New Soi Ldmos High Voltage Devices

SOI (Silicon On Insulator) is called the mainstream of silicon integrate technique for high speed and low power using in twenty-first century, and become the trend of the Power Integrated Circuit (PIC) due to the improved isolation, reduced leakage current, high speed performance, low power dissipation, and perfect irradiation hardness. As one of the key device in SOI HVIC, SOI high voltage device has been deeply investigated in this field. Because of the depletion region can't spread into substrate SOI device suffer from low breakdown voltage. In this thesis, new SOI high voltage structure is proposed, and the breakdown voltage problem is also analyzed from device structure. The main study is as follows:The request of SOI device obtaining uniform surface electric field is derived from analyzing SOI RESURF(REduced SURface Field ) principle. The RESURF criterion is obtained by solving poisson equation of full depletion SOI LDMOS, and the conclusion of SOI device must considering the influence of buried oxide is proved. Then the relation between the parameters is given when SOI device could obtained uniform surface electronic field. This relation is the product of drift doping and characteristic thickness needs to meet a step value. From this, several possible methods for improving vertical breakdown voltage of SOI device is proposed.A new step thickness drift region SOI structure is firstly proposed. In this structure the drift region is divided into different parts with thickness increasing from source to drain, surface electric field is improved due to modulation effect of step drift region on the electric field, and therefore the breakdown voltage is increased. The optimized relation for the structure parameters to breakdown voltage and conduct resistance is studied by 2-D simulation soft MEDICE. It is found the proposed structure can also improve vertical breakdown voltage and have smaller conduct resistance for higher optimized doping of drift region. Numerical simulation indicates the breakdown voltage increases by 76% and the on-resistance decreases by 31.3% in comparison with that of conventional SOI structure at the 1μm buried oxide thickness and 0.5μm drift region thickness. The proposed structure can use thinner buried oxide thickness at the same breakdown voltage, this helps to alleviate self–heating effect.A novel SOI high voltage device with charge captured trench is studied. The high voltage model is established by introducing interface charge on the buried oxide which can improve vertical breakdown voltage. Then, a new method to improving vertical breakdown voltage of SOI device by introducing charge captured trench on buried oxide is proposed. For the novel structure on the reverse-biased state, high density interface charge increasing from the source to the drain are distributed in the trenches of the buried oxide for the restriction of the captured trench to the carriers. As a result, an improved vertical breakdown voltage can be achieved due to the enhancement of the electric field in the buried oxide layer. Furthermore, the lateral breakdown voltage can also be improved due to the considerable uniform surface electric field modulated by the interface charges. A detailed investigation is performed to analyze the impact of the structure parameters (drift length and doping concentration, top silicon thickness, buried oxide layer thickness),and the trench parameters(height, width, space of the trench and the degree of the interlaced trenches for double-surface charge captured trench structure ).Finally, the hexagon pattern of buried oxide layer is proposed. Its request and impact to breakdown voltage are analyzed when there are trenches for the drain at any position and any direction.