I am trying to fit the DC characteristics of the real BJT which I had attached in my earlier email using a pspice BJT model. I am using the gummel poon model parameters to model the real BJT . I don't know which parameters to change . Since there is drop of gain at low currents I have to figure out how to model it in pspice model of BJt the same loss of gain BF . Since the equation for IB in the book had Ib (nonideal)= C2 Is(0) (e^(q*Vbe/nelkT) -1) I changed the value of C2 and nel to describe the nonideal component of Ib which is dominant at low currents.So let me know if i am going wrong and please refer me to someone who knows about this.