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 600V-class Semi-SJ MOSFET, and discuss about the design optimization by comparing the trade-off characteristics between the specific on-resistance (R on A) 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 V B =750V, the R on A=24.6mΩcm 2 , the maximum avalanche current density J AP =292A/cm 2 (I AP =7.6A, E AS =1.25J/cm 2 ), and softness factor=0.277 were obtained with the structure of T BAL -ratio=27% and N BAL =1.0x10 15 cm -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.