Trench Type Integrable SOI LDMOS And Its Protection Circuit

SemiconductorOnInsulator (SOI) Lateral Double diffusion MOSFET (LDMOS)has the characteristics of low leakage current, high speed, easy to integrated, andsimple drive circuit. However,thedevelopment of SOI LDMOS is limited by the‘silicon limit’.Based on the Moore’s law, the integrated density of the IC(integrated circuit)enhances continually as well as the power density of IC, especially the power IC.Mistakes or accidents will lead to the overload or output short-circuit, which resultsinto losing efficacy. Hence, integrated the protect circuits in IC are necessary.In this paper, we propose a trench-type SOI LDMOS to eliminate thecontradictionbetween the breakdown voltage (BV) andthe specific on-resistance(Ron,sp). It has two features: an oxide trench embedded into the drift region, a trenchgate extended to the buried oxide. The permittivityof oxide is samller than that of thesilicon.Thus the oxide trench improves the device BV and decreases the cell pitch byfolding the drift region. The accumulation layer on the surface of the trench gatereduces the Ron,spandthe power MOSFETs and drive circuits are isolated by the trenchgate.The BV, Ron,sp, and dynamic characteristics are studied in this paper. Comparingthe trench-type SOI LDMOS and SOI C-LDMOS (conventional LDMOS), the BVincreases by156%and Ron,spdecreases by35%at the same cell pitch, or the cell pitchand Ron,spdecrease54%and74%at the same BV, respectively.An over temperature protection circuit and an over current protection circuitwithlow sampling error are proposed in this paper.Using the positivetemperaturecharacteristic of ΔVGSand negative temperature characteristic of the threshold voltageVTH, we design an over temperature protection circuit. It features high Power SupplyRejection Ratio (PSRR), high precision hysteresis, and low power dissipation. It canintegrate in the p-well CMOS circuit because the circuit doesn’t use any BJT.ThePSRR of the circuit reaches to90dB, the over temperature threshold(T_H) is150℃，andthe thermal hysteresis threshold(T_L) is110℃.The over current protection circuittakessingle MOS as a comparer, of which the source is the input. It has the advantages ofsimple structure, low sampling error, and be available in the low side power MOS.The sampling transistor samples the output current and the resistor translates thecurrent signal to voltage signal. The circuit has low sampling errordue to the low overcurrent threshold voltage, which is only0.1V~0.3V. When the output current is150%of the rated current, the over current protection circuit will turn on.A CMOS compatible process is proposed in this paper. The layout of the power IChas been designed and the tape-out has been finished. The size of the trench in thetrench-type SOI LDMOS is3×3μm. Based on test, the BV of trench-type SOILDMOS is ranging from170V to190V, the threshold is about3V, and the outputcurrent is22mA at VGS=10V. In this paper, we also analyzed the issues which werefound after the test, and then proposed the solutions. The layout had been modified andbeing taped-out.