On the High Temperature Operation of High Voltage ...

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of a triac silicon die, the same P+NP+ structure with the two identical blocking junctions is exhibited. The purpose of this paper is to provide further experimental ...
The Junction Edge Leakage Current and the Blocking I-V Characteristics of Commercial Glass Passivated Thyristor Devices Vasile V.N. Obreja, Elena Manea, Cecilia Codreanu, Marioara Avram and Cecilia Podaru National R&D Institute for Microtechnology (IMT), P.O. Box 38-160, Bucharest, Romania Tel: +4021-490-8212, Fax: +4021-490-8238,E-mail: [email protected], [email protected], [email protected] Abstract Unreliable performance of thyristor devices at junction temperature higher than 125–150 oC, cannot be understood if non-negligible leakage current flow at the junction edge is not taken into consideration. Current-voltage blocking (off-state) characteristics have been investigated for medium power thyristor devices available on the market. Typical results are shown at room and high temperature. A split of the two blocking characteristics is attributed to the edge junction leakage current component. Leakage current voltage dependence like V1/n where n varies in the range 1-5 is possible. At high temperature, no saturation tendency of the blocking leakage current is observed. Such results are not understandable by considering only the bulk component of the junction current. 1.

current leakage at the junction edge (IDLs or IRLs in Figs. 1b –1c) of commercial thyristor devices. K

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Introduction

The maximum operation temperature of commercial silicon thyristor devices available at this time is limited to 125-150 oC in spite of improved junction passivation techniques like glass passivation used in the manufacturing technology. Some results already reported, [1-3], indicated that leakage current flowing at the junction edge, can induce electrical characteristic instability due to non-uniform junction temperature, followed by device failure. The surface component of the blocking current has also influence on the maximum specified working blocking voltage, because premature surface breakdown may take place, [4]. Pertinent experimental evidence is still required to prove the presence of the surface component and this is not an easy task for available devices on the market. The surface component of silicon junction leakage current is usually neglected in theory and practice in favor of the bulk component. A typical thyristor structure for glass passivated devices is shown in Fig.1a. When bias voltage is applied between anode and cathode but without voltage on the gate, the device is in the off state. In such situation a low blocking (leakage current) direct current, IDL, or reverse current, IRL, flows through the device (Figs. 1b -1c). In the case of a triac silicon die, the same P+NP+ structure with the two identical blocking junctions is exhibited. The purpose of this paper is to provide further experimental evidence in favor of non-negligible

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c) Fig.1. – Glass passivated thyristor structure for medium power devices; 1 – junction termination; A – anode; K – cathode; G-gate; a) no bias voltage; b) anode positive bias voltage; c) anode negative bias voltage; 2. Experimental results. Discussion Typical blocking I-V characteristics found for glass passivated commercial thyristors are shown in Figs. 2 - 3. These devices are packaged in TO220 molded case. One can see a split of the two

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Fig.2: I–V blocking characteristics for 12A, 600V commercial thyristors supplied by a leading manufacturer of semiconductor devices; Top plot: log-log scale; Lower plot: log–linear scale; blocking characteristics at higher voltage. This behavior cannot be explained only by a dominant bulk leakage current for the two identical P+N junctions. Significant deviation from voltage dependence like V1/n of the blocking current, visible above 100 V, is not supported by the generation or diffusion bulk current components. One can see in Figs. 2-3 current–voltage dependence approximately like V1/2 or V1/3 at room temperature.

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Fig.3: I-V blocking characteristics for 12A, 600V commercial thyristors supplied by another manufacturer; Top plot: log-log scale; Lower plot: log-linear scale; Nevertheless at room temperature voltage dependence like V1/4 or V1/5 have been found for other device samples. Such voltage dependences of the junction leakage current are also exhibited when the surface component is the dominant one, [5 –6]. At high temperature (above 125 oC), lack of saturation of the blocking current is rather attributed to the surface component than to the bulk component. The results shown in Figs.2 -3 are based on investigation carried out on a few device samples. Taking into

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Fig. 4: Variation of the blocking current level found from a sample to another for 20 A, 600V commercial thyristors

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account that a typical value of 0.1 mA (0.5 mA maximum) is specified in the data sheet at 600Vand 125 oC for IDL or IRL, lower or higher level of the blocking current is possible for other device samples in comparison with the situation in Figs. 2-3. For higher level of the direct blocking current due to excessive flow at the junction edge, a breakover may take place below 600V at 150 oC and not towards 200 oC like in Fig.2. Consequently, this explains specification in the data sheet, limited to 125 oC for the maximum operation temperature. Apparently, a higher value for IDL above 600 V in Fig.2 could be attributed to some influence from the cathode junction. Nevertheless for a good device design, this is not possible as it is confirmed by the results shown in Fig.3. A charge accumulation layer at the junction periphery, [7], which shunts the junction bulk blocking capability (Fig.1) could explain the shape of the I-V blocking characteristics in Figs.2-3. The state of the semiconductor – dielectric interface from the junction periphery may differ for the two identical junctions. An example of spreading of the blocking current level is given in Fig.4 where the results are based only the measurements taken on 4 device samples. A split of the blocking I-V characteristics is also visible like in Figs. 2-3. In this case, a typical value of 0.2mA (1mA maximum) is specified in the data sheet at 600V and 125 oC. Some commercial devices do not exhibit a split of the I-V characteristic, because the dominant junction surface current is the same for the two identical junctions. An example is given in Fig. 5 for a 40A, 600 V triac. At room temperature current voltage dependence like V1/3 is manifested up to 1000 V, whereas in Figs. 2 – 3, similar dependence is exhibited only up to 200 - 300 V. Nonetheless at 175oC significant deviation from approximately V1/4 linear dependence is exhibited, and this behavior hardly could be attributed to the bulk component of the junction current. For the same device type but manufactured by different companies, significant difference has been found for the level of the blocking leakage current. An example is given in Fig.6 for two triac samples of 12A, 600 V. For the sample No.1 a split of the I-V characteristics is exhibited. In the case of the sample No.2 where the split is practically not visible, significant higher level of the current flows through the blocking junctions of about the same area as for the sample No.1 case. No saturation tendency of the leakage current is observed in both cases. The accentuated voltage dependence observed above 100V cannot be explained by the bulk generation or diffusion current components. Excessive high current flowing at the junction edge in the case of the sample No.2, causes breakover in one direction at lower voltage than in the case of sample No.1.

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Fig.5: I-V blocking characteristics for a 40 A, 600 V commercial triac; Upper plot: log-linear scale; lower plot: log-log scale;

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Fig.6: I-V blocking characteristics for triac samples of 12A, 600V supplied by two different manufacturers (plots in log–linear and log–log scale) For lower breakdown voltage devices, similar behavior has been found as above. No saturation of the blocking current is observed at high temperature, although identical blocking characteristics have been found for both bias voltage directions. 3.

Conclusions

Electrical blocking characteristics at low and high junction temperature, from thyristor and triac devices manufactured at this time by known companies in the field have been investigated. For some device samples, a split of the two identical blocking characteristics is observed. No saturation of the blocking characteristics is manifested at high

temperature. Voltage dependence of the junction blocking leakage current like V1/n, with n in a range of 2…5, is exhibited. Significant difference is possible in the level of the blocking current from sample to sample of the same type of thyristor device. This behavior cannot be understood if it is considered that the leakage current flowing at the junction edge has no contribution to the blocking current level. The junction edge current component is governed by phenomena at the silicon – passivation dielectric interface. The glass passivation method used at this time is not in full control of these phenomena. References [1] V.V.N. Obreja, C. Codreanu, K.I. Nuttall, O. Buiu, C. Podaru, "The Operation Temperature of Silicon Power Thyristors and the Blocking Leakage Current" in Proceedings 35th IEEE Power Electronics Specialists Conference(PESC2004), June 2004, Aachen, Germany, pp.2990-2993 [2] V.V.N. Obreja, C. Codreanu, K.I. Nuttall, O Buiu ,“Reverse Current Instability of Power Silicon Diodes (Thyristors) at High Temperature and the Junction Surface Leakage Current” in Proceedings IEEE International Symposium on Industrial Electronics(ISIE2005), June 2005, Dubrovnik, Croatia [3] V.V.N. Obreja, C. Codreanu, K.I. Nuttall, I. Codreanu, “Peaks in Temperature Distribution over the Area of Operating Power Semiconductor Junctions Related to the Surface Leakage Current” in Proceedings 6th International Conference on Thermal, Mechanical and Multiphysics Simulation and Experiments in M icro-electronics and Microsystems (EuroSIME2005), April 2005, Berlin, pp. 584 -589 [4} V.V.N. Obreja, C. Codreanu, D. Poenar ,O. Buiu, “Semiconductor PN Junction Failure at Operation Near or in the Breakdown Region of the Reverse I-V Characteristic”, in Proceedings 12th International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA2005), June-July 2005, Singapore [5] V.V.N. Obreja, C. Codreanu, K.I. Nuttall, “Reverse Leakage Current Instability of Power Fast Switching Diodes Operating at High Junction Temperature” in Proceedings 36th IEEE Power Electronics Specialists Conference(PESC2005), June 2005, Recipe, Brazil, pp. 537-540 [6] V.V.N. Obreja, “On the Leakage Current of Present-Day Manufactured Semiconductor Junctions”, Solid State Electronics, vol.49, No.1, pp.49 - 57, 2000 [7] V.V.N. Obreja, “An Experimental Investigation on the Nature of Reverse Current of Power Silicon PN Junctions”, IEEE Trans. Electron Devices, vol. ED-49, pp.155-163, 2002