4.2.3 Simulation Results

Reducing the on-resistance while maintaining a desired BV rating has been the main issue in the development of lateral power devices. The on-resistance of high-voltage SOI-LDMOSFETs strongly depends on the doping of the drift layer. In order to increase the BV of RESURF devices the doping of the drift layer must be reduced and the drift layer length increased. $R_\mathrm{sp}$ and BV are inversely related to each other.

In conventional SOI-LDMOSFETs a large voltage drops in the buried oxide, and it prevents potential lines from spreading into the substrate. Because the maximum electric field strength of the buried oxide is determined by the electric field strength of silicon at the interface between the silicon and the buried oxide, in the thin film SOI almost all the voltage drops in the buried oxide layer. To achieve a higher BV the surface electric field must be reduced by optimizing the drift length and the doping.

Figure 4.14 shows the potential distribution of the lateral trench gate SOI-LDMOSFET at a drain-source voltage of $V_\mathrm{DS}$ = 110V. The equipotential lines are uniformly spaced along the surface of the drift region. It exhibits a similar potential distribution as that of the conventional device, the lateral trench does not affect the RESURF condition. With the same BV as the conventional device it helps to decrease the on-resistance by increasing the current spreading area at the channel region. Three-dimensional numerical analysis have been performed to investigate the BV, $R_\mathrm{sp}$, and self-heating effects as a function of the lateral trench depth, and the space between the trenches.

Figure 4.14: Potential distribution of a lateral trench gate SOI-LDMOSFET at $V_\textrm {DS}$ $=$ 110 V.