4.4.1 Introduction

We describe high-voltage SJ SOI-LDMOSFETs which have a trench oxide in the drift region. With the proposed device structure a reduction of the on-resistance of the $n$-drift layer can be achieved.

The BV and the specific on-resistance of the suggested devices as a function of the trench oxide depth, the $p$-column width, and the doping are studied. We confirm that the specific on-resistance of the device proposed is lower than that of conventional SOI-LDMOSFETs, and the drift length is reduced to 65% compared to conventional devices.

To obtain the best trade-off between $R_\mathrm{sp}$ and the BV, we suggest a SJ SOI-LDMOSFET which has an extra $p$-column and a trench oxide in the drift region. The extra $p$-column is doped to achieve a balanced charge condition which means that the net depletion layer charge is zero. The trench oxide in the $p$-column helps to reduce the drift length without further decreasing the conduction area (only the $n$-column contributes to the current conduction).

The $R_\mathrm{sp}$ of the proposed structure is effectively reduced by the SJ concept together with the trench oxide. The SJ helps to increase the doping concentration of the $n$-drift layer, and the trench oxide in the drift allows to reduce the device size. Lowering $R_\mathrm{sp}$ without degrading the BV gives rise to a reduction in silicon area, which supports the economic fabrication of smart power devices.