4.1.2 Device Structures

The schematic cross sections of the simulated devices are shown in Figure 4.1 to Figure 4.3. Generally, the BV of conventional SOI LDMOSFETs, is limited by the buried oxide thickness, SOI thickness and the drift layer length. Figure 4.1 shows a cross-sectional view of a conventional $n$-channel SOI-LDMOSFET designed for a BV of 300V with an SOI thickness $t_\mathrm{soi}$ of 7$\mu$m, and with a buried oxide thickness $t_\mathrm{ox}$ of 2$\mu$m. The drift region of the device is doped according to the RESURF principle to achieve a maximum BV. $n$-channel P-SOI LDMOSFETs with a silicon window under the drain and under the source are shown in Figure 4.2 and Figure 4.3, respectively. As shown in the figures, silicon is used as a window instead of some part of the buried oxide layer. The major variable parameters are the $p$-substrate doping concentration and the $n$-drift layer length. The $t_\mathrm{soi}$, $t_\mathrm{ox}$, and SOI layer doping concentration of the P-SOI LDMOSFETs are the same as those of conventional SOI LDMOSFETs shown in Figure 4.1.

Figure 4.1: Potential distribution of a conventional SOI-LDMOSFET.

Figure 4.2: Potential distribution of a P-SOI LDMOSFET (Silicon window: Under drain).