Slide 5 of 64
This slide shows a simplified cross section of the Rockwell GaAs HBT (original picture by Peter Asbeck) showing the usual reasons why bipolar is interesting for high speed and driving capacitive loads such as interconnections on a chip. Transit time through the base is determined by the base thickness which is readily formed by epitaxy. It can be made much thinner than the width of lateral feature sizes that require lithography to set their size. Current flow to the load is vertical through the area of the emitter stripe. Hence when desired, large currents can be switched to drive capacitive loads.