Super-junction transistors bypass this limit, and bipolar transistor are not subject to it, but both require p-type semiconductor layers in otherwise n-type unipolar devices.
To create p-type SiC, it has to be doped with aluminium (usually), by epitaxial growth or ion implantation – both of which can cause defects that alter the way the devices drift layer works.
Researchers at the Nagoya Institute of Technology have been studying the relative merits of epitaxial growth or ion implantation in forming p-inclusive devices, and creating guidelines for minimising detrimental effects.
They studied SiC PiN diodes, investigating the depth distribution of defects using ‘deep level transient spectroscopy’ (DLTS) and cathodoluminescence (CL).
“Our findings will help with the optimum design of SiC power devices,” said Nagoya researcher Masashi Kato. “These results will ultimately help improve the performance, as well as the size and energy consumption of traction systems in vehicles and trains.”
They found that p-type layer deposition by epitaxial growth did not cause damage in adjacent n-type layers, but the growth showed slight instability that led to the formation of deep level defects. “The specific on-resistance of this diode was also low, thanks to the effects of conductivity modulation,” according to the University.
For the diode formed by ion implantation, the researchers found high specific on-resistance without influencing conductivity modulation. And defects that penetrated at least 20µm from the implantation region. “Our study shows that the ion implantation in SiC bipolar devices need to be processed at least 20µm away from the active regions,” said Kato.
The work is covered by ‘Depth distribution of defects in SiC PiN diodes formed using ion implantation or epitaxial growth‘ published in Physica Status Solidi (b).
Image courtesy of Masashi Kato, Nagoya Institute of Technology