3.3.2 SJ $pn$-Diode

The SJ concept is based on the idea to achieve charge balance during the off-state between alternatively stacked $n$- and $p$-regions (pillars). The stacks can be made vertically and (or) laterally. In the off-state the $n$- and $p$-pillars is completely depleted before breakdown, and the electric field distribution becomes flat in the depletion region. This causes a linear relationship between $R_\mathrm{sp}$ and BV instead of the power relationship in (3.39). Figure 3.14 shows a schematic view of the SJ diode. With alternating $p$- and $n$-columns in the drift region the doping in this region can be increased drastically. In the off-state, increased cathode bias first extends the depletion layer of the $n$- and $p$-column junction in the horizontal direction, and both $n$- and $p$-columns are completely depleted at a larger cathode voltage.

Figure 3.15 shows the potential distribution of the SJ $pn$-diode at a reverse voltage of 300V. Potential lines are uniformly distributed throughout the drift region. This structure is designed to obtain a BV of 300V with an $n$- and $p$-column width $W_\mathrm{N}$ $=$ $W_\mathrm{P}$ $=$ 0.5$\mu$m and drift length $L_\mathrm{d}$ $=$ 15$\mu$m. With the SJ concept drift doping can be increased by decreasing the pillar width. However, considering the built-in depletion region, the width cannot be decreased indefinitely and there exists a minimum pillar width which is comparable to the built-in depletion width. Further decrease the pillar width below certain critical value the pillar doping cannot be increased. One can expect a square shape of the electric field distribution instead of the triangular shape for the case of conventional $pn$-diodes. Figure 3.16 shows the electric field distribution of the SJ $pn$-diode. It shows a rather high electric field along the $pn$-junction (and $p^+ n$- and $n^+ p$-junction) compared to that at each side of the device, but the electric field distribution is nearly square shaped throughout the drift region.

Figure 3.14: Schematic structure of SJ $pn$-diode.

Figure 3.15: Potential distribution of the SJ $pn$-diode at 300 V.

Figure 3.16: Electric field in the SJ $pn$-diode at 300 V.

Because of the lateral $p$- and $n$-column junction at the middle of the drift region (and the thin layer thickness of each column), a large depletion region can be observed at a relatively low voltage. For the conventional $pn$-junction diode the delpetion region expends gradually as the voltage increases. Fot the SJ structure, on the other hand, relatively thin $n$- and $p$-pillars become completely depleted at a low voltage (about 10V in this example). Although the doping concentrations of each column are much more higher compared to that of the drift region for the conventional $pn$-diode, it behaves similar to a lightly doped drift layer.

Figure 3.17 shows the electric field along the cut lines A, B, and C. As expected from Figure 3.16 higher electric field is observed along the $n$- and $p$-pillar junction (cut line C), and peak electric fields are observed at the $p^+ n$- and $n^+ p$-junction (peak positions at cut lines A and B). Figure 3.18 shows the electric field along the $pn$-junction at the middle of the device (cut line C in Figure 3.15) for reverse voltages from 10V to 300V, and it is flat throughout the junction. Figure 3.19 shows the electric field at the vertical direction of the device (along the cut line D in Figure 3.15). High electric field for each voltage step is observed at the middle of the cut line, and the voltage difference between the middle and the side of the structure becomes lower at high reverse voltage. Even at a low applied voltage of 10V, one can see a fairly high electric field at the side of each column because of the fully depleted $n$- and $p$-columns.

Figure 3.20 shows the BV versus drift doping concentration. The $n$- and $p$-column doping concentration over the critical value will reduce the BV of the device abruptly.

Figure 3.17: Electric field at the middle and each side of the SJ $\textrm {pn}$-diode at a reverse voltage of 300 V.

Figure 3.18: Electric field along the cut line C in Fig. 3.15 with the reverse voltage stepped from 10 V to 300 V.

Figure 3.19: Electric field along the cut line D in Fig. 3.15 with the reverse voltage stepped from 10 V to 300 V.

Figure 3.20: BV versus $n$-column doping of the SJ $pn$-diode. Drift length $L_\textrm {d}$ $=$ 15.0 $\mu$m.

Figure 3.21: BV of SJ and conventional $pn$-diodes. Drift length $L_\textrm {d}$ $=$ 9.5 $\mu$m. SJ $pn$-diode is built in $n$- and $p$-column width of 1.0 $\mu$m and 3.0 $\mu$m, respectively, and $n$- and $p$-column doping of 1.0 $\times$ $10^{16}$ $\textrm {cm}^{-3}$ and 3.3 $\times$ $10^{15}$ $\textrm {cm}^{-3}$, respectively.

Figure 3.21 shows the BV comparison for the SJ and conventional $pn$-diodes. Drift length of the $pn$-diode is $L_\mathrm{d}$ $=$ 9.5$\mu$m. SJ $pn$-diode is built in $n$- and $p$-column width of 1.0$\mu$m and 3.0$\mu$m, respectively, and $n$- and $p$-column doping of 1.0 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$ and 3.3 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$, respectively. For the simulation of SJ $pn$-diode complete charge balance of the drift region was assumed, this can be the major reason that the simulated BV is a little higher than the measured data. Because of the reduced drift length with high doping concentration in the drift region, conventional $pn$-diodes (drift region doping concentration of 2.5 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$ and 1.0 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$, respectively) have lower breakdown voltages compared to the SJ $pn$-diode.