Shigehisa Yamamoto

Development of 3.3 kV SiC-MOSFET: Suppression of Forward Voltage Degradation of the Body Diode

In order to achieve cost reduction or shrinkage of power devices, an internal body diode, which forms in a MOSFET parasitically, can be designed as a free-wheeling diode in substitution for an external Schottky barrier diode (SBD). However, in a SiC p-i-n diode, forward current stress causes reliability degradation due to expansion of the electron-hole recombination-induced stacking faults. Applying the process optimization of the epitaxial layer for the reduction of recombination-induced stacking faults and the body diode screening method to 3.3 kV SiC-MOSFETs, we obtained more stable devices under forward current operation.

Stacking fault expansion from basal plane dislocations converted into threading edge dislocations in 4H-SiC epilayers under high current stress

We evaluate the stacking faults (SFs) expansion from basal plane dislocations (BPDs) converted into threading edge dislocations (TEDs) under the current stress to the pn devices and analyzed the nucleation site of the SF by combined polishing, chemical etching in molten KOH, photoluminescence imaging, Focus ion beam, transmission electron microscopy, and Time-of-Flight secondary ion mass spectrometer techniques. It was found that the formation of SFs occurs upon the current stress levels of 400 A/cm2 where the diode area is not including BPDs in the drift layer after the high current stress, and the high current stress increases the SFs expansion density. It was also found the dependence of the junction temperature. The estimated activation energy for the expansion of SFs is Ea = 0.46 eV. The SF extends from the conversion point of the BPD into the TED within buffer layer. Even though BPDs converted into TEDs within the high doped buffer layer, SFs expand under high current stress.

Demonstration of SiC-MOSFET embedding Schottky barrier diode for inactivation of parasitic body diode

External Schottky barrier diodes (SBD) are generally used to suppress the conduction of the body diode of MOSFET. A large external SBD is required for a high voltage module because of its high specific resistance, while the forward voltage of SBD should be kept smaller than the built-in potential of the body diode. Embedding SBD into MOSFET with short cycle length increases maximum source-drain voltage where body diode remains inactive, resulting in high current density of SBD current. We propose a MOSFET structure where an SBD is embedded into each unit cell and an additional doping is applied, which allows high current density in reverse operation without any activation of body diode. The proposed MOSFET was successfully fabricated and much higher reverse current density was demonstrated compared to the external SBD. We can expect to reduce total chip size of high voltage modules using the proposed MOSFET embedding SBD.

Driving Force of Stacking Fault Expansion in 4H-SiC PN Diode by In Situ Electroluminescence Imaging

We evaluate the velocity of stacking faults (SFs) expansion under various current and temperature levels on the pn diodes by electroluminescence (EL) observation in situ. The driving force of the SFs expansion is analyzed on the basis of the experimental results. The velocity of the SFs expansion increases in proportional to the current density at the every junction temperature levels. The activation energy for the velocity of the SFs expansion is estimated.