4.5.1 Introduction

The trench oxide at the drain/anode region effectively suppresses the snap-back voltage inherent in conventional SA-LIGBTs without increasing the anode length of the device. A weak negative differential resistance region is observed with the proposed device. SA-LIGBTs on SOI support a larger current density compared to SOI-LDMOSFETs, and the shorted-anode helps to achieve a faster turn-off time compared to the conventional SOI-LIGBT. With the proposed structure we confirm that the snap-back voltage of the proposed SOI SA-LIGBT is reduced by about 20% compared to the conventional SOI SA-LIGBT.

Lateral IGBTs on SOI have attracted much attention for high-voltage ICs and smart power applications, because they simultaneously handle a high voltage and a large current. In order to reduce the chip size of HVICs it is important to increase the current density of the output power devices. By means of dielectric isolation high-voltage LIGBTs on SOI allow to increase the operating current density due to the conductivity modulation by the minority carrier injection.

Figure 4.42: Turn-off time as a function of anode current for the LIGBT and SA-LIGBT.

However, LIGBTs have a slow turn-off time and a parasitic thyristor latch-up of the devices. It is necessary for lateral power devices to realize fast switching without the use of a lifetime control process which is not included in conventional CMOS technologies.

In order to fulfill these requirements new structures such as the hybrid SOI LDMOS-IGBT [160,161], the multi-channel approach [35], and the dual gate lateral inversion layer emitter transistor (DGLILET) [26] have been proposed.

One of the most efficient methods to achieve fast switching is to introduce a shorted-anode structure to the LIGBT. The SA-LIGBT (Shorted-Anode LIGBT) offers design flexibility with respect to a trade-off between switching speed and on-resistance. The $n^+$-anode short provides an electron extraction path during turn-off.

Generally, the SA-LIGBT has a much higher current density compared to LDMOSFETs and a faster switching speed compared to conventional LIGBTs. Figure 4.42 shows the turn-off time as a function of anode current for the LIGBT and SA-LIGBT [162]. This figure shows a faster turn-off time compared to conventional LIGBT. The major drawback of the SA-LIGBT is its negative differential resistance (NDR) region which is due to the two different conduction mechanisms responsible for the current flow in SA-LIGBTs [163]. To suppress the NDR one needs to increase the $p^+$-anode length, but this results in a larger chip size. We propose a new SA-LIGBT, which has a trench oxide at the drain/anode region. With this structure it is possible to reduce the snap-back voltage and a similar turn-off time as that of the LDMOSFET can be obtained. Even the reverse characteristics of the proposed structure are similar to that of conventional devices.