4.2.1 Introduction

The lateral trench gate allows the channel current to flow laterally on the trench side walls, which decreases its on-resistance, because the current spreading area of the device is increased [148,36]. The specific on-resistance ( $R_\mathrm{sp}$) strongly depends on the trench depth, which affects the channel area on the side wall of the trench and the space between the trenches also affects the channel area of the device. The $R_\mathrm{sp}$ of the suggested devices as a function of the lateral trench depth and the space between the trenches is studied.

Conventional SOI-LDMOSFETs have their channel regions on the surface. The channel is obtained by a double diffusion process [149,150]. The maximum BV of conventional SOI-LDMOSFETs is limited by the buried oxide thickness and the SOI thickness. It has been shown that by a proper choice of the $n$-drift doping and length, an optimal on-resistance can be achieved for a given BV requirement. The trench structure is used only for the isolation of power devices and low-voltage circuitry.

We present a lateral trench gate SOI-LDMOSFET which uses narrow trenches as channels. Contrary to conventional vertical trench MOSFETs with current flow in vertical direction, the lateral trench gate is formed laterally on the side wall of a trench and the channel current flows in lateral direction through the trench side walls. This results in an increased channel area compared to that of conventional SOI LDMOSFETs. Even the reverse characteristics of the proposed device is similar to that of the conventional device. Using three-dimensional simulations it is possible to see the vertical and lateral electric field distribution near the gate and the lateral current spreading in the channel.