techniques an air bridge 70 as shown in fig2 g is used to connect all collector contacts 60 passive circuit elements such as capacitors and transmission lines are fabricated the wafer is lapped to the desired thickness and via holes 18 see fig1 are fabricated these steps follow standard fabrication techniques the first preferred embodiment of the present invention has the advantage that an hbt can be formed with low base resistance and therefore high operating frequency by the use of the heavily zinc doped base layer 28 if not for the inclusion of the carbon doped base diffusion barrier layer 30 zinc diffusion into the emitter finger 52 driven by heat dissipated by the device and the forward biased base emitter p n junction would degrade the device performance irreversibly over its operating lifetime producing unstable and unpredictable device performance a few preferred embodiments have been described in detail hereinabove it is to be understood that the scope of the invention also comprehends embodiments different from those described yet within the scope of the claims for example the carbon doped diffusion barrier can be used in relation to any two semiconductor layers where it is desired to control the diffusion of dopants from one layer into the other such as diodes transistors lasers etc words of inclusion are to be interpreted as nonexhaustive in considering the scope of the invention internal and external connections can be ohmic capacitive direct.