Design concept of n-buffer layer (n-Bottom Assist Layer) for 600V-class Semi-Super Junction MOSFET

We report the experimental results detailed about the n-buffer layer (n-BAL: n-bottom assist layer) of 600 V-class semi-SJ MOSFET, and discuss about the design optimization by comparing the trade-off characteristics between the specific on-resistance (R_onA) and the breakdown voltage (V_B), the avalanche capability and the body diode characteristic for the first time. As design parameters, the thickness ratio T_BAL-ratio and the doping concentration N_BAL were varied in this work. As a result, the VB=750 V, the R_onA=24.6 mOmegacm², the maximum avalanche current density J_AP=292 A/cm² (I_AP=7.6A, E_AS=1.25 J/cm²), and softness factor=0.277 were obtained with the structure of T_BAL-ratio=27% and N_BAL=1.0x10¹⁵cm^-3. The demonstration results showed that NPT (non punch through)-type design (with high T_BAL-ratio and high N_BAL) realized the larger avalanche capability and the softer reverse recovery characteristic compared with PT (punch through)-type design.