Design And Research On Electrical Characteristics Of Dual Material Gate LDMOS

In recent years, communication market which give priority to honeycomb mobile communication increases explovsively. So the demand for RF LDMOS became more and more extensively such as emission amplifier whose operating frequency is from 900MHz to 2.4GHz and voltage about 30V. To RF LDMOS, more attation has been paid to device frequency besides intrinicⅠ-Ⅴcharacteristics. And both the high voltage and high frequency should be concerned when design a novel LDMOS used in communication IC. According to this, a new Dual Material Gate) LDMOS was proposed in this dissertation for RF amplifier whose high voltage and high frequency characteristics were improved simultaneously.Firstly, the application and superiority of LDMOS are introduced followed by its structure and characteristics. Considering for the different application fields, some concrete LDMOS structures were presented in conjunction with the comparison of main performance parameters.The proposed dual-material gate(DMG) LDMOS employed an effective concept of gate engineering. The gates of the DMG-LDMOS consist of S-gate with high workfunction material p+ poly and D-gate with low workfunction material n+ poly and are parallel to each other. Meanwhile, process of the composite gate is simple and feasible, which can be realized by only S-gare mask through compensation implanting and simulated by process simulation soft Tsuprem4. The compact process flow is presented afterward.To make the circuit simulation soft more and more accurately, it is indispensable to set up a precise model for threshold voltage which is a basic parameter of MOS device. But because the gate of DMG-LDMOS is made up of two kinds of marerials and channel is doped uniformly, it is hard to obtain the analytical model by solving poisson's equatin. Consequently, we analysed the relations of threshold voltage to length ration of the composite gate and workfunction. And then the experiential model of threshold voltage is built up based on the model of normal LDMOS, in which the parameters are concerned with substrate's material, composite gate's material and structure. Comparisons with MEDICI's simulation results show that the error is so small that can be used for circuit simulation.Because S-gate and D-gate are composed of low work function n+polysilicon and high work function p+polysilicon, a step electric potential which results in a peak electric field is generated. The peak electric field can improve the average speed of channel carriers, which make the device a higher transconductance and cut-off frequency. Meanwhile, the step electric potential not only increases breakdown voltage by a more uniform surface electric field but alleviates hot carrier effects and improves device life time by screening electric field of drain side.