4.3.1 Introduction

We describe low on-resistance lateral trench gate SJ LDMOSFETs on SOI. The specific on-resistance $R_\mathrm{sp}$ of SOI-LDMOSFETs is effectively improved by the SJ concept together with the lateral trench gate. The SJ helps to increase the doping concentration of the $n$-drift layer and the lateral trench gate allows to increase the channel area. Therefore, a reduction of the on-resistance is expected in the both of the $n$-drift (by the increased doping concentration) and channel (by the large channel area) regions. We confirm that $R_\mathrm{sp}$ of the proposed lateral trench gate SJ SOI-LDMOSFETs is about 60% of that of conventional SOI-LDMOSFETs. With the $n$-column width larger than that of the $p$-column the doping in the drift region can be reduced to 70% of the value of standard SJ devices without degrading the on-resistance. As a result the sensitivity of the BV to the charge imbalance is improved in the proposed device.

Conventionally, the optimun drift layer doping concentration of SOI-LDMOSFETs is determined by the RESURF principle [106]. In order to increase the BV of RESURF devices the doping of the drift layer must be reduced and the drift layer length increased. New concepts such as SJ [130,33,158] and lateral trench gate [36,148] are proposed to improve $R_\mathrm{sp}$ of MOSFETs. Most of the SJ devices such as COOLMOS [131,132] and MDmesh [32] assume complete charge balance. This can be achieved by introducing alternating $n$- and $p$-columns in the drift region, and the doping in this region can be increased drastically. There is an inverse relationship with the width of the $n$- and $p$-columns. Even the current conduction area is reduced by the additional $p$-columns which do not contribute to the on-state conduction. This results in a significant reduction in $R_\mathrm{sp}$.

We present a lateral trench gate SJ SOI-LDMOSFET to obtain a low on-resistance. Contrary to conventional vertical trench MOSFETs, the gate is formed laterally on the side wall of a trench and the channel current flows in the lateral direction through the trench side walls. This allows to increase the channel area and decreases the on-state resistance of the devices. To increase the on-state conduction area in the drift region an unbalanced structure is examined where the width of the $n$-column is larger than that of the $p$-column.