FIGS. 4a through 4c are views useful in explaining the advantages of the present invention, wherein FIG. 4a is a fragmental enlarged cross sectional view of a conventional bipolar transistor such as shown in FIG. 1g; FIG. 4b is a view similar to FIG. 4a but illustrating the present invention, and FIG. 4c is a graph illustrating the electric characteristic of the present invention as compared with