4.4.2 PiN Junction Diodes

As the desired blocking voltage increases, the n-type blocking layer must become thicker and more lightly doped, as illustrated in Fig. 4.4. This leads to increased on-state resistance and reduced the maximum forward current density. Inverting (4.9) gives an expression for on-resistance as a function of blocking voltage

$$R_\mathrm{on,sp}=\displaystyle\frac{V^2_\mathrm{B}}{\mu_{n}\cdot\varepsilon_{s}}\cdot\left(\frac{3}{2E_\mathrm{B}}\right)^{3}$$ (4.10)

Strictly speaking, (4.9) and (4.10) apply only to unipolar devices such as the Schottky diode and the MOSFET. In bipolar devices such as the PiN diode, BJT, and GTO, the injection of minority carriers into the blocking layer is accompanied by a corresponding increase in the density of majority carriers to maintain charge neutrality [37,40]. These additional carriers increase the conductivity of the region (conductivity modulation) and the on-resistance is lower than given by (4.10). This represents a major advantage of PiN diodes over SBDs at high blocking voltages.
T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation