In spite of the advancements in the field of semiconductor technology, traditional power electronic systems comprising of Silicon are still being used today. Silicon-insulated bipolar transistors have always been used in voltage source conversion to create flexible ac transmission systems (FACTS) and high voltage direct current transmission (HVDC). To further improve efficiency in terms of performance and keeping in mind the overall cost of the device, silicon carbide (SiC) is the new trend in the field of electronics.

SiC has superior physical and electrical properties to Si and is thus used in a wide range of power electronic applications. Semiconductor devices that comprise of SiC have a strong eclectic breakdown field strength due to which a reduced drift region thickness with higher blocking voltage capabilities can be seen.

During the experiment, the performance of high voltage SiC devices like MOSFETs, IGBTs, SB-diodes, and PiN diodes were tested under high voltage conditions and blocking voltages of 20 kV. Tests were also done to further extend the blocking voltages for instance SiC MOSFETs up to 20 kV as well as SiC GTO.

Ⅰ. Understanding various Experimentaional Models

While this experiment was conducted there were four models that were studied and analyzed in the conduction of ultra-high voltages.

A. General Modelling Approach: In the general modeling approach, there was an initial assumption that was made wherein various electrical parameters like gate drivability, gate resistance, and stray inductance. In this approach, the conduction loss, and switching loss are used in the calculation of total power loss. With this, the maximum allowed current density and switching frequency for a given maximum power dissipation limit for eg: PD,MAX=300 W/cm2, can be determined.

The above equation shows the derived total power and the calculated maximum frequency respectively.  

Fig 1 Blocking voltage as a function of drift region doping concentration and the estimated drift region doping concentration with drift region width.

Fig 1 shows the theoretical blocking voltage as a function of drift layer doping concentration and Fig 2 shows particular drift region doping concentration and drift region width for the devices used in this model.

B. SiC MOSFET Model: To determine the ON state characteristics of the SiC MOSFETs, parameters like MOS channel resistance, drift region resistance, and substrate resistance are inherited in the device. For MOSFET devices with blocking voltage above 1.2kV, the resistance contribution raised from the drift region is higher than the other contributing resistances.     

Fig 2 Temperature dependence of SiC MOSFET peak transconductance and threshold voltage, simulated results of SiC MOSFET conduction power loss density, and switching power loss density                                                

The above figure shows the temperature-dependent and peak transconductance and voltage parameters for 10 kV SiC MOSFET. The switching voltage is kept at 60 % of the blocking voltage(VDS=0.6VB). Furthermore, simulation results of the conduction power loss density and switching power loss density have also been presented. The estimated conduction power loss density is 30 W/cm2 to 60 W/cm2 in the range of 10 kV to 20 kV when J=15 A/cm2 and T=300 K.

C. SiC IGBT Model: The conduction losses of SiC devices that are bipolar have been comprised of 2 parts - knee voltage (VKNEE) which is placed near the and the inherent resistance elements present in the device similar to RON.

Fig 3 SiC IGBT knee voltage, simulated SiC IGBT conduction power loss density, and switching power loss density.

SiC has many state-of-the-art high voltage characteristics which can be used in the extraction of  VKNEE and RON parameters, as shown in Fig. 3. The forward voltage drop for SiC IGBT can be presented as: -

From the above equation, LCH is the MOS channel length and μni is the channel mobility, p is the cell pitch, VG is the gate voltage and COX is the oxide capacitance which can be determined by COX=εOX/tOX. The reverse recovery of the diode was not considered instead the turn ON energy loss has been assumed equal to the turn OFF energy loss. Switching characteristics of this device depends on the reverse recovery behavior of the freewheeling diode when turn ON and charge carrier are removed within the IGBT drift structure region.

D. SiC GTO Thyristor Model: Similar to the SiC IGBT, the SiC GTO thyristor model also allows conduction loss in the same modeling approach. To compare the results of SiC IGBT and SiC GTO, the same knee voltage model is adopted as of  “Mixed model”

From the above equation, EG is the bandgap energy of SiC and A is the area of the device. Just like the SiC IGBT, the losses in the SiC GTO are comprised of losses during voltage rise time and the tail current. The region thickness (WP) is set at 50 % of the (WN) region thickness.

Fig 4 Simulated results of SiC GTO thyristor conduction power loss density and switching power loss density, GTO turn-ON, and turn-OFF switching energy loss with maximum current gain.

The above equation shows the simulation results for various parameter testing of the SiC GTO thyristor in different conditions. It was seen that switching power loss was increasing as blocking voltage increased. On the other hand, turn-on losses, which were considered the main part of the total loss, showed a clear decline in trend.

Ⅱ. Testing various Parametric Sensitivity levels under different conditions

Devices made of SiC MOSFETs, SiC IGBTs and SiC GTO thyristors were tested and experimented with under the same conditions where J=15 A/cm2 , T=300 K, Ta=10 μs and f=150 Hz. The experiment was done to check the capabilities of ultra-high voltage SiC-based devices which are not very popular in the semiconductor industry today. Due to this, there may be certain uncertainties in the modeling parameters and hence a Parametric Sensitivity test was conducted.

Fig 5 Maximum current density and maximum switching frequency

As shown in figure 5, the maximum allowed current density and the maxim switching frequency are derived under given conditions of T=300 K, IJa=10 μs and f=150 Hz. from the above simulations, it could be clear that SiC MOSFETS can be employed at switching frequencies higher than SiC IGBTs and SiC GTO thyristors.

Ⅲ. Conclusion

In this experiment, different modeling approaches have been proposed which examine the use of ultra-high voltage  SiC MOSFETs, SiC IGBTs and SiC GTO thyristors. Several simulations were conducted which could prove that SiC MOSFETs have the largest current handling capability up to 15 kV. SiC IGBTs can be the device to be used for blocking voltages in the range of 15 to 35 kV and the SiC GTO thyristor is more suited for voltages more than 35 kV.