4.4.3.2 MPS Diode Simulation

To evaluate the MPS diode concept it is important to compare the on-state voltage and the reverse bias I-V characteristics of SBD, PiN, and MPS diodes fabricated on the same wafer, and simulate these devices under the same conditions. The MPS diode shown in Fig. 4.12 is simulated and compared with measurement results extracted from [177].

Fig. 4.13 illustrates the on-state characteristics for the MPS diode for different temperatures. The result shows the excellent rectifier ability of the MPS diode. The on-state characteristics of the MPS and Schottky diodes are almost identical, indicating excellent current spreading and minimal increase in the on-state voltage due to the introduction of the p+ implanted grid. As with Schottky diode, the increase in the on-state voltage with temperature in MPS diode is indicative of the reduction of the mobility with temperature for a majority carrier device. Although this positive temperature coefficient of the resistance increases the on-state power loss at high temperatures, it is beneficial for paralleling two or more devices to increase the power by large area current sharing. The current flow in MPS diode occurs primarly accross the Schottky region as shown in Fig. 4.14. A high current density of 600 A/cm$^2$ for an on-state voltage drop of only 3 V was obtained.

The reverse bias characteristics of the MPS

Figure 4.13: Forward voltage (left), and reverse voltage (right) characteristics of 4H-SiC MPS diode at different temperatures.

Figure 4.14: Current density in the 4H-SiC MPS diode (left) under forward bias operation, and a lateral and vertical cut (right).

Figure 4.15: Profile of the electric field in 4H-SiC MPS diode at 1000 V reverse voltage operation (left), a horizontal and vertical cut (right).

diode displayed in Fig. 4.13 (right) is much more similar to the PiN diode than to the Schottky diodes. However, the high temperature leakage current for the MPS diode is larger than that of the PiN diode due to the Schottky region. At a leakage current density of 2 $\mu$A/cm$^2$, the operating temperature of Schottky, MPS, and PiN diodes are predicted by simulation to be 500 K, 350 K, 300 K, respectively. A blocking voltage of 1350 V which is 90% of the desired value with negligible leakage current density was achieved for the MPS diode operating at room temperature. To evaluate the effectiveness of the p+ region on the reverse bias operation, the ratio of the electric field at the Schottky interface to the peak electric field (at the bottom of the p+ region) is analyzed by simulation. It was found to be 26% for the optimized device geometry as shown in Fig. 4.15. When the space between the p+ regions is increased, this ratio also increases significantly. T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation