4.1.1 Introduction

The high-voltage and self-heating behavior of P-SOI LDMOSFETs were studied numerically. Different locations of the silicon window were considered to investigate the electrical and thermal effects. It was found that the potential distribution inside P-SOI LDMOSFETs with a silicon window under the drain is similar to that of standard junction isolation devices. With a silicon window under the source the potential distribution is similar to that of conventional SOI-LDMOSFETs. We confirm that the BV of P-SOI LDMOSFETs with a silicon window under the source is higher than that of P-SOI LDMOSFETs with a silicon window under the drain.

One of the key issues in the realization of `smart power' technology is the isolation of power devices and low-voltage circuitry [135,136,137,138,139,140]. SOI technology constitutes an attractive alternative to the traditional junction isolation. When high-voltage devices over 100V are integrated on an SOI wafer, the isolation area between devices shrinks and lower leakage currents result in greatly improved high temperature performance.

In general, the breakdown voltage of an SOI structure is a function of the thickness of the silicon and the buried oxide layer [89,94]. The buried oxide helps to sustain a high electric field which results in a higher BV. However, the operation of SOI power devices is limited by self-heating effects during switching and some fault conditions such as short circuit [141]. Since the buried oxide underneath the device is a good thermal insulator, the temperature rise inside SOI power devices can be much higher than that of bulk silicon devices [142,143,144].

Self-heating in SOI power devices can result in serious reliability problems during operation in a high temperature environment. To solve this problem, the P-SOI technology was suggested [145,146,42,43,147], where a silicon window helps to reduce self-heating. In addition, potential lines spread into the substrate. Therefore it is possible to obtain higher breakdown voltages than those of conventional SOI devices, because the depletion layer in the substrate helps to increase the breakdown voltage.