overvoltages and spurious oscillations that may arise at switch-off and jeopardize the diode’s reliability. We evaluated different proton irradiation profiles in order to extract indications on the optimum trade-off among switching speed, recovery softness, overvoltage and spurious oscillation damping. I. INTRODUCTION Fast recovery diodes are today one of the most important and common devices in power electronics circuits, with freewheeling, snubber or clamp function. Due to the increasing power and speed of state-of-the-art power semiconductor switches, such as IGBTs, the diodes are subjected to ever more critical switch-off conditions: the higher the current gradient, the larger the overvoltage across the diode during reverse recovery. In order to limit these dangerous overvoltages, snubbers are often used, but their increasing complexity and cost lead to a demand for diodes able to operate without snubbers. Fig. 1. Inductive reverse recovery measured for p-i-n diodes with different structure. Fig. 1 shows two cases of snubberless inductive reverse recovery: “hard” recovery and “soft” recovery. A soft recovery prevents the so-called “snap-off” condition, which refers to the arising of anomalous overvoltage oscillations, highly dangerous for device and circuit. In this respect, the problem is that of designing diode structures that ensure self-limitation of the peak reverse voltage at the moment of current switch-off [1]-[6], a problem that is clearly reliability-related, because a well-designed diode, able to cope with the possible worst-case conditions encountered during circuit operation, can avoid catastrophic failure of the diode itself and of the surrounding elements. In electron-irradiated p-i-n diodes, softer recovery and oscillation avoidance can be obtained by a good compromise between irradiation dose, resistivity and width of the drift region, as shown in [7], [8]. Nevertheless, as suggested by many authors [9]-[11], a better compromise between switching speed and softness can be reached by selectively reducing the carrier lifetime across the p-n junction. In this way, the turn-off transient can be improved, with softer switching and a smaller impact on the forward voltage drop. There are various methods to obtain selective lifetime control: one of the popular ones in today’s technology is proton irradiation, whereby the lattice damage can be confined within a region 10-20 µm large across the junction. The goal of this work is the experimental and numerical study of the effect of the proton irradiation on the reverse recovery of fast-switching p-i-n diodes. II. EXPERIMENTAL AND NUMERICAL TECHNIQUES A. Samples The samples studied in this work are proton-irradiated p-i-n diodes for 1000 A, 4500 V applications, which may withstand di/dt’s higher than 1000 A/µs. Fig. 2 is a sketch of the device structure; the dashed line represents a schematic lifetime profile as obtained implanting H + from the anode side. A few identical wafers were H + -irradiated with different doses and then electrically characterized in order to evaluate the influence of this parameter. B. Measurements The carrier lifetime under high-injection conditions was measured by the OCVD (Open Circuit Voltage Decay) technique [12]. We used a fast mercury relay to obtain P. Cova, and R. Menozzi are with the Department of Information Technology, University of Parma, Parco Area delle Scienze, 181/A, I-43100 Parma, Italy, E-mail: cova@ee.unipr.it M. Portesine is with POSEICO S.p.A., via N. Lorenzi, 8, 16152 Genova, Italy, E-mail: portesine.marco@poseico.com