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The major advances in heterojunction bipolar transistor (HBT) technology since the mid‐1980s is covered in this article. The cutoff frequency ( f T ) and the maximum oscillation frequency ( f max ) have reached record highs. Indium phosphide (InP) HBTs have shown ( f T ) and ( f max ) in excess of 450 GHz and gate delays of 1.95 ps. Silicon–germanium (SiGe) HBTs have demonstrated 350 GHz ( f T ) and gate delays of 3.6 ps. The improved performance of the devices has resulted in impressive circuit results. Static frequency dividers operating at 152 GHz has been reported in InP HBT technology. SiGe HBT technology has demonstrated dynamic frequency dividers running at 110 GHz. Multiplexer/demultiplexer (MUX/DMUX) circuits operating at 40 Gbps (gigabits per second) were demonstrated with InP HBTs. On the other hand, 50‐Gbps 4–1 MUX/DMUX circuits have been shown with SiGe HBTs.