Monte Carlo evaluation of electron transport in heterojunction bipolar transistor base structures

Electron transport through base structures of AlxGa1-xAs heterojunction bipolar transistors is evaluated by Monte Carlo simulation. Simulation results demonstrate the effectiveness of both ballistic launching ramps and graded bases for reducing base transit time. Both techniques are limited, however, in their ability to maintain short transit times across the wide bases that are desirable for reduction of base resistance. Simulation results demonstrate that neither technique is capable of maintaining a 1-ps transit time across a 0.25-µm base. The physical mechanisms responsible for limiting the performance of each structure are identified and a promising hybrid structure is described.