Research On Reliability And Temperature Characteristic Of LDMOS

In recent years, power semiconductor devices are making great progress rapidly with the fast development of power integrated circuits, and correspondingly, their applications in markets are extending step by step. As one lateral power DMOS device, the High-voltage bulk LDMOS is very suitable for power integrated circuits because of high breakdown voltage and great on-state characteristics. Furthermore, the electrode gate, source and drain are lead on the surface of LDMOS, which bring on easy integration in the standard CMOS processand very low cost. Therefore, high-voltage bulk LDMOS has been investigated widely.The dissertation selects the topic from 863 project significant spacial topic (High Voltage Drive Circuits Module). The high-voltage and large-current transistor are required in the Plasma Displaying Panel(PDP) data driver to drive and inspire the PDP. Among the whole cost of the PDP, the drive circuits have occupied quitea large part of it. In addition, the costs of the data driver and the scan chips are highest, so it is necessary to investigate these chips. With the improving technology of displaying panels, the working voltage of data driver chip have been reduced from 150V, 120V to 100V, 80V even less(50V) and the saturation current is only 40mA. The design of high-voltage transistor directly determines the performance of chips. The data driveris produced by the epitaxial technology abroad, but this technology costs a lot. If the single well process compatible to low-voltage process is substituted for the epitaxial technology to produce the drift region of LDMOS, the manufacture cost and difficulty can be cut down. Considering the requirements of high breakdown voltage, large current, low on-resistance in data driver, main structural parameters of LDMOS, including the length, the doping concentration and the junction depth of drift region and the length of the field plate, were obtained with the help of numerical simulation software -MEDICI by analyzing the mutual influences among the on-resistance, breakdown voltage and saturation current.Device models are essential for circuits simulations. Whether the results of simulations could predict the PIC' sperformance exactly, the precise HV device model was important and wanted. Because of the variety and complexity, at present, the amount of HV models is limited for PIC Computer Aided Design (CAD). Therefore, it is necessary and urgent to establish the SPICE modeling for PICCAD. In this dissertation, acurrent model of LDMOS is established and the expressions which include improved on-resistance model for double-diffused channel and drift region with field plate in LDMOS ispresented. The physical models of the capacitances in LDMOS are also built up to support the analysis of large signal and transient properties.After designing the basic parameters of devices, the designer should take the following problem of reliability into account. As a very synthetical concept, the reliability involves the structure of devices, manufacture process and applying conditions. The original design needs further to optimize. In view of the reliability of LDMOS, the author investigates the breakdown property which affects the reliability. As far as the power devices are concerned, the power dissipation and its distribution also have great effects on the device performances. The author builds up a macro-model and calculates the power dissipation and gain of an LDMOS invertor by using the model. The power dissipation and safe operation area of LDMOS are both discussed in details.In the high temperature microelectronics area, it is very natural to manufacture high-temperature silicon devices and Ies. Overseas researchers early developed this aspect work. Since the end of 70's, some scholars have carried on the investigations of high-temperature silicon devices and ICs positively. The early work took aim to design and manufacture high-temperature bipolar devices and thick film ICs. After 1984, some scholars started to research on high-temperature MOS transistor and CMOS IC systematically. They have given out several experiential formulae and equations of high-temperature electrical characteristics about the threshold voltage, surface mobility of carriers, sub-threshold current, drain current and leakage current variations under 25～300℃. At the end of 80's, high-temperature and large power transistor and SOI MOS have been studied thoroughly and LDMOS have entered historical arena henceforth and obtained more and more applications.The key to accurately evaluate the performances of devices lies on the simulation and analysis of LDMOS properties under various temperatures. Under such a background, it is significant to investigate the temperature characteristic and obtain the temperature model and provide a set of design methods under high-temperature. The temperature effects of threshold voltage, mobility, saturation current, leakage current and on-resistance are discussed here in the case of equal temperature and a linear expression of threshold voltage is proposed. Then the conclusion is drawn that the temperature coefficient can be regarded as a constant whose expression is also derived. At last, according to the equivalent circuits of LDMOS, the expression describing relationship between the on-resistance and temperature is presented which can applied in the temperature range from 27℃to 300℃. The results of proposed model make an agreement with simulated results by MEDICI.