4.4.2 Device Structures and Operations

Figure 4.31 shows the schematic structure of the proposed SJ SOI-LDMOSFET which has a trench oxide in the drift region.

Figure 4.31: Schematic of the SJ SOI-LDMOSFET with a trench oxide in the drift region. It has a buried $p$-column in the $n$-drift region.

This structure can be made by introducing buried $p$-columns in the drift region and by an additional trench process. Generally, the maximum BV of conventional SOI-LDMOSFETs is limited by the thickness of the buried oxide. The optimum drift length must be ensured to get the best trade-off between $R_\mathrm{sp}$ and the BV. With the structure proposed it is possible to reduce the drift length drastically without degrading the maximum BV by increasing the surface path of the drift layer. This buried $p$-column can be connected to the $p$-body directly or indirectly. The optimum $p$-column doping concentration is determined by the width of the $p$-column and the net charge of the $n$-column.

Our device is designed to achieve a BV of 300V with an SOI thickness $t_\mathrm{soi}$ of 7.0$\mu$m and with a buried oxide thickness $t_\mathrm{ox}$ of 2.0$\mu$m. With these structure parameters the maximum BV of conventional SOI-LDMOSFETs is 300V at the minimum allowable drift length of 20.0$\mu$m.

The main focus of the study is to optimize the device parameters of the proposed structure shown in Figure 4.31. The trench oxide depth affects the BV and it must be designed to ensure a long enough surface path of the device. It is important to minimize the $p$-column width, because it shrinks the conduction area of the device. The $n$- and $p$-column doping concentrations are a function of the column width. The $n$-column doping must be increased to lower the on-resistance of the SJ devices.

Simulations are performed to find optimum device parameters with a trench oxide depth from 2.0 to 3.0$\mu$m and a $p$-column width from 0.3 to 1.3$\mu$m. With an $n$-column width $W_\mathrm{N}$ of 4.0$\mu$m, a $p$-column width $W_\mathrm{P}$ of 0.3$\mu$m and a drift length $L_\mathrm{d}$ of 13.0$\mu$m the doping concentration of the $n$-column can be raised up to 6.0 $\times$ $10^{15}$ $\mathrm{cm}^{-3}$. As shown in Figure 4.32, the current of the proposed structure flows through the $n$-column. The figure shows clearly that only the $n$-column contributes to the current conduction.

Figure 4.32: Current distribution of a SJ SOI-LDMOSFET with a trench oxide at $V_\textrm {GS}$ $=$ 15 V and $V_\textrm {DS}$ $=$ 20 V. The $p$-column below the trench oxide does not contribute to the current conduction. The arrows show the current flow in the drift region.