Charging Time of Double-Layer Emitter in Heterojunction Bipolar Transistor Based on Transmission Formalism

We theoretically examine the charging time of a double-layer emitter in heterojunction bipolar transistors (HBTs) on the basis of transmission formalism. It is indicated that the charging time is shorter in the double-layer emitter with an n ++ /n + combination than in the single-layer n + emitter. The nonparabolic band structure is responsible for this finding. Thus, the inclusion of a second layer in the emitter is essential for high-speed operation. The theory also explains the experimentally reported charging times in state-of-the-art high-speed HBTs.