1. A method for modeling a semiconductor device by analyzing simultaneous equations consisting of electron- and hole-transport equations and Poisson's equation by means of a computer, said method comprising the steps of: obtaining a representation of at least a structure and impurity concentrations of the semiconductor device; rewriting the simultaneous equations to the following equations (a), (b), (c) containing time differential terms dp/dt, dn/dt, d.psi./dt and sensitivity coefficients .lambda..sub.p, .lambda..sub.n, and .lambda..sub..psi. : EQU dp/dt=-.lambda..sub.p f.sub.p (a) EQU dn/dt=.lambda..sub.n f.sub.n (b) EQU d.psi./dt=.lambda..sub..psi. f.sub..psi. (c) determining meshpoints (M,N) having spatial position dependency corresponding to the at least a structure and impurity concentrations of the semiconductor device; transforming equations (a), (b) and (c) to the following equations (d), (e) and (f): EQU dp(M,N)/dt=-.lambda..sub.p (M,N)f.sub.p (M,N) (d) EQU dn(M,N)/dt=.lambda..sub.n (M,N)f.sub.n (M,N) (e) EQU d.psi.(M,N)/dt=.lambda..sub..psi. (M,N)f.sub..psi. (M,N) (f) linearizing equations (d), (e) and (f), discreting the same with respect to time, and transforming the same to the following vector equation: ##EQU51## time-integrating equation (g) after obtaining such the sensitivity coefficients that an eigen value of an error propagation matrix A contained in equation (g) is less than 1, thereby solving the simultaneous equations; and producing a model of said semiconductor device having physical values (p,n) at the respective meshpoints (M,N) obtained by using the solved simultaneous equations.