4.5.3.1 Forward Blocking Characteristics

Figure 4.45 shows a comparison of the BV of conventional SOI-LIGBTs, which have an $n$-drift length $L_\mathrm{d}$ $=$ 8.5 $\mu$m, and the SOI SA-LIGBT with a trench oxide at the drain/anode. In the forward blocking state the leakage current of conventional SOI-LIGBTs depends both on the space charge carrier generation and the common base current gain of the $pnp$-transistor, as a result a larger leakage current can be seen (dotted-dashed line in Figure 4.45). As shown in the figure the BV of the SOI SA-LIGBT is higher than that of the conventional LIGBT. Off-state current gain of the $pnp$-transistor affects the blocking voltage of the LIGBT, which is lower compared to the diode with same drift doping concentration and length. Because the shorted-anode by the $n^+$-drain effectively reduces the common base current gain of the $pnp$-transistor(some amount of electrons flow towards the $n^+$-drain contact and hole injection from the $p$-anode will be suppressed), SA-LIGBT has a similar high blocking capability to that of a conventional SOI-LDMOSFET.

The conventional SOI-LIGBT has a BV of 90V at $L_\mathrm{d}$ $=$ 8.5$\mu$m and $N_\mathrm{D}$ $=$ 1.0 $\times$ $10^{16}$ $\mathrm{cm}^{-3}$. The solid line in the figure shows the BV of the proposed SOI SA-LIGBT with a trench oxide at the drain/anode region. A BV of 130V is obtained with the same $L_\mathrm{d}$ as that of the conventional SOI-LIGBT. Because of the reduced common base current gain of the $pnp$-transistor, the BV is much higher compared to that of the conventional SOI-LIGBT. As can be seen in the figure it is the same BV as that of the conventional SA-LIGBT. To prevent a punch through of the depletion layer at the drain/anode, an $n$-buffer layer is added to all the device structures. Thereby higher forward blocking voltages due to RESURF effect are obtained. Figure 4.46 shows the potential distribution of the SOI SA-LIGBT at an anode-cathode voltage $V_\mathrm{A}$ $=$ 120V. The potential lines are suppressed by the highly doped $n$-buffer and the dense potential distribution can be seen at the buried oxide layer. It exhibits a similar potential distribution to that of the conventional SOI SA-LIGBT, the trench oxide at the drain/anode region does not affect the RESURF condition.

Figure 4.45: BV comparison of the conventional SOI-LIGBT, the conventional, and the proposed SOI SA-LIGBT. Lower leakage currents can be seen for the SA-LIGBTs.

Figure 4.46: Potential distribution of a proposed SA-LIGBT at the $V_\textrm {DS}$ of 120 V. The potential lines are suppressed by the highly doped $n$-buffer layer.