Model And Optimization Of Superjunction Devices

The power MOSFETs are widely employed as the main power semiconductor switching devices in low and medium power applications, which are also the main electron devices in the DC/DC converters and occupy the largest share of the power semiconductor market products. Based on the principle of charge compensation, the superjunction MOSFETs have much lower specific on-resistance than the conventional MOSFETs. Although the main drawbacks of fabrication difficulty and hard reverse-recovery of the body diode are existed in the S J MOSFETs, the introduction of some new structures, to some degree, alleviates these problems, and accordingly, a deep inside into the SJ MOSFETs is still important.In this thesis, the two-dimensional analytical models of the electric fields for the superjunction(SJ) and semisuperjunction (SemiSJ) MOSFETs are established. The relations of the specific on-resistance and the breakdown voltage of these two devices are also studied. The mechanisms of the avalanche breakdown are explained, and the impact on the breakdown voltages by the charge imbalance in the SJ layers is analyzed. Moreover, as a comparison, new design expressions for the abrupt parallel plain nonpunchthrough and punchthrough junctions, which are used as the sustaining layers of the VDMOSFETs, are proposed. The main innovations of this thesis are included as follows:1. Based on the Chynoweth law of the ionization rate, new design expressions for the NPT and PT junctions are proposed by using the numerical calculation method. The expressions are more accurate in terms of the MEDICI device simulations as compared with the results by using Fulop model. The new expressions allow more accurate evaluation in the design of the breakdown voltages and the parameters choices for the device designers.2. A useful approximation on the2-D electric field for the interdigitated balanced symmetric SJ devices is proposed, and the breakdown voltage mechanisms are explained by the doping concentration and depth in the SJ columns. It is found that the condition to obtain the minimum specific on-resistance for the SJ structure is that, when the voltage-sustaining layer is not fully depleted, the ionization integrals along different field lines approach one simultaneously. For the design of BV larger than600V, the optimized specific on-resistance can be lower down by larger than13%, compared with the conventional suboptimal design. By using the Catalan constant, G, the electric fields along the vertical direction and along the field line across the middle point in the boundary of P and N columns are approximated as the exponential function, and the previous conclusion that obtained by simulations is theoretically verified.3. Analytical models of the2-D electric field distributions of the interdigitated balanced symmetric SemiSJ structure, based on the charge superposition method, are derived. An accurate approximation of the exact analytical solution of the vertical electric field is also proposed and demonstrated by device simulation. The optimization method and its numerical calculation results of the specific on-resistance of the SemiSJ trench MOSFETs with constant aspect ratio are presented and verified by simulations. The calculation results show that the optimized specific on-resistance of SemiSJ trench MOSFETs can be reduced by larger than12%as aspect ratio equaling three, as compared with that of SJ trench MOSFETs. The effect of charge imbalance on the breakdown voltage is also investigated. It is found that the maximum BV is achieved at positive charge imbalance condition rather than at charge balance condition.