First SiGe diode avoids thermal runaway
Nexperia has developed the first SiGe diode rectifiers that avoid the problems of thermal runaway in power designs.
“We are the first company in the world to offer SiGe rectifiers,” said Jan Fischer, product manager at Nexperia. “After a study a few years back we saw the materials were beneficial for discrete rectifiers,” he said. “Usually you want the best efficiency with the highest thermal stability with a safety margin. Schottky suffer from high leakage and unstable at high temperatures and stop at 150 to 200V when you move to silicon PN diodes. SiGe matches the performance of a Schottky diode with low leakage current and we think this is particularly interesting in the 100V to 200V range.”
The PMEG SiGe diode 1 to 3A devices (PMEGxGxELR/P) are housed in the Clip-bonded FlatPower CFP3 and CFP5 packages that use a solid copper clip. These are also used for the company’s Schottky and fast recovery diodes, making drop in replacement possible.
The SiGe diode process is more complex than a Schottky diode but avoids the thermal runaway problem. “You grow SiGe layer on the epi with the metal layer, and the function is set by the Si-SiGe junction rather than the si-metal junction,” he said. “We have tested the chip to 200˚C and you cannot provoke thermal runaway but beyond 175˚C the issues are the clip interfaces and the mold compound so it’s more a package failure.”
This means the designer doesn’t have to worry about the thermal runaway.
“A recovery rectifier at 150 or 160 ˚C in a motor control or applications with a hard limit on leakage current such as a current sensor, these are reasons for using a recovery rectifier,” he said. “The SiGe rectifier has a similar leakage level but 20 percent lower conduction loss so we see SiGe as a drop in replacement for recovery rectifiers in applications such as high temperature LED lighting, ECUs and electrification as well as reverse battery protection in laser systems. The first devices are qualified to AECQ101 for automotive designs.
The next step is to go to 15A in a CFP15B (SOT1289) package. We need to see if 200V makes sense in that package thermally. That’s the plan we have at the moment,” he said. “We are also looking at 60 to 100V for lower voltage.”
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