4.4.1.2 SBD Simulation

Fig. 4.6 shows simulation results of static current-voltage (I-V) characteristics of SiC Schottky diodes formed using different metals on the same blocking epilayer. The results affirm that the conduction voltage and the output current strongly depend upon the metal/semiconductor barrier height $\Phi_\mathrm{Bn}$. The saturation of the I-V characteristics at high forward currents shown in Fig. 4.6 (right) is due to the series resistance of the n-type blocking layer, which in this case is 6.5 $\mu$m thick with a doping concentration of 2$\times$10$^{16}$ cm$^{-3}$. The lower the barrier height, the lower is the forward voltage and the larger is the reverse current, see Fig. 4.7. As general rule it is desirable to keep the reverse current density below about 1 mA/cm$^2$ at the maximum specified reverse voltage. Using this criterion, the metal which has a lower $\Phi_\mathrm{Bn}$ could only be used for lower reverse voltage operation.

The Ni SBD provides the best overall compromise between forward and reverse characteristics for 1200 V operation as shown in Fig. 4.8.

Figure 4.6: Forward voltage characteristics of SiC SBD for different metal contact (left), and saturation characteristics at high forward currents (right).

Figure 4.7: Reverse voltage characteristics of SiC SBD at room temperature as a function of the barrier height (left), and influence of temperature on the leakage current for a cathode voltage of 1000 V (right).

The effect of the temperature on both forward and reverse voltage characteristics is analyzed and compared with the measurement results extracted from literature for commercially available SBD from Cree Inc. [175]. This diode will carry 1.5 A/cm at a forward voltage of 1.5 V for room temperature and reduces approximately to 0.7 A/cm at 500$~^{\circ}$C. This negative temperature coefficient will allow to connect more than one die in parallel in a package without any unequal current sharing issues. This behavior is unlike for high voltage Si PiN diodes. Fig. 4.8 (right) shows the reverse characteristics. The leakage current is less than 1 $\mu$A/cm at room at temperature and rises to 10 $\mu$A/cm at 500$~^{\circ}$C which is a very nominal increase for such a wide temperature range. The simulation results also show that the breakdown voltage increases slightly with elevated temperature, which is the expected characteristics of SiC power devices resulting from the reduced impact ionization coefficients with increasing temperature as explained in Section 3.5.3.

Figure 4.8: influence temperature on the forward voltage characteristics (left), and reverse voltage characteristics (right) in Ni/4H-SiC SBD.

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation