Patent ID: 7777255

Claim:
A bipolar transistor on a substrate comprising a base ( 4 , 12 ) with an inner base portion ( 4 a ) and an outer base portion ( 4 b ) which adjoins the inner base portion ( 4 a ) in a lateral direction in parallel relationship with the substrate surface and in a heightwise direction facing perpendicularly with respect to the substrate surface and is of a greater extent than the inner base portion ( 4 a ), wherein the base includes a first epitaxial base layer ( 4 ) and a second base layer ( 12 ) disposed on the first layer ( 4 ) the second base layer ( 12 ) being opened in a lateral region above the inner base portion ( 4 a ) to form an emitter window ( 9 , 4 a ), an emitter ( 9 ) of a T-shaped cross-sectional profile, which is separated from the outer base portion ( 4 b ) laterally by a spacer ( 8 ) of insulating material, wherein a lower end of a portion of the emitter ( 9 ), which corresponds to a vertical bar of the T-shaped cross-sectional profile, adjoins in the heightwise direction the inner base portion ( 4 a ), in which beneath a portion of the emitter ( 9 ), which corresponds to a horizontal bar of the T-shaped cross-sectional profile, a lateral extent (d) of the spacer ( 8 ) increases with increasing height over the inner base portion ( 4 a ), having a lowest value at an interface between the spacer ( 8 ) and the inner base portion ( 4 a ), wherein a first interface formed by the emitter ( 9 ) and the spacer ( 8 ) meets a second interface formed by the emitter and the inner base portion ( 4 a ) at a first angle (α) which is either a right angle or an obtuse angle, and a third interface formed by the spacer ( 8 ) and the outer base portion ( 4 b ) meets the second interface at a second obtuse angle which is larger than the first angle.