4.4.2.2 PiN Diode Simulation

The PiN diode shown in Fig. 4.9 is simulated. The device is optimized and model parameters are calibrated to the experimental data obtained from  [177]. For calibration the dimensions of the experimental device are used to define the simulated device dimension, doping profile, contact, and mesh griding.

Figure 4.10: Forward voltage characteristics of 4H-SiC (left), and 6H-SiC (right) PiN diode for different temperatures.

The physical model parameters are adjusted until good agreement between the simulated and measured static IV characteristics is obtained.

Fig. 4.10 shows the simulated (solid lines) and measured (circles) on-state characteristics of SiC PiN diodes for different temperatures in the range of 300 to 500 K. The diffusion current in a p-n diode scales as the square of the intrinsic carrier concentration $n_i$ (see (2.2)). Because of the wider bandgap of SiC, $n_i$ is approximately 18 orders of magnitude lower than silicon $n_i$ [37,40]. This requires a greater reduction of the built-in potential barrier to achieve the same current density. Typical junction voltage drops in a forward-biased SiC PiN diode are on the order of 3 V as shown in Fig. 4.10 for a 4H- and 6H-SiC on a 10.5 $\mu$m blocking layer doped 7.2$\times$10$^{15}$ cm$^{-3}$. The total on-state power dissipation of a diode is determined by the voltage drop across the junction and the on-state resistance of the blocking layer. The diode has a lower on-resistance, but a higher junction voltage drop as compared to the SBD.

The advantage of lower on-resistance is partially offset by the higher junction voltage required to achieve the same current density in a PiN diode compared to an SBD. The decrease in the on-state voltage with temperature is due to the decrease in the built-in potential with increasing temperature and the increase in the life time with temperature.

Figure 4.11: Reverse voltage characteristics of 4H-SiC (left), and 6H-SiC (right) PiN diodes for different temperatures.

For comparison Fig. 4.10 (right) displays the forward bias characteristics of 6H-SiC. The result shows that the 6H-SiC polytype has a lower output current due to its lower and less isotropic electron mobility compared to its 4H-SiC counter part (see Section 3.3).

As the blocking voltage is increased, the on-resistance of the blocking layer eventually becomes the dominant resistance, giving the PiN diode an advantage over the SBD. Fig. 4.11 shows the reverse voltage characteristics of SiC PiN diode. This diode blocks 1600 V for 4H-SiC and a slightly higher for 6H-SiC due to its higher breakdown electric field strength. A positive temperature coefficient can be observed on the blocking characteristics of the PiN diode. The leakage current is, however, orders of magnitudes lower than in the SBD. T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation