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AN-1086 Datasheet(PDF) 4 Page - International Rectifier |
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AN-1086 Datasheet(HTML) 4 Page - International Rectifier |
4 / 9 page www.irf.com 4 AN-1086 When the edge of depletion layer width of the reverse biased P+/N- junction approaches the P+ collector of the IGBT, injection of holes will occur causing and results in breakdown when the undepleted drift region portion (Wn) is less than one hole diffusion length. In this case open base transistor BVceo occurs when the product of electron multiplication coefficient and base transport factor becomes equal to unity. It can be demonstrated (Ref 1, pg 352) that BV B ceoB = VBBB (1-αBTB)P 1/6 Were V B BB = Avalanche Breakdown Voltage of the PP + P N P - P junction, α B TB = Base Transport Factor = 1 / cosh [ (WBnB / Sqrt (DBpBτBpB) ] W B nB= Undepleted Base Width, DBpB = Hole Diffusion Coefficient, τBp = BHole Carrier Lifetime. In order to reduce conduction and switching losses, short N P - P base and N Depletion Stop are required, along with high carrier lifetime to insure sufficient conductivity modulation of the N- base. As a result high base transport factor is achieved. The short N- base could also result in BVceo-type of characteristics, which cause looping behavior in static BV. The looping phenomenon is non destructive as long as the current is limited within the SOA capability of the device. If the current source used in an automatic tester (typically used in final probe and assembly) to perform the BV test has a high Voltage compliant, the variable load will result in uncontrolled repetitive current spikes that can damage the device. See Figure 4 for normal and loopy static BV waveforms as seen on a curve tracer. When the loopy BV phenomenon occurs, it can be seen that the leakage current is low (40~60nA at 25C) up to the onset of avalanche breakdown, approx 700V. As soon as the impact ionization process starts, the secondary holes are injected into the base triggering the PNP transistor into BVceo mode. After the open base transistor breakdown the current through the N-drift region increases and this can result in depletion extension and further reduction of undepleted portion of the base (Wn). The depletion extension can be estimated by comparing the leakage current to the Kirk current density (Ref 2 p 224). Overall impact of this is increase in α B TB and further reduction in voltage across the IGBT. This leads to device exhibiting negative resistance where voltage decreases with increasing current. Due to the constant power provided by the curve tracer, the negative resistance developed as the avalanche current increases results in Current / Voltage looping. This looping will result in inconsistent break down voltage readings. Repetitive measurements can potentially damage the IGBT under test. |
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