Patent ID: 7750438

Claim:
A semiconductor device comprising: a first conductivity type drift region; a first conductivity type emitter region formed on one side of said drift region; a second conductivity type base region arranged between said drift region and said emitter region; a gate electrode layer arranged to be electrically isolated from and opposed to said base region held between said drift region and said emitter region; a second conductivity type collector region formed on another side of said drift region; and a first conductivity type buffer region arranged between said drift region and said collector region with a higher impurity concentration than that of said drift region, wherein the ratio (D C /D B ) of the net dose D C of said collector region with respect to the net dose D B of said buffer region is at least a assuming that a represents the ratio (WTA/WTB) between WTA expressed as: WTA = 2 ⁢ ɛ s ⁢ ɛ 0 ⁢ V qNd (where ∈ S represents the dielectric constant of silicon, ∈ 0 represents the dielectric constant of a vacuum, q represents the charge quantity of electrons, Nd represents the impurity concentration of said drift region and V represents the avalanche voltage) and the thickness WTB of said drift region held between said base region and said buffer region.