Heat localization mechanism of temperature field nonlinear evolution in semiconductor material acting as an element of electronic circuit with a constant E.M.F., is proposed. Using this mechanism, nonlinear mathematical model of SB formation in the material film is built. The model is constructed on the basis of nonlinear thermal conductivity equation having a heat source. The equation nonlinearity is connected with the temperature dependencies of electrical conductivity and of thermal one. The instability of spatially uniform solution is shown. It is manifested by means of the sharpening regime (solution tends to ∞, but time is finite), if initial nonuniformity lies in a localized spatial region having a characteristic size (fundamental length). This solutions property describes the spontaneous origin of thermal structure characterizing by strong heat localization in domains having the size of fundamental length order. Due to this, the sharpening is interpreted as SB. The SB formation takes place during the time connected with the sharpening one. SB is physically manifested by appearence of channels melted through the film when sub-microsecond duration pulse applying. Due to this, material metallization and poly-crystallization take place. Breakdown time t^* and localization size r^* are estimated on the basis of parabolic equations maximum principle by means of standard solutions technique in frameworks of one-dimensional model.