Bipolar degradation is a known problem in the development of SiC MOSFETs when the body diodes (p+ body/ n-drift layer) are forward biased. Mostly higher voltage classes like the 1.7 kV or 3.3 kV SiC MOSFETs have been studied in literature resulting with significant Rdson increase [1-2]. In this work, body diode stress was conducted for 1.2kV SiC MOSFETs, which were mapped with Infra-Red photoluminescence (IR-PL) to determine and localize the exact number of BPDs present in the drift layers of each die [3, 4] and grouped by this criterion. Devices were stressed at extremely high current densities (1200 – 1700 A/cm2) under pulsed conditions. The post-stress analysis shows non-negligible increase of Rdson and Vf. Bipolar degradation occurring from stressing the body diodes at high forward current densities was confirmed by electroluminescence analysis.
In this work, body diode stress has been carried out for 1700 V 25 mΩ planar SiC MOSFETs. The epitaxial wafers were mapped with Infra-Red photoluminescence (IR-PL) to determine and localize the exact number of basal plane dislocations present in the drift layers of each die. The SiC MOSFETs were then packaged in groups with individual BPD counts in different bins ranging from 0 up to more than 30 per device. Pulsed body diode measurements with high currents of 250-400 A (about 1000-1600 A/cm2) were then performed with electrical characterization before and after to check for drift in key electrical parameters. Significantly increased RDSon was found after high current stress from about 300 A for devices with BPDs. A physical analysis of the degraded devices by backside electroluminescence show the presence of several trapezoid-shaped patterns indicating the occurrence of bipolar degradation.
Similar charge to failure distributions with mean values of about 50 C/cm2 were measured for planar SiC MOSFETs and MOS capacitors. Fast occurring and saturating negative flatband and threshold voltage drops were found in time resolved 1 second long pulsed gate current stress with IG=1 mA/cm2 at T=150 °C. No substantial difference in VTH drift rate with VGS=28 V at T=150 °C was found after about 10 s recovery period for IG stressed devices compared with unstressed devices. Additionally, IG stressed and unstressed devices did not differ in final VTH shift at T=25 °C after VGS=28 V stress (during 3 hrs or 31 hrs). More gate oxide reliability characterization is important to determine if 1 mA/cm2 pulsed gate current stress creates any permanent changes to the SiC MOSFET device behaviour. Additionally, parametric shifts in VTH and RDSon was examined after long-term AC gate bias stress by a gate driver switching between-8V and 20V for four different commercially available SiC MOSFETs.
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