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.