2.5 Crystal face dependence

All results discussed above are extracted from in-house MOSFETs with the active channel on the a-face, (110)-plane. To obtain a complete picture, we also investigated MOSFETs with the channel on the Si-face, (0001)-plane. For these devices, the same trend is observed, although is in the range of millivolts and therefore not as pronounced in the ID -VG curves (not shown). This is a surprising outcome if one considers the, in general, much lower mobility for devices with the active channel on the Si-face. Compared to their a-face counterparts, the typical mobility of Si-face MOSFETs, which were subjected to an identical interface passivation, is 2 to 3 times lower. In our case, the extracted low-field mobility of the in-house Si-face devices is , whereas the a-face devices show a 3 times higher mobility of , which is consistent with recent studies [114].

From , the number of trapped charges per device area , can be extracted using the the formula of the capacitance and is given by

with the relative permittivity of SiO₂, , the vacuum permittivity , the oxide thickness and the electronic charge . Fig. 2.24 shows extracted from via (2.12) for both crystal faces as a function of the up-sweep starting voltage. It is important that, despite the lower channel mobility, also the interface trap density calculated from is about one order of magnitude lower on Si-face (diamonds, blue) devices than on a-face (circles, green) MOSFETs. For both device designs, no permanent component is present in the hysteresis. Even after millions of charging-discharging cycles, which occur during AC-use conditions, and therefore the hysteresis remains constant [111].

To summarize the dependence of the subthreshold sweep hysteresis on the SiC crystal plane, both Si-face and a-face MOSFETs show the same trend. However, the total amount of trapped charges extracted from the ID -VG curves is smaller on the Si-face, which is in contrast to the lower mobility of these devices. An explanation for the discrepancy between a lower trap density and lower mobility is given in the next section.